Publikationen von Prof. Dr. Thomas Stöhlker

Alle Publikationen des HI Jena

2022

S. Ringleb, M. Kiffer, N. Stallkamp, S. Kumar, J. Hofbrucker, B. Reich, B. Arndt, G. Brenner, M. Ruiz-Lopéz, S. Düsterer, M. Vogel, K. Tiedtke, W. Quint, T. Stöhlker, and G. G. Paulus
High-intensity laser experiments with highly charged ions in a Penning trap
Physica Scripta 97, 084002 (2022)

Abstract: We have conceived and built the HILITE (High-Intensity Laser-Ion Trap Experiment) Penning-trap setup for the production, confinement and preparation of pure ensembles of highly charged ions in a defined quantum state as a target for various high-intensity lasers. This enables a broad suite of laser-ion interaction studies at high photon energies and/or intensities, such as non-linear photo-ionisation studies. The setup has now been used to perform experiments at one such laser facility, namely the FLASH Free-Electron Laser at DESY in Hamburg, Germany. We describe the experimental possibilities of the apparatus, the results of the first measurements and future experiments at other laser facilities.

B. Zhu, A. Gumberidze, T. Over, G. Weber, Z. Andelkovic, A. Bräuning-Demian, R. J. Chen, D. Dmytriiev, O. Forstner, C. Hahn, F. Herfurth, M. O. Herdrich, P.-M. Hillenbrand, A. Kalinin, F. M. Kröger, M. Lestinsky, Yu. A. Litvinov, E. B. Menz, W. Middents, T. Morgenroth, N. Petridis, Ph. Pfäfflein, M. S. Sanjari, R. S. Sidhu, U. Spillmann, R. Schuch, S. Schippers, S. Trotsenko, L. Varga, G. Vorobyev, and Th. Stöhlker
X-ray emission associated with radiative recombination for Pb⁸²⁺ ions at threshold energies
Physical Review A 105, 052804 (2022)

Abstract: For decelerated bare lead ions at a low beam energy of 10 MeV/u, the x-ray emission associated with radiative recombination (RR) at threshold energies has been studied at the electron cooler of CRYRING@ESR at GSI, Darmstadt. In our experiment, we observed the full x-ray emission pattern by utilizing dedicated x-ray detection chambers installed at 0∘ and 180∘ observation geometry. Most remarkably, no line distortion effects due to delayed emission are present in the well-defined x-ray spectra, spanning a wide range of x-ray energies (from about 5 to 100 keV), which enables us to identify fine-structure resolved Lyman, Balmer, and Paschen x-ray lines along with the RR transitions into the K, L, and M shells of the ions. For comparison with theory, an elaborate theoretical model is established taking into account the initial population distribution via RR for all atomic levels up to Rydberg states with principal quantum number n=165 in combination with time-dependent feeding transitions. Within the statistical accuracy, the experimental data are in very good agreement with the results of rigorous relativistic predictions. Most notably, this comparison sheds light on the contribution of prompt and delayed x-ray emission (up to 70 ns) to the observed x-ray spectra, originating in particular from yrast transitions into inner shells.

K. Schulze, B. Grabiger, R. Loetzsch, B. Marx-Glowna, A. Schmitt, A. Garcia, W. Hippler, L. Huang, F. Karbstein, Z. Konopková, H.-P. Schlenvoigt, J.-P. Schwinkendorf, C. Strohm, T. Toncian, I. Uschmann, H.-C. Wille, U. Zastrau, R. Röhlsberger, T. Stöhlker, T. Cowan, and G. Paulus
Towards perfectly linearly polarized x-rays
Physical Review Research 4, 013220 (2022)

Abstract: In recent years, high-precision x-ray polarimeters have become a key method for the investigation of fundamental physical questions from solid-state physics to quantum optics. Here, we report on the verification of a polarization purity of better than 8×10−11 at an x-ray free-electron laser, which implies a suppression of the incoming photons to the noise level in the crossed polarizer setting. This purity provides exceptional sensitivity to tiny polarization changes and offers intriguing perspectives for fundamental tests of quantum electrodynamics.

J. Stierhof, S. Kühn, M. Winter, P. Micke, R. Steinbrügge, C. Shah, N. Hell, M. Bissinger, M. Hirsch, R. Ballhausen, M. Lang, C. Gräfe, S. Wipf, R. Cumbee, G. L. Betancourt-Martinez, S. Park, J. Niskanen, M. Chung, F. S. Porter, T. Stöhlker, T. Pfeifer, G. V. Brown, S. Bernitt, P. Hansmann, J. Wilms, J. R. Crespo López-Urrutia, and M. A. Leutenegger
A new benchmark of soft X-ray transition energies of Ne, CO2, and SF6: paving a pathway towards ppm accuracy
The European Physical Journal D 76, 38 (2022)

Abstract: A key requirement for the correct interpretation of high-resolution X-ray spectra is that transition energies are known with high accuracy and precision. We investigate the K-shell features of Ne, CO2, and SF6 gases, by measuring their photo ion-yield spectra at the BESSY II synchrotron facility simultaneously with the 1s–np fluorescence emission of He-like ions produced in the Polar-X EBIT. Accurate ab initio calculations of transitions in these ions provide the basis of the calibration. While the CO2 result agrees well with previous measurements, the SF6 spectrum appears shifted by ∼0.5 eV, about twice the uncertainty of the earlier results. Our result for Ne shows a large departure from earlier results, but may suffer from larger systematic effects than our other measurements. The molecular spectra agree well with our results of time-dependent density functional theory. We find that the statistical uncertainty allows calibrations in the desired range of 1–10 meV, however, systematic contributions still limit the uncertainty to ∼40–100 meV, mainly due to the temporal stability of the monochromator energy scale. Combining our absolute calibration technique with a relative energy calibration technique such as photoelectron energy spectroscopy will be necessary to realize its full potential of achieving uncertainties as low as 1–10 meV.

P.-M. Hillenbrand, S. Hagmann, Y. S. Kozhedub, E. P. Benis, C. Brandau, R. J. Chen, D. Dmytriiev, O. Forstner, J. Glorius, R. E. Grisenti, A. Gumberidze, M. Lestinsky, Yu. A. Litvinov, E. B. Menz, T. Morgenroth, S. Nanos, N. Petridis, Ph. Pfäfflein, H. Rothard, M. S. Sanjari, R. S. Sidhu, U. Spillmann, S. Trotsenko, I. I. Tupitsyn, L. Varga, and Th. Stöhlker
Single and double K-shell vacancy production in slow Xe⁵⁴⁺,⁵³⁺-Xe collisions
Physical Review A 105, 022810 (2022)

Abstract: We present an experimental and theoretical study of symmetric Xe54++Xe collisions at 50, 30, and 15 MeV/u, corresponding to strong perturbations with vK/vp=1.20, 1.55, and 2.20, respectively (vK is the classical K-shell orbital velocity and vp is the projectile velocity), as well as Xe53++Xe collisions at 15 MeV/u. For each of these systems, x-ray spectra are measured under a forward angle of 35∘ with respect to the projectile beam. Target satellite and hypersatellite radiation Kαs2,1 and Kαhs2,1, respectively, are analyzed and used to derive cross-section ratios for double-to-single target K-shell vacancy production. We compare our experimental results to relativistic time-dependent two-center calculations.

2021

W. Middents, G. Weber, U. Spillmann, T. Krings, M. Vockert, A. Volotka, A. Surzhykov, and T. Stöhlker
Possible Polarization Measurements in Elastic Scattering at the Gamma Factory Utilizing a 2D Sensitive Strip Detector as Dedicated Compton Polarimeter
Annalen der Physik 2100285, 2100285 (2021)
F. Kröger, G. Weber, S. Hirlaender, R. Alemany-Fernandez, M. Krasny, T. Stöhlker, I. Tolstikhina, and V. Shevelko
Charge‐State Distributions of Highly Charged Lead Ions at Relativistic Collision Energies
Annalen der Physik -, 2100245 (2021)
A. Gumberidze, D. Thorn, A. Surzhykov, C. Fontes, D. Banas, H. Beyer, W. Chen, R. Grisenti, S. Hagmann, R. Hess, P.-M. Hillenbrand, P. Indelicato, C. Kozhuharov, M. Lestinsky, R. Märtin, N. Petridis, R. Popov, R. Schuch, U. Spillmann, S. Tashenov, S. Trotsenko, A. Warczak, G. Weber, W. Wen, D. Winters, N. Winters, Z. Yin, and T. Stöhlker
Angular Distribution of Characteristic Radiation Following the Excitation of He-Like Uranium in Relativistic Collisions
Atoms 9, 20 (2021)

Abstract: In this paper, we present an experimental and theoretical study of excitation processes for the heaviest stable helium-like ion, that is, He-like uranium occurring in relativistic collisions with hydrogen and argon targets. In particular, we concentrate on angular distributions of the characteristic K alpha radiation following the K -> L excitation of He-like uranium. We pay special attention to the magnetic sub-level population of the excited 1s2l(j) states, which is directly related to the angular distribution of the characteristic K alpha radiation. We show that the experimental data can be well described by calculations taking into account the excitation by the target nucleus as well as by the target electrons. Moreover, we demonstrate for the first time an important influence of the electron-impact excitation process on the angular distributions of the K alpha radiation produced by excitation of He-like uranium in collisions with different targets.

D. Winzen, V. Hannen, M. Bussmann, A. Buß, C. Egelkamp, L. Eidam, Z. Huang, D. Kiefer, S. Klammes, Th. Kühl, M. Loeser, X. Ma, W. Nörtershäuser, H.-W. Ortjohann, R. Sánchez, M. Siebold, Th. Stöhlker, J. Ullmann, J. Vollbrecht, Th. Walther, H. Wang, Ch. Weinheimer, and D. F. A. Winters
Laser spectroscopy of the 2S1/2-2P1/2, 2P3/2 transitions in stored and cooled relativistic C³⁺ ions
Scientific Reports 11, 9370 (2021)

Abstract: The 2S1/2−2P1/2 and 2S1/2−2P3/2 transitions in Li-like carbon ions stored and cooled at a velocity of beta=0.47 in the experimental storage ring (ESR) at the GSI Helmholtz Centre in Darmstadt have been investigated in a laser spectroscopy experiment. Resonance wavelengths were obtained using a new continuous-wave UV laser system and a novel extreme UV (XUV) detection system to detect forward emitted fluorescence photons. The results obtained for the two transitions are compared to existing experimental and theoretical data. A discrepancy found in an earlier laser spectroscopy measurement at the ESR with results from plasma spectroscopy and interferometry has been resolved and agreement between experiment and theory is confirmed.

M. Stapelfeld, F. Schmidl, P. Seidel, S. Stück, V. Tympel, T. Stöhlker, D. Haider, M. Schwickert, T. Sieber, M. Schmelz, T. Schönau, and R. Stolz
Application Driven Optimization of Cryogenic Current Comparators (CCC) for Beam Storage Rings
IEEE Transactions on Applied Superconductivity 31, 1600504 (2021)

Abstract: Non-destructive measurements of nA beam currents in particle beam storage rings by detecting the azimuthal magnetic field generated by moving charged particles with a Cryogenic Current Comparator (CCC) are well established. The detection of beam currents with small amplitudes with a CCC in a storage ring demands a high slew rate which is caused by the rapid change of the beam current exceeding the operational limit of the SQUID in flux-locked loop mode. Previous solutions to increase the slew rate used a LCR first-order low-pass filter were a small resistor, unfortunately, dominated the current noise of the CCC. In this work we present a novel take by adding a second resonator into the CCC which in turn allows for higher resistances of the LCR low-pass filter and therefore lower thermal current noise. A second challenge connected with this CCC approach is the residual magnetization of the highly permeable magnetic core and the resulting shielding currents in the superconducting circuits of the CCC. The timing of a storage ring in the range of minutes opens a way to reduce these DC currents using a LR high-pass filter. Using serial sub-micro ohm resistors, time constants in the hour range can be achieved to improve the stability and performance of the CCC system.

W. Nörtershäuser, A. Surzhykov, R. Sánchez, B. Botermann, G. Gwinner, G. Huber, S. Karpuk, T. Kühl, C. Novotny, S. Reinhardt, G. Saathoff, T. Stöhlker, and A. Wolf
Polarization-dependent disappearance of a resonance signal: Indication for optical pumping in a storage ring?
Physical Review Accelerators and Beams 24, 024701 (2021)

Abstract: We report on laser spectroscopic measurements on Li+ ions in the experimental storage ring ESR at the GSI Helmholtz Centre for Heavy Ion Research. Driving the 2s 3S1(F=3/2)↔2p 3P2(F=5/2)↔2s 3S1(F=5/2) Λ-transition in 7Li+ with two superimposed laser beams it was found that the use of circularly polarized light leads to a disappearance of the resonance structure in the fluorescence signal. This can be explained by optical pumping into a dark state of polarized ions. We present a detailed theoretical analysis of this process that supports the interpretation of optical pumping and demonstrates that the polarization induced by the laser light must then be at least partially maintained during the round trip of the ions in the storage ring. Such polarized ion beams in storage rings will provide opportunities for new experiments, especially on parity violation.

B. Marx-Glowna, I. Uschmann, K. Schulze, H. Marschner, H.-C. Wille, K. Schlage, T. Stöhlker, R. Röhlsberger, and G. Paulus
Advanced X-ray polarimeter design for nuclear resonant scattering
Journal of Synchrotron Radiation 28, 120 (2021)

Abstract: This work presents the improvements in the design and testing of polarimeters based on channel-cut crystals for nuclear resonant scattering experiments at the 14.4 keV resonance of Fe-57. By using four asymmetric reflections at asymmetry angles of alpha(1) = -28 degrees, alpha(2) = 28 degrees, alpha(3) = -28 degrees and alpha(4) = 28 degrees, the degree of polarization purity could be improved to 2.2 x 10(-9). For users, an advanced polarimeter without beam offset is now available at beamline P01 of the storage ring PETRA III.

2020

W. Płaczek, A. Abramov, S. E. Alden, R. Alemany Fernandez, P. S. Antsiferov, A. Apyan, H. Bartosik, E. G. Bessonov, N. Biancacci, J. Bieroń, A. Bogacz, A. Bosco, R. Bruce, D. Budker, K. Cassou, F. Castelli, I. Chaikovska, C. Curatolo, P. Czodrowski, A. Derevianko, K. Dupraz, Y. Dutheil, K. Dzierżęga, V. Fedosseev, N. Fuster Martinez, S. M. Gibson, B. Goddard, A. Gorzawski, S. Hirlander, J. M. Jowett, R. Kersevan, M. Kowalska, M. W. Krasny, F. Kroeger, D. Kuchler, M. Lamont, T. Lefevre, D. Manglunki, B. Marsh, A. Martens, J. Molson, D. Nutarelli, L. J. Nevay, A. Petrenko, V. Petrillo, S. Redaelli, Y. Peinaud, S. Pustelny, S. Rochester, M. Sapinski, M. Schaumann, R. Scrivens, L. Serafini, V. P. Shevelko, T. Stoehlker, A. Surzhykov, I. Tolstikhina, F. Velotti, G. Weber, Y. K. Wu, C. Yin-Vallgren, M. Zanetti, F. Zimmermann, M. S. Zolotorev, and F. Zomer
The Gamma Factory Project at CERN: a New Generation of Research Tools Made of Light
Acta Physica Polonica B 13, 645 (2020)
V. Zaytsev, A. Surzhykov, V. Serbo, V. Kosheleva, M. Groshev, V. Yerokhin, V. Shabaev, and T. Stöhlker
Atomic processes with twisted electrons
Journal of Physics: Conference Series 1412, 052013 (2020)

Abstract: The present status of the fully-relativistic nonperturbative calculations of the fundamental atomic processes with twisted electrons is presented. In particular, the elastic (Mott) scattering, the radiative recombination, and for the very first time, the Bremsstrahlung processes are considered. The electron-ion interaction is accounted for in a nonperturbative manner, that allows obtaining reliable results for heavy systems. We investigate the influence of the "twistedness" of the incoming electron on the angular and polarization properties of the emitted electrons and photons for the elastic and inelastic scattering, respectively. It is found that these properties exhibit a strong dependence on the opening angle of the vortex electron beam in all processes considered.

M. Kiffer, S. Ringleb, N. Stallkamp, S. Kumar, B. Arndt, M. Vogel, W. Quint, and T. Stöhlker
Characterisation of ion bunches by a single-pass non-destructive charge counter
Journal of Physics: Conference Series 1412, 242004 (2020)

Abstract: Synopsis We present non-destructive single-pass ion bunch detection and characterisation by measuring the induced image charge in a detection electrode. The presented technique allows direct determination of ion kinetic energy, absolute ion number and spatial ion bunch length. We will show the results of corresponding measurements with bunches of low-energy highly charged ions and discuss the minimum detectable number of charges.

S. Hagmann, P. Hillenbrand, Y. Litvinov, U. Spillmann, and T. Stöhlker
Coincident mapping of e+and e-from free-free pair production in a magnetic toroidal lepton spectrometer
Journal of Physics: Conference Series 1412, 232004 (2020)

Abstract: We discuss the electron-optical properties of a toroidal magnetic sector spectrometer and its suitablilty for electron-positron pair spectroscopy in relativistic ion-atom collisions in the future HESR storage ring at FAIR. With the simultaneous mapping of electrons and positrons and geometric invariants in the lepton trajectorties this instrument offers a very high efficiency for studies of vector momentum correlation in free-free pair production.

P. Jagodziłski, D. Banaś, M. Pajek, A. Warczak, H. Beyer, A. Gumberidze, G. Weber, Th. Stöhlker, and M. Trassinelli
Concept and simulations of a high-resolution asymmetric von Hamos X-ray spectrometer for CRYRING@ESR electron cooler
Journal of Physics: Conference Series 1412, 132031 (2020)

Abstract: A concept of a high resolution asymmetric von Hamos X-ray spectrometer for the CRYRING@ESR electron cooler is described and its characteristics obtained by ray-tracing Monte-Carlo simulations are presented. The spectrometer will be used to study the QED e-ects in H-like medium-Z ions by measuring the energies of X-rays from radiative recombination of highly charged ions with cooling electrons, with a ppm precision of energy determination.

A. Borovik, G. Weber, V. Hilbert, H. Lin, P. Pfäfflein, B. Zhu, C. Hahn, M. Lestinsky, S. Schippers, Th. Stöhlker, and J. Rothhardt
Development of a detector to register low-energy, charge-changed ions from ionization experiments at CRYRING@ESR
Journal of Physics: Conference Series 1412, 242003 (2020)

Abstract: A detector setup for registering ion species between the poles of a dipole magnet at CRYRING@ESR has been developed. It is based on a scintillator delivering light via a quartz light guide onto a semiconductor photomultiplier. The detector is capable of operating in a strong magnetic field. It can be swiftly retracted from the exposition area during the beam injection into the ring and repositioned back for the measurement cycle to avoid unnecessary exposition and, thus, to increase the scintillator life time.

N. Stallkamp, S. Ringleb, B. Arndt, M. Kiffer, S. Kumar, G. Paulus, W. Quint, T. Stöhlker, and M. Vogel
HILITE-A well-defined ion target for laser experiments
Journal of Physics: Conference Series 1412, 092009 (2020)

Abstract: We present a Penning-trap-based setup for the study of light-matter interactions in the high-power and/or high-intensity laser regime, such as multi-photon ionization and field ionization. The setup applies ioncloud formation techniques to highly charged ions to the end of specific target preparation, as well as nondestructive detection techniques to identify and quantify the interaction educts and products.

A. Gumberidze, C. Kozhuharov, R. Zhang, S. Trotsenko, Y. Kozhedub, R. Du, H. Bois, F. Beyer, K.-H. Blumenhagen, C. Brandau, A. Bräuning-Demian, W. Chen, O. Forstner, B. Gao, T. Gassner, R. Grisenti, S. Hagmann, P.-M. Hillenbrand, P. Indelicato, A. Kumar, M. Lestinsky, Y. Litvinov, N. Petridis, D. Schury, U. Spillmann, C. Trageser, M. Trassinelli, X. Tu, and T. Stöhlker
Impact parameter sensitive study of inner-shell atomic processes in Xe54+, Xe52+ → Xe collisions
Journal of Physics: Conference Series 1412, 142015 (2020)

Abstract: In this work, we present a pilot experiment in the experimental storage ring (ESR) at GSI devoted to impact parameter sensitive studies of inner shell atomic processes for bare and He-like xenon ions (Xe54+, Xe52+) colliding with neutral xenon gas atoms. The projectile and target x-rays have been measured at different observation angles for all impact parameters as well as for the impact parameter range of ∼35 - 70 fm.

Y. Litvinov, T. Stöhlker, X. Ma, Y. Zhang, and T. Yamaguchi
Nuclear physics research at heavy ion accelerators: Precision studies with stored and cooled exotic nuclei
Journal of Physics: Conference Series 1401, 012001 (2020)

Abstract: This contribution is based on the plenary presentation at the 14th International Conference on Heavy Ion Accelerator Technology (HIAT-2018) in Lanzhou, China. Heavy-ion storage rings offer unparalleled opportunities for precision experiments in the realm of nuclear structure, atomic physics and astrophysics. A brief somewhat biased review of the presently ongoing research programs is given as well as the future projects are outlined. The limited space does not allow for detailed description of individual experiments, which shall - to some extent - be compensated by extended bibliography.

E. Menz, C. Hahn, P. Pfäfflein, G. Weber, and T. Stöhlker
Performance of a scintillator-based ion detector for CRYRING@ESR
Journal of Physics: Conference Series 1412, 232006 (2020)

Abstract: A detector based on the scintillator material YAP:Ce and capable of counting single ions is presented. The detector consists of a YAP:Ce crystal and a light guide operating in ultra high vacuum and a conventional photomultiplier outside the vacuum. The crystal demonstrated the necessary radiation hardness against heavy ion irradiation. The detector has been commissioned at CRYRING@ESR and its detection capabilities have been confirmed with beam from the local source.

M. Lestinsky, Z. Andelkovic, M. Bai, A. Bräuning-Demian, V. Chetvertkova, S. Fedotova, O. Geithner, W. Geithner, O. Gorda, F. Herfurth, A. Kalinin, A. Källberg, S. Litvinov, E. Menz, K. Mohr, R. Sanchez, T. Stöhlker, G. Vorobjev, and U. Weinrich
Status of CRYRING@ESR and preparations for first experiments
Journal of Physics: Conference Series 1412, 23007 (2020)

Abstract: CRYRING was moved from Stockholm to Darmstadt, modernized and integrated into the GSI/FAIR beamline topology behind ESR. As CRYRING@ESR, it will receive and store heavy, highly charged ions from all species the present accelerator chain is capable of producing. An extensive research program on low-energy atomic collisions, spectroscopy and nuclear reactions was proposed. The facility is gradually completing commissioning, ion beams from the local injector branch have already been stored and prototype experiments performed. We present the machine status and highlight some planned experiments.

D. Schury, A. Méry, J. Ramillon, L. Adoui, J.-Y. Chesnel, A. Lévy, S. Macé, C. Prigent, J. Rangama, P. Rousseau, S. Steydli, M. Trassinelli, D. Vernhet, A. Gumberidze, T. Stöhlker, A. Bruning-Demian, C. Hahn, U. Spillmann, and E. Lamour
The low energy beamline of the FISIC experiment: Current status of construction and performance
Journal of Physics: Conference Series 1412, 162011 (2020)

Abstract: Ion-ion collisions between slow (kev/u) and fast (MeV/u) ions play an important role in for example astrophysical or inertial fusion plasmas as well as in ion-matter interaction. In this regime the energy transfer is maximum, as all primary electronic processes reach their maximum. At the same time up to today no reliable experimental data exists while being difficult to treat accurately by theory. We present the current status and performance of the low energy beam-line of the FISIC experiment which aims at filling in the blanks in this regime.

L. Varga, K. Blaum, T. Davinson, J. Glorius, B. Jurado, C. Langer, C. Lederer-Woods, Y. Litvinov, R. Reifarth, Z. Slavkovská, T. Stöhlker, P. Woods, and Y. Xing
Towards background-free studies of capture reactions in a heavy-ion storage ring
Journal of Physics: Conference Series 1412, 232011 (2020)

Abstract: Stored and cooled highly-charged ions offer unprecedented capabilities for precision studies in realm of atomic-, nuclear-structure and astrophysics. In context of the latter, after the successful investigation of the cross section of 96Ru(p,γ) in 2009, in 2016 the first measurement of the 124Xe(p,γ)125Cs reaction was performed at the Experimental Storage Ring (ESR) at GSI.

Y. Xing, J. Glorius, L. Varga, L. Bott, C. Brandau, B. Brückner, R. Chen, X. Chen, S. Dababneh, T. Davinson, P. Erbacher, S. Fiebiger, T. Gaßner, K. Göbel, M. Groothuis, A. Gumberidze, G. Gyürky, M. Heil, R. Hess, R. Hensch, P. Hillmann, P.-M. Hillenbrand, O. Hinrichs, B. Jurado, T. Kausch, A. Khodaparast, T. Kisselbach, N. Klapper, C. Kozhuharov, D. Kurtulgil, G. Lane, C. Langer, C. Lederer-Woods, M. Lestinsky, S. Litvinov, Y. Litvinov, B. Löher, N. Petridis, U. Popp, M. Reed, R. Reifarth, M. Sanjari, H. Simon, Z. Slavkovská, U. Spillmann, M. Steck, T. Stöhlker, J. Stumm, T. Szücs, T. Nguyen, A. Zadeh, B. Thomas, S. Torilov, H. Törnqvist, C. Trageser, S. Trotsenko, M. Volknandt, M. Wang, M. Weigand, C. Wolf, P. Woods, Y. Zhang, and X. Zhou
Determination of luminosity for in-ring reactions: A new approach for the low-energy domain
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 982, 164367 (2020)

Abstract: Luminosity is a measure of the colliding frequency between beam and target and it is a crucial parameter for the measurement of absolute values, such as reaction cross sections. In this paper, we make use of experimental data from the ESR storage ring to demonstrate that the luminosity can be precisely determined by modelling the measured Rutherford scattering distribution. The obtained results are in good agreement with an independent measurement based on the x-ray normalization method. Our new method provides an alternative way to precisely measure the luminosity in low-energy stored-beam configurations. This can be of great value in particular in dedicated low-energy storage rings where established methods are difficult or impossible to apply.

M. A. Leutenegger, S. Kühn, P. Micke, R. Steinbrügge, J. Stierhof, C. Shah, N. Hell, M. Bissinger, M. Hirsch, R. Ballhausen, M. Lang, C. Gräfe, S. Wipf, R. Cumbee, G. L. Betancourt-Martinez, S. Park, V. A. Yerokhin, A. Surzhykov, W. C. Stolte, J. Niskanen, M. Chung, F. S. Porter, T. Stöhlker, T. Pfeifer, J. Wilms, G. V. Brown, J. R. C. López-Urrutia, and S. Bernitt
High-Precision Determination of Oxygen Kα Transition Energy Excludes Incongruent Motion of Interstellar Oxygen
Physical Review Letters 125, 243001 (2020)

Abstract: We demonstrate a widely applicable technique to absolutely calibrate the energy scale of x-ray spectra with experimentally well-known and accurately calculable transitions of highly charged ions, allowing us to measure the K-shell Rydberg spectrum of molecular O2 with 8 meV uncertainty. We reveal a systematic ∼450  meV shift from previous literature values, and settle an extraordinary discrepancy between astrophysical and laboratory measurements of neutral atomic oxygen, the latter being calibrated against the aforementioned O2 literature values. Because of the widespread use of such, now deprecated, references, our method impacts on many branches of x-ray absorption spectroscopy. Moreover, it potentially reduces absolute uncertainties there to below the meV level.

M. Togawa, S. Kühn, C. Shah, P. Amaro, R. Steinbrügge, J. Stierhof, N. Hell, M. Rosner, K. Fujii, M. Bissinger, R. Ballhausen, M. Hoesch, J. Seltmann, S. Park, F. Grilo, F. S. Porter, J. P. Santos, M. Chung, T. Stöhlker, J. Wilms, T. Pfeifer, G. V. Brown, M. A. Leutenegger, S. Bernitt, and J. R. C. López-Urrutia
Observation of strong two-electron–one-photon transitions in few-electron ions
Physical Review A 102, 052831 (2020)

Abstract: We resonantly excite the K series of O5+ and O6+ up to principal quantum number n=11 with monochromatic x rays, producing K-shell holes, and observe their relaxation by soft-x-ray emission. Some photoabsorption resonances of O5+ reveal strong two-electron–one-photon (TEOP) transitions. We find that for the [(1s2s)15p3/2]3/2;1/2 states, TEOP relaxation is by far stronger than the radiative decay and competes with the usually much faster Auger decay path. This enhanced TEOP decay arises from a strong correlation with the near-degenerate upper states [(1s2p3/2)14s]3/2;1/2 of a Li-like satellite blend of the He-like Kα transition. Even in three-electron systems, TEOP transitions can play a dominant role, and the present results should guide further research on the ubiquitous and abundant many-electron ions where electronic energy degeneracies are far more common and configuration mixing is stronger.

D. Dmytriiev, M. S. Sanjari, Yu. A. Litvinov, and Th. Stöhlker
Software defined radio for Schottky analysis in storage rings
Journal of Physics: Conference Series 1668, 012014 (2020)

Abstract: Resonant Schottky cavity pickups have applications in the measurements of beam parameters in a storage ring. Apart from that they can be used in nondestructive in-ring decay studies of radioactive ion beams. In order to obtain the results of an experiment suitable Data Acquisition System (DAQ) is necessary. Several DAQs were used at Experimental Storage Ring (ESR) at GSI based on the different hardware and software solutions such as TCAP or NTCAP. The goal of this work is to design a prototype of a DAQ using open hardware and open source software defined radio (SDR) and conduct the test measurements at ESR.

F. M. Kröger, G. Weber, M. O. Herdrich, J. Glorius, C. Langer, Z. Slavkovská, L. Bott, C. Brandau, B. Brückner, K. Blaum, X. Chen, S. Dababneh, T. Davinson, P. Erbacher, S. Fiebiger, T. Gaßner, K. Göbel, M. Groothuis, A. Gumberidze, Gy. Gyürky, S. Hagmann, C. Hahn, M. Heil, R. Hess, R. Hensch, P. Hillmann, P.-M. Hillenbrand, O. Hinrichs, B. Jurado, T. Kausch, A. Khodaparast, T. Kisselbach, N. Klapper, C. Kozhuharov, D. Kurtulgil, G. Lane, C. Lederer-Woods, M. Lestinsky, S. Litvinov, Yu. A. Litvinov, B. Löher, F. Nolden, N. Petridis, U. Popp, M. Reed, R. Reifarth, M. S. Sanjari, H. Simon, U. Spillmann, M. Steck, J. Stumm, T. Szücs, T. T. Nguyen, A. T. Zadeh, B. Thomas, S. Yu. Torilov, H. Törnqvist, C. Trageser, S. Trotsenko, M. Volknandt, M. Weigand, C. Wolf, P. J. Woods, V. P. Shevelko, I. Yu. Tolstikhina, and Th. Stöhlker
Electron capture of Xe54+ in collisions with H2 molecules in the energy range between 5.5 and 30.9 MeV/u
Physical Review A 102, 042825 (2020)

Abstract: The electron-capture process was studied for Xe54+ colliding with H2 molecules at the internal gas target of the Experimental Storage Ring (ESR) at GSI, Darmstadt. Cross-section values for electron capture into excited projectile states were deduced from the observed emission cross section of Lyman radiation, being emitted by the hydrogenlike ions subsequent to the capture of a target electron. The ion beam energy range was varied between 5.5 and 30.9 MeV/u by applying the deceleration mode of the ESR. Thus, electron-capture data were recorded at the intermediate and, in particular, the low-collision-energy regime, well below the beam energy necessary to produce bare xenon ions. The obtained data are found to be in reasonable qualitative agreement with theoretical approaches, while a commonly applied empirical formula significantly overestimates the experimental findings.

V. Popov, V. M. Shabaev, D. A. Telnov, I. I. Tupitsyn, I. A. Maltsev, Y. S. Kozhedub, I. Bondarev, V. Kozin, X. Ma, G. Plunien, T. Stöhlker, D. A. Tumakov, and V. A. Zaytsev
How to access QED at a supercritical Coulomb field
Physical Review D 102, 076005 (2020)
P. Boller, A. Zylstra, P. Neumayer, L. Bernstein, C. Brabetz, J. Despotopulos, J. Glorius, J. Hellmund, E. Henry, J. Hornung, J. Jeet, J. Khuyagbaatar, L. Lens, S. Roeder, Th. Stöhlker, A. Yakushev, Y. Litvinov, D. Shaughnessy, V. Bagnoud, T. Kühl, and D. Schneider
First on-line detection of radioactive fission isotopes produced by laser-accelerated protons
Scientific Reports 10, 17183 (2020)

Abstract: The on-going developments in laser acceleration of protons and light ions, as well as the production of strong bursts of neutrons and multi-MeV photons by secondary processes now provide a basis for novel high-flux nuclear physics experiments. While the maximum energy of protons resulting from Target Normal Sheath Acceleration is presently still limited to around 100MeV, the generated proton peak flux within the short laser-accelerated bunches can already today exceed the values achievable at the most advanced conventional accelerators by orders of magnitude. This paper consists of two parts covering the scientific motivation and relevance of such experiments and a first proof-of-principle demonstration. In the presented experiment pulses of 200J at ≈500fs duration from the PHELIX laser produced more than 10 12 protons with energies above 15MeV in a bunch of sub-nanosecond duration. They were used to induce fission in foil targets made of natural uranium. To make use of the nonpareil flux, these targets have to be very close to the laser acceleration source, since the particle density within the bunch is strongly affected by Coulomb explosion and the velocity differences between ions of different energy. The main challenge for nuclear detection with high-purity germanium detectors is given by the strong electromagnetic pulse caused by the laser-matter interaction close to the laser acceleration source. This was mitigated by utilizing fast transport of the fission products by a gas flow to a carbon filter, where the γ -rays were registered. The identified nuclides include those that have half-lives down to 39s. These results demonstrate the capability to produce, extract, and detect short-lived reaction products under the demanding experimental condition imposed by the high-power laser interaction. The approach promotes research towards relevant nuclear astrophysical studies at conditions currently only accessible at nuclear high energy density laser facilities.

M. Sanjari, D. Dmytriiev, Y. Litvinov, O. Gumenyuk, R. Hess, R. Joseph, S. Litvinov, M. Steck, and Th. Stöhlker
A 410 MHz resonant cavity pickup for heavy ion storage rings
Review of Scientific Instruments 91, 083303 (2020)

Abstract: An improved design of a longitudinally sensitive resonant Schottky cavity pickup for the heavy ion storage rings of the Facility for Antiproton and Ion Research in Europe (FAIR) project is reported. The new detector has a higher measured Q value of ∼3000 and a higher simulated shunt impedance of 473.3 kω. It is possible to vary the sensitivity of the cavity with a motorized mechanism by inserting a dissipative blade during the operation based on experimental needs. Apart from a lower price tag, the new design features a more robust and production-friendly mechanical structure suitable for a series production in the future FAIR project. The manufactured cavity was built temporarily into the experimental storage ring and had delivered its first results using stored heavy ion beams. The structure, simulation results, and performance of this cavity are presented in this work.

S. Kühn, C. Shah, J. López-Urrutia, K. Fujii, R. Steinbrügge, J. Stierhof, M. Togawa, Z. Harman, N. Oreshkina, C. Cheung, M. Kozlov, S. Porsev, M. Safronova, J. Berengut, M. Rosner, M. Bissinger, R. Ballhausen, N. Hell, S. Park, M. Chung, M. Hoesch, J. Seltmann, A. Surzhykov, V. Yerokhin, J. Wilms, F. Porter, T. Stöhlker, C. Keitel, T. Pfeifer, G. Brown, M. Leutenegger, and S. Bernitt
High Resolution Photoexcitation Measurements Exacerbate the Long-Standing Fe XVII Oscillator Strength Problem
Physical Review Letters 124, 225001 (2020)

Abstract: For more than 40 years, most astrophysical observations and laboratory studies of two key soft x-ray diagnostic 2p-3d transitions, 3C and 3D, in Fe XVII ions found oscillator strength ratios f(3C)/f(3D) disagreeing with theory, but uncertainties had precluded definitive statements on this much studied conundrum. Here, we resonantly excite these lines using synchrotron radiation at PETRA III, and reach, at a millionfold lower photon intensities, a 10 times higher spectral resolution, and 3 times smaller uncertainty than earlier work. Our final result of f(3C)/f(3D)=3.09(8)(6) supports many of the earlier clean astrophysical and laboratory observations, while departing by five sigmas from our own newest large-scale ab initio calculations, and excluding all proposed explanations, including those invoking nonlinear effects and population transfers.

H. Bernhardt, A. Schmitt, B. Grabiger, B. Marx-Glowna, R. Loetzsch, H.-C. Wille, D. Bessas, A. Chumakov, R. Rüffer, R. Röhlsberger, T. Stöhlker, I. Uschmann, G. Paulus, and K. Schulze
Ultra-high precision x-ray polarimetry with artificial diamond channel cuts at the beam divergence limit
Physical Review Research 2, 023365 (2020)

Abstract: We report on the use of synthetic single-crystal diamonds for high definition x-ray polarimetry. The diamonds are precision mounted to form artificial channel-cut crystals (ACCs). Each ACC supports four consecutive reflections with a scattering angle 2ΘB of 90°. We achieved a polarization purity of 3.0×10−10 at beamline ID18 of the European Synchrotron Radiation Facility (ESRF). When the x-ray beam's horizontal divergence was reduced through additional collimation from 17 to 8.4μrad, the polarization purity improved to 1.4×10−10. Precision x-ray polarimetry thus has reached the limit, where the purity is determined by the divergence of the beam. In particular, this result is important for polarimetry at fourth generation x-ray sources, which provide diffraction-limited x-ray beams. The sensitivity expected as a consequence of the present work will pave the way for exploring new physics such as the investigation of vacuum birefringence.

C. Hahn, E. Menz, P. Pfäfflein, G. Weber, and T. Stöhlker
A scintillator‐based particle detector for CRYRING@ESR
X-Ray Spectrometry 49, 338 (2020)

Abstract: With the unprecedented range of ion species and energies offered by the newly commissioned CRYRING facility, the availability of single ion detectors is of significant importance as part of standard instrumentation as well as for novel experiments. A detector system was constructed on the basis of the YAP:Ce crystal scintillator, which is at once radiation‐hard, fast, and affordable. Results of a characterization experiment confirmed the feasibility of the setup for incident ion rates on the order of MHz and found a critical fluence of some 10¹³ cm⁻² upon which the crystal is rendered locally blind to further ion irradiation. The device was first used in CRYRING commissioning runs in August and November 2018. Future efforts will complete the integration of the detector into the GSI control and data acquisition system MBS.

P.-M. Hillenbrand, S. Hagmann, M. Groshev, D. Banaś, E. Benis, C. Brandau, E. De Filippo, O. Forstner, J. Glorius, R. Grisenti, A. Gumberidze, D. Guo, B. Hai, M. Herdrich, M. Lestinsky, Y. Litvinov, E. Pagano, N. Petridis, M. Sanjari, D. Schury, U. Spillmann, S. Trotsenko, M. Vockert, G. Weber, V. Yerokhin, and Th. Stöhlker
Radiative electron capture to the continuum in U89+ + N2 collisions: Experiment and theory
Physical Review A 101, 022708 (2020)

Abstract: For U89+ projectiles colliding at a beam energy of 75.91 MeV/u with a N2 target, we present a coincidence measurement between the cusp electrons emitted under an angle of 0° with respect to the projectile beam and the photons emitted under a polar angle of 90°. This radiative-electron-capture-to-continuum cusp directly probes the theory of electron-nucleus bremsstrahlung up to the high-energy endpoint in inverse kinematics. In the present study, significant improvement with respect to the experimental accuracy has been achieved, resulting in a finer agreement between experimental and theoretical results.

N. Stallkamp, S. Ringleb, B. Arndt, M. Kiffer, S. Kumar, T. Morgenroth, G. G. Paulus, W. Quint, Th. Stöhlker, and M. Vogel
HILITE—A tool to investigate interactions of matter and light
X-Ray Spectrometry 49, 188 (2020)

Abstract: Detailed investigations of laser–ion interactions require well‐defined ion targets and detection techniques for high‐sensitivity measurements of reaction educts and products. To this end, we have designed and built the High‐Intensity Laser‐Ion Trap Experiment Penning trap setup, which features various ion‐target preparation techniques including selection, cooling, compression, and positioning as well as destructive and non‐destructive measurement techniques to determine the number of stored ions for all charge states individually and simultaneously. We have recently performed first commissioning experiments of ion deceleration and dynamic ion capture with highly charged ion bunches from an electron beam ion source. We have characterized our single‐pass non‐destructive ion counter in detail and were able to determine the ion velocity as well as the number of ions from the signals acquired.

O. Forstner, D. Bemmerer, T. Cowan, R. Dressler, A. Junghans, D. Schumann, T. Stöhlker, T. Szücs, A. Wagner, and K. Zuber
Opportunities for measurements of astrophysical-relevant alpha-capture reaction rates at CRYRING@ESR
X-Ray Spectrometry 49, 129 (2020)

Abstract: The heavy-ion storage ring CRYRING@ESR has recently been installed and commissioned at GSI as one of the first installations of the upcoming Facility for Antiproton and Ion Research (FAIR). It is designed to store highly charged ions in the energy range between 300?keV/u and about 10?MeV/u. It will incorporate a gas-jet target providing high-density jets of, among other gases, hydrogen and helium. This will allow to study alpha-capture reaction rates of astrophysical interest in the energy range of the Gamow window for core-collapse supernovae. Special interest comes from the long-lived radio-isotope 44Ti (t1/2?=?58.9?years), which is supposed to be produced in the alpha-rich freeze-out during such an event. The nucleosynthesis of this isotope is of great interest, as the amount of material produced can be estimated by direct observation in remnants of recent supernovae. The disagreements between the observations and the estimations from astrophysical models show the need of more experimental data for the production and consumption reactions in the energy range of a core-collapse supernova. In this article, we will describe the proposed method of injecting beams of 44Ti into CRYRING@ESR and performing the actual reaction rate measurements.

G. Weber, A. Gumberidze, M. Herdrich, R. Märtin, U. Spillmann, A. Surzhykov, D. Thorn, S. Trotsenko, N. Petridis, C. Fontes, and Th. Stöhlker
Towards a determination of absolute cross sections for projectile excitation of hydrogen‐like uranium in collisions with neutral atoms
X-Ray Spectrometry 49, 239 (2020)

Abstract: Recently, the contribution of the generalized Breit interaction to electron impact ionization was identified for the first time in a high‐Z system, namely, hydrogen‐like uranium. This study employed a measurement of the relative population of the j = 1/2 and j = 3/2 states of the L shell by projectile excitation in collision of U91+ with hydrogen and nitrogen targets. However, for a rigorous test of ion–atom collision theory, also the absolute excitation cross sections are of great importance. In the present work, we report on our efforts to extend the previous study to a determination of the absolute projectile excitation cross sections by normalization to the well‐known radiative electron capture process.

R. Sánchez, A. Braeuning-Demian, J. Glorius, S. Hagmann, P.-M. Hillenbrand, A. Kalinin, T. Köhler, Y. A. Litvinov, N. Petridis, S. Sanjari, U. Spillmann, and T. Stöhlker
Towards experiments with highly charged ions at HESR
X-Ray Spectrometry 49, 33 (2020)

Abstract: The atomic physics collaboration SPARC is a part of the APPA pillar at the future Facility for Antiproton and Ion Research. It aims at atomic‐physics research across virtually the full range of atomic matter. An emphasis of this contribution are the atomic physics experiments addressing the collision dynamics in strong electro‐magnetic fields as well as the fundamental interactions between electrons and heavy nuclei at the HESR. Here we give a short overview about the central instruments for SPARC experiments at this storage ring.

D. Dmytriiev, M. Sanjari, Y. Litvinov, and T. Stöhlker
Position sensitive resonant Schottky cavities for heavy ion storage rings
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 463, 320 (2020)

Abstract: Resonant Schottky pick-up cavities are sensitive beam monitors. They are indispensable for the beam diagnostics in storage rings. Apart from their applications in the measurements of beam parameters, they can be used in nondestructive in-ring decay studies of radioactive ion beams. In addition, position sensitive Schottky pick-up cavities enhance precision in the isochronous mass measurement technique. The goal of this work is to construct and test such a position sensitive cavity (e.g. Schottky detector) based on previous theoretical calculations and simulations. These cavities will allow measurement of position using the monopole mode using a non-circular(elliptic) geometry. This information can be further analyzed to increase the performance in isochronous mass spectrometry. A brief description of the detector system from elliptical and cylindrical cavities is described in this work.

S. Hagmann, P. Hillenbrand, Y. A. Litvinov, U. Spillmann, and T. Stöhlker
A magnetic spectrometer for electron‐positron pair spectroscopy in storage rings
X-Ray Spectrometry 49, 115 (2020)

Abstract: We report an analysis of electron‐optical properties of a toroidal magnetic sector spectrometer and examine parameters for its implementation in a relativistic heavy‐ion storage ring, for example the High Energy Storage ring (HESR) at the future Facility for Antiproton and Ion Research (FAIR) facility. For studies of free–free pair production in heavy‐ion atom collisions, this spectrometer exhibits very high efficiencies for coincident e+–e− pair spectroscopy over a wide range of momenta of emitted lepton pairs. The high coincidence efficiency of the spectrometer is the key for stringent tests of theoretical predictions for the phase space correlation of lepton vector momenta in free–free pair production.

F. M. Kröger, G. Weber, V. P. Shevelko, S. Hirlaender, M. W. Krasny, and Th. Stöhlker
Charge state tailoring of relativistic heavy ion beams for the Gamma Factory project at CERN
X-Ray Spectrometry 49, 25 (2020)

Abstract: We present charge‐state evolution studies for Pb⁵⁴⁺ ion beams passing through stripper foils at relativistic energies of 5.9 GeV/u. The purpose of this investigation is to determine the optimum target material and non‐equilibrium thickness for the efficient production of few‐electron lead ions, that is, Pb⁸⁰⁺ and Pb⁸¹⁺, at the present European Organization for Nuclear Research, CERN, accelerator facility at energies as high as 5.9 GeV/u. Based on these predictions, an Al stripper foil has been selected for a proof‐of‐principle measurement in the frame of the Gamma Factory study group. The experimental data confirms a substantial yield of non‐bare Pb ions. In addition, a charge‐state evolution study for the production of Li‐like lead ions Pb⁷⁹⁺ is presented, which will be subject of a follow‐up experiment in the near future.

M. Herdrich, A. Fleischmann, D. Hengstler, S. Allgeier, C. Enss, S. Trotsenko, T. Morgenroth, R. Schuch, G. Weber, and T. Stöhlker
High-precision X-ray spectroscopy of Fe ions in an EBIT using a micro-calorimeter detector: First results
X-Ray Spectrometry 49, 184 (2020)

Abstract: A micro-calorimeter X-ray detector of the maXs-30 type was used to record the X-ray radiation from Fe ions, being produced in the S-EBIT-I electron beam ion trap at the site of GSI. The resulting spectra demonstrate the superior energy resolving power of micro-calorimeter detectors compared with conventional semiconductor detectors. The experiment serves as another proof of principle for the application of calorimeters as dedicated high-resolution X-ray spectrometers at an ion facility. Together with the development of an improved analysis algorithm for online readout, these results present a step towards the use of maXs-type detectors as standard instrumentation at GSI/FAIR.

T. Gassner, A. Gumberidze, M. Trassinelli, R. Heß, U. Spillmann, D. Banaś, K.-H. Blumenhagen, F. Bosch, C. Brandau, W. Chen, C. Dimopoulou, E. Förster, R. Grisenti, S. Hagmann, P.-M. Hillenbrand, P. Indelicato, P. Jagodzinski, T. Kämpfer, M. Lestinsky, D. Liesen, Y. Litvinov, R. Lötzsch, B. Manil, R. Märtin, F. Nolden, N. Petridis, M. Sanjari, K. Schulze, M. Schwemlein, A. Simionovici, M. Steck, Th. Stöhlker, C. Szabo, S. Trotsenko, I. Uschmann, G. Weber, O. Wehrhan, N. Winckler, D. Winters, N. Winters, E. Ziegler, and H. Beyer
High-resolution wavelength-dispersive spectroscopy of K-shell transitions in hydrogen-like gold
X-Ray Spectrometry 49, 204 (2020)

Abstract: We present a measurement of K‐shell transitions in H‐like gold (Au78+) using specially developed transmission type crystal spectrometers combined with Ge(i) microstrip detectors. The experiment has been carried out at the Experimental Storage Ring at GSI in Darmstadt. This is a first high‐resolution wavelength‐dispersive measurement of a K‐shell transition in a high‐Z H‐like ion, thus representing an important milestone in this field. Ideas on possible future improvements are discussed as well.

J. Rothhardt, M. Bilal, R. Beerwerth, A. Volotka, V. Hilbert, T. Stöhlker, S. Fritzsche, and J. Limpert
Lifetime measurements of ultrashort-lived excited states in Be-like ions
X-Ray Spectrometry 49, 165 (2020)

Abstract: We propose to measure the lifetime of short-lived excited states in highly charged ions by pump-probe experiments. Utilizing two synchronized and delayed Femtosecond pulses allows accessing these lifetimes with Femtosecond precision. Such measurements could provide sensitive tests of state-of-the art atomic structure calculations beyond the capabilities of established methods.

M. Stapelfeld, F. Schmidl, P. Seidel, S. Stück, V. Tympel, T. Stöhlker, D. Haider, M. Schwickert, T. Sieber, M. Schmelz, T. Schönau, and R. Stolz
The Dual-Cryogenic Current Comperator (DCCC) as a new Prototype CCC for Beamline Monitoring
(2020)

2019

S. Hagmann, P. Hillenbrand, Y. Litvinov, U. Spillmann, and Th. Stöhlker
The magnetic toroidal sector as a broad-band electron–positron pair spectrometer I. lepton trajectories
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 946, 162641 (2019)

Abstract: We report an analysis of electron-optical properties of a toroidal magnetic sector spectrometer and examine parameters for its implementation in a relativistic heavy-ion storage ring like HESR. For studies of free-free pair production in heavy-ion atom collisions this spectrometer exhibits very high efficiencies for coincident e(+)- e(-) pair spectroscopy over a wide range of momenta of emitted lepton pairs. The high coincidence efficiency of the spectrometer is the key for stringent tests of theoretical predictions for the coincident positron- and electron emission characteristics and for the phase space correlation of lepton vector momenta in free-free pair production.

I. Engin, Z. Chitgar, O. Deppert, L. Lucchio, R. Engels, P. Fedorets, S. Frydrych, P. Gibbon, A. Kleinschmidt, A. Lehrach, R. Maier, D. Prasuhn, M. Roth, F. Schlüter, C. Schneider, T. Stöhlker, K. Strathmann, and M. Büscher
Laser-induced acceleration of Helium ions from unpolarized gas jets
Plasma Physics and Controlled Fusion 61, 115012 (2019)

Abstract: In order to develop a laser-driven spin-polarized He-3-ion beam source available for nuclear-physics experiments as well as for the investigation of polarized nuclear fusion, several challenges have to be overcome. Apart from the provision of a properly polarized He-3 gas-jet target, one of the biggest milestones is the demonstration of the general feasibility of laser-induced ion acceleration out of gas-jet targets. Of particular importance is the knowledge about the main ion-emission angles as well as the achievable ion-energy spectra (dependent on the optimal set of laser and target parameters). We report on the results of such a feasibility study performed at PHELIX, GSI Darmstadt. Both He-3- and He-4-gas jets (n(gas) similar to 10(19) cm(-3)) were illuminated with high-intensity laser pulses, I-L similar to O(10(19) W cm(-2)). The main ion-emission angles could be identified (+/- 90 degrees with respect to the laser-propagation direction) and the ion-energy spectra for all ion species could be extracted: for the optimal laser and target parameters, the high-energy cut-offs for He-2+,He-1+ ions were 4.65 MeV (with a normalized energy uncertainty of Delta epsilon epsilon(-1) = 0.033) and 3.27 MeV (Delta epsilon epsilon(-1) = 0.055), respectively.

M. Kiffer, S. Ringleb, N. Stallkamp, B. Arndt, I. Blinov, S. Kumar, S. Stahl, T. Stöhlker, and M. Vogel
Single-pass non-destructive electronic detection of charged particles
Review of Scientific Instruments 90, 113301 (2019)

Abstract: We have devised an experimental method and apparatus for the simultaneous nondestructive determination of the absolute ion number, ion kinetic energy, and length of bunches of charged particles. We have built and operated a corresponding electronic detector that is based on induced charges and their subsequent low-noise amplification at cryogenic temperatures. We have performed measurements with bunches of low-energy highly charged ions from an electron-beam ion source that show the capability of the methods and their implementation. We discuss requirements for, and applications of, such detectors with a particular view on the obtainable information and their sensitivity.

F. Ozturk, B. Akkus, D. Atanasov, H. Beyer, F. Bosch, D. Boutin, C. Brandau, P. Bühler, R. Cakirli, R. Chen, W. Chen, X. Chen, I. Dillmann, C. Dimopoulou, W. Enders, H. Essel, T. Faestermann, O. Forstner, B. Gao, H. Geissel, R. Gernhäuser, R. Grisenti, A. Gumberidze, S. Hagmann, T. Heftrich, M. Heil, M. Herdrich, P.-M. Hillenbrand, T. Izumikawa, P. Kienle, C. Klaushofer, C. Kleffner, C. Kozhuharov, R. Knöbel, O. Kovalenko, S. Kreim, T. Kühl, C. Lederer-Woods, M. Lestinsky, S. Litvinov, Y. Litvinov, Z. Liu, X. Ma, L. Maier, B. Mei, H. Miura, I. Mukha, A. Najafi, D. Nagae, T. Nishimura, C. Nociforo, F. Nolden, T. Ohtsubo, Y. Oktem, S. Omika, A. Ozawa, N. Petridis, J. Piotrowski, R. Reifarth, J. Rossbach, R. Sánchez, M. Sanjari, C. Scheidenberger, R. Sidhu, H. Simon, U. Spillmann, M. Steck, Th. Stöhlker, B. Sun, L. Susam, F. Suzaki, T. Suzuki, S. Torilov, C. Trageser, M. Trassinelli, S. Trotsenko, X. Tu, P. Walker, M. Wang, G. Weber, H. Weick, N. Winckler, D. Winters, P. Woods, T. Yamaguchi, X. Xu, X. Yan, J. Yang, Y. Yuan, Y. Zhang, and X. Zhou
New test of modulated electron capture decay of hydrogen-like ¹⁴²Pm ions: Precision measurement of purely exponential decay
Physics Letters B 797, 134800 (2019)

Abstract: An experiment addressing electron capture (EC) decay of hydrogen-like ¹⁴²Pm⁶⁰⁺ ions has been conducted at the experimental storage ring (ESR) at GSI. The decay appears to be purely exponential and no modulations were observed. Decay times for about 9000 individual EC decays have been measured by applying the single-ion decay spectroscopy method. Both visually and automatically analysed data can be described by a single exponential decay with decay constants of 0.0126(7) s⁻¹ for automatic analysis and 0.0141(7) s⁻¹ for manual analysis. If a modulation superimposed on the exponential decay curve is assumed, the best fit gives a modulation amplitude of merely 0.019(15), which is compatible with zero and by 4.9 standard deviations smaller than in the original observation which had an amplitude of 0.23(4).

I. A. Maltsev, V. M. Shabaev, R. V. Popov, Y. S. Kozhedub, G. Plunien, X. Ma, T. Stöhlker, and D. A. Tumakov
How to Observe the Vacuum Decay in Low-Energy Heavy-Ion Collisions
Physical Review Letters 123, 113401 (2019)

Abstract: In slow collisions of two bare nuclei with the total charge larger than the critical value Zcr≈173, the initially neutral vacuum can spontaneously decay into the charged vacuum and two positrons. The detection of the spontaneous emission of positrons would be direct evidence of this fundamental phenomenon. However, the spontaneously produced particles are indistinguishable from the dynamical background in the positron spectra. We show that the vacuum decay can nevertheless be observed via impact-sensitive measurements of pair-production probabilities. The possibility of such an observation is demonstrated using numerical calculations of pair production in low-energy collisions of heavy nuclei.

N. A. Zubova, I. S. Anisimova, M. Y. Kaygorodov, Y. S. Kozhedub, A. V. Malyshev, V. M. Shabaev, I. I. Tupitsyn, G. Plunien, C. Brandau, and T. Stöhlker
Isotope shifts of the 1s²2s2p (J)–1s²2s²$transition energies in Be-like thorium and uranium Journal of Physics B: Atomic, Molecular and Optical Physics 52, 185001 (2019) Abstract: Precise calculations of the isotope shifts in berylliumlike thorium and uranium ions are presented. The main contributions to the field and mass shifts are calculated within the framework of the Dirac–Coulomb–Breit Hamiltonian employing the configuration-interaction Dirac–Fock–Sturm method. These calculations include the relativistic, electron–electron correlation, and Breit-interaction effects. The QED, nuclear deformation, and nuclear polarization corrections are also evaluated. F. Aumayr, K. Ueda, E. Sokell, S. Schippers, H. Sadeghpour, F. Merkt, T. F. Gallagher, F. B. Dunning, P. Scheier, O. Echt, T. Kirchner, S. Fritzsche, A. Surzhykov, X. Ma, R. Rivarola, O. Fojon, L. Tribedi, E. Lamour, J. R. C. Lopez-Urrutia, Y. A. Litvinov, V. Shabaev, H. Cederquist, H. Zettergren, M. Schleberger, R. A. Wilhelm, T. Azuma, P. Boduch, H. T. Schmidt, and T. Stöhlker Roadmap on photonic, electronic and atomic collision physics: III. Heavy particles: with zero to relativistic speeds Journal of Physics B: Atomic, Molecular and Optical Physics 52, 171003 (2019) Abstract: We publish three Roadmaps on photonic, electronic and atomic collision physics in order to celebrate the 60th anniversary of the ICPEAC conference. Roadmap III focusses on heavy particles: with zero to relativistic speeds. Modern theoretical and experimental approaches provide detailed insight into the wide range of many-body interactions involving projectiles and targets of varying complexity ranging from simple atoms, through molecules and clusters, complex biomolecules and nanoparticles to surfaces and crystals. These developments have been driven by technological progress and future developments will expand the horizon of the systems that can be studied. This Roadmap aims at looking back along the road, explaining the evolution of the field, and looking forward, collecting nineteen contributions from leading scientists in the field. S. Kumar, W. Quint, S. Ringleb, C. P. Safvan, N. Stallkamp, T. Stöhlker, and M. Vogel Properties of a cylindrical Penning trap with conical endcap openings Physica Scripta 94, 075401 (2019) Abstract: We describe the results of analytical calculations and numerical simulations of the confinement properties of a mechanically compensated cylindrical Penning trap which has conical endcap openings for large-solid-angle access for example with highly focused laser beams. While the analytical calculations show that under the common geometrical conditions the harmonicity of the confining fields near the centre of the trap does not change when a conical shape of the endcap electrodes is introduced, numerical simulations show significant changes when the opening angle of the cone exceeds a certain critical angle. We also show that these sharp features are due to the fringe-field effects above the critical angle, which are not described by the analytical calculations. These effects are also observed in a cylindrical Penning trap when the length of the endcap electrodes is reduced below a certain critical value. A. V. Volotka, M. Bilal, R. Beerwerth, X. Ma, Th. Stöhlker, and S. Fritzsche QED radiative corrections to the ²P₁/₂-²P₃/₂ fine structure in fluorinelike ions Physical Review A 100, 010502 (2019) Abstract: calculations of QED radiative corrections to the 2P1/2 - 2P3/2 fine-structure transition energy are performed for selected F-like ions. These calculations are nonperturbative in αZ and include all first-order and many-electron second-order effects in α. When compared to approximate QED computations, a notable discrepancy is found especially for F-like uranium for which the predicted self-energy contributions even differ in sign. Moreover, all deviations between theory and experiment for the 2P1/2 - 2P3/2 fine-structure energies of F-like ions, reported recently by Li et al., Phys. Rev. A 98, 020502(R) (2018), are resolved if their highly accurate, non-QED fine-structure values are combined with the QED corrections ab initially evaluated here. A. Bondarev, Y. Kozhedub, I. Tupitsyn, V. Shabaev, G. Plunien, and Th. Stöhlker Differential cross sections for ionization of atomic hydrogen by antiprotons Hyperfine Interactions 240, 60 (2019) Abstract: We consider ionization of atomic hydrogen by antiproton impact. Doubly differential cross sections for ionization calculated within a recently developed semiclassical approach by Bondarev et al. (Phys. Rev. A 95, 052709, 2017) are presented. Comparison of the obtained results with other theoretical predictions is given. D. A. Glazov, A. V. Volotka, O. V. Andreev, V. P. Kosheleva, S. Fritzsche, V. M. Shabaev, G. Plunien, and Th. Stöhlker Ground-state hyperfine splitting of B-like ions in the high-Z region Physical Review A 99, 062503 (2019) Abstract: The hyperfine splitting of the ground state of selected B-like ions within the range of nuclear charge numbers Z=49–83 is investigated in detail. The rigorous QED approach together with the large-scale configuration-interaction Dirac-Fock-Sturm method are employed for the evaluation of the interelectronic-interaction contributions of first and higher orders in 1/Z. The screened QED corrections are evaluated to all orders in αZ by using an effective potential. The influence of nuclear magnetization distribution is taken into account within the single-particle nuclear model. W. Nörtershäuser, J. Ullmann, L. V. Skripnikov, Z. Andelkovic, C. Brandau, A. Dax, W. Geithner, C. Geppert, C. Gorges, M. Hammen, V. Hannen, S. Kaufmann, K. König, F. Kraus, B. Kresse, Y. A. Litvinov, M. Lochmann, B. Maaß, J. Meisner, T. Murböck, A. F. Privalov, R. Sánchez, B. Scheibe, M. Schmidt, S. Schmidt, V. M. Shabaev, M. Steck, T. Stöhlker, R. C. Thompson, C. Trageser, M. Vogel, J. Vollbrecht, A. V. Volotka, and C. Weinheimer The hyperfine puzzle of strong-field bound-state QED Hyperfine Interactions 240, 51 (2019) Abstract: The hyperfine splitting in heavy highly charged ions provide the means to test QED in extremely strong magnetic fields. In order to provide a meaningful test, the splitting has to be measured in H-like and Li-like ions to remove uncertainties from nuclear structure. This has been achieved at the experimental storage ring ESR but a discrepancy to the theoretical prediction of more than 7s was observed. We report on these measurements as well as on NMR measurements that were performed to solve this issue. M. Vockert, G. Weber, H. Bräuning, A. Surzhykov, C. Brandau, S. Fritzsche, S. Geyer, S. Hagmann, S. Hess, C. Kozhuharov, R. Märtin, N. Petridis, R. Hess, S. Trotsenko, Yu. A. Litvinov, J. Glorius, A. Gumberidze, M. Steck, S. Litvinov, T. Gassner, P.-M. Hillenbrand, M. Lestinsky, F. Nolden, M. S. Sanjari, U. Popp, C. Trageser, D. F. A. Winters, U. Spillmann, T. Krings, and Th. Stöhlker Radiative electron capture as a tunable source of highly linearly polarized x rays Physical Review A 99, 052702 (2019) Abstract: The radiative electron capture (REC) into the K shell of bare Xe ions colliding with a hydrogen gas target has been investigated. In this study, the degree of linear polarization of the K-REC radiation was measured and compared with rigorous relativistic calculations as well as with the previous results recorded for U92+. Owing to the improved detector technology, a significant gain in precision of the present polarization measurement is achieved compared to the previously published results. The obtained data confirms that for medium-Z ions such as Xe, the REC process is a source of highly polarized x rays which can easily be tuned with respect to the degree of linear polarization and the photon energy. We argue, in particular, that for relatively low energies the photons emitted under large angles are almost fully linear polarized. V. Hannen, J. Vollbrecht, Z. Andelkovic, C. Brandau, A. Dax, W. Geithner, C. Geppert, C. Gorges, M. Hammen, S. Kaufmann, K. König, Y. Litvinov, M. Lochmann, B. Maaß, J. Meisner, T. Murböck, R. Sánchez, M. Schmidt, S. Schmidt, M. Steck, T. Stöhlker, R. Thompson, C. Trageser, J. Ullmann, C. Weinheimer, and W. Nörtershäuser Lifetimes and g -factors of the HFS states in H-like and Li-like bismuth Journal of Physics B: Atomic, Molecular and Optical Physics 52, 085003 (2019) Abstract: The LIBELLE experiment performed at the experimental storage ring at the GSI Helmholtz Center for Heavy Ion Research in Darmstadt, Germany, has successfully determined the ground state hyperfine (HFS) splittings in hydrogen-like (Bi-209(82+)) and lithium-like (Bi-209(80+)) bismuth. The study of HFS transitions in highly charged ions enables precision tests of QED in extreme electric and magnetic fields otherwise not attainable in laboratory experiments. Besides the transition wavelengths the time-resolved detection of fluorescence photons following the excitation of the ions by a pulsed laser system also allows the extraction of lifetimes of the upper HFS levels and g-factors of the bound 1s and 2s electrons for both charge states. While the lifetime of the upper HFS state in Bi-209(82+) has already been measured in earlier experiments, an experimental value for lifetime of this state in Bi-209(80+) is reported for the first time in this work. A. Gumberidze, D. B. Thorn, A. Surzhykov, C. J. Fontes, B. Najjari, A. Voitkiv, S. Fritzsche, D. Banaś, H. F. Beyer, W. Chen, R. E. Grisenti, S. Hagmann, R. Hess, P.-M. Hillenbrand, P. Indelicato, C. Kozhuharov, M. Lestinsky, R. Märtin, N. Petridis, R. V. Popov, R. Schuch, U. Spillmann, S. Tashenov, S. Trotsenko, A. Warczak, G. Weber, W. Wen, D. F. A. Winters, N. Winters, Z. Yin, and Th. Stöhlker Electron- and proton-impact excitation of heliumlike uranium in relativistic collisions Physical Review A 99, 032706 (2019) Abstract: We have studied the K-shell excitation of He-like uranium (U90+) in relativistic collisions with hydrogen and argon atoms. Performing measurements with different targets, as well as with different collision energies, enabled us to explore the proton- (nucleus-) impact excitation as well as the electron-impact excitation process for the heaviest He-like ion. The large fine-structure splitting in uranium allowed us to partially resolve excitation into different L-shell levels. State-of-the-art relativistic calculations which include excitation mechanisms due to the interaction with both protons (nucleus) and electrons are in good agreement with the experimental findings. Moreover, our experimental data clearly demonstrate the importance of including the generalized Breit interaction in the treatment of the electron-impact excitation process. J. Glorius, C. Langer, Z. Slavkovská, L. Bott, C. Brandau, B. Brückner, K. Blaum, X. Chen, S. Dababneh, T. Davinson, P. Erbacher, S. Fiebiger, T. Gaßner, K. Göbel, M. Groothuis, A. Gumberidze, G. Gyürky, M. Heil, R. Hess, R. Hensch, P. Hillmann, P.-M. Hillenbrand, O. Hinrichs, B. Jurado, T. Kausch, A. Khodaparast, T. Kisselbach, N. Klapper, C. Kozhuharov, D. Kurtulgil, G. Lane, C. Lederer-Woods, M. Lestinsky, S. Litvinov, Yu. A. Litvinov, B. Löher, F. Nolden, N. Petridis, U. Popp, T. Rauscher, M. Reed, R. Reifarth, M. S. Sanjari, D. Savran, H. Simon, U. Spillmann, M. Steck, T. Stöhlker, J. Stumm, A. Surzhykov, T. Szücs, T. T. Nguyen, A. Taremi Zadeh, B. Thomas, S. Yu. Torilov, H. Törnqvist, M. Träger, C. Trageser, S. Trotsenko, L. Varga, M. Volknandt, H. Weick, M. Weigand, C. Wolf, P. J. Woods, and Y. M. Xing Approaching the Gamow Window with Stored Ions: Direct Measurement of ¹²⁴Xe(p,γ) in the ESR Storage Ring Physical Review Letters 122, 092701 (2019) Abstract: We report the first measurement of low-energy proton-capture cross sections of 124Xe in a heavy-ion storage ring. 124Xe^54+ ions of five different beam energies between 5.5 and 8 AMeV were stored to collide with a windowless hydrogen target. The 125Cs reaction products were directly detected. The interaction energies are located on the high energy tail of the Gamow window for hot, explosive scenarios such as supernovae and x-ray binaries. The results serve as an important test of predicted astrophysical reaction rates in this mass range. Good agreement in the prediction of the astrophysically important proton width at low energy is found, with only a 30% difference between measurement and theory. Larger deviations are found above the neutron emission threshold, where also neutron and γ widths significantly impact the cross sections. The newly established experimental method is a very powerful tool to investigate nuclear reactions on rare ion beams at low center-of-mass energies. M. Durante, P. Indelicato, B. Jonson, V. Koch, K. Langanke, U.-G. Meißner, E. Nappi, T. Nilsson, T. Stöhlker, E. Widmann, and M. Wiescher All the fun of the FAIR: fundamental physics at the facility for antiproton and ion research Physica Scripta 94, 033001 (2019) Abstract: The Facility for Antiproton and Ion Research (FAIR) will be the accelerator-based flagship research facility in many basic sciences and their applications in Europe for the coming decades. FAIR will open up unprecedented research opportunities in hadron and nuclear physics, in atomic physics and nuclear astrophysics as well as in applied sciences like materials research, plasma physics and radiation biophysics with applications towards novel medical treatments and space science. FAIR is currently under construction as an international facility at the campus of the GSI Helmholtzzentrum for Heavy-Ion Research in Darmstadt, Germany. While the full science potential of FAIR can only be harvested once the new suite of accelerators and storage rings is completed and operational, some of the experimental detectors and instrumentation are already available and will be used starting in summer 2018 in a dedicated research program at GSI, exploiting also the significantly upgraded GSI accelerator chain. The current manuscript summarizes how FAIR will advance our knowledge in various research fields ranging from a deeper understanding of the fundamental interactions and symmetries in nature to a better understanding of the evolution of the Universe and the objects within. V. Zakosarenko, M. Schmelz, T. Schönau, S. Anders, J. Kunert, V. Tympel, R. Neubert, F. Schmidl, P. Seidel, T. Stöhlker, D. Haider, M. Schwickert, T. Sieber, and R. Stolz Coreless SQUID-based cryogenic current comparator for non-destructive intensity diagnostics of charged particle beams Superconductor Science and Technology 32, 014002 (2019) Abstract: We report on a novel concept and prototype development of a coreless SQUID-based charged-particle beam monitor as a non-destructive diagnostic tool for accelerator facilities. Omitting the typically used pickup coil with a high magnetic permeability core leads to a significant improvement in low-frequency noise performance. Moreover, a revised shielding geometry allows for very compact and rather lightweight device designs. Based on highly sensitive SQUIDs featuring sub-micron cross-type Josephson tunnel junctions, our prototype device exhibits a current sensitivity of about 6 pA Hz(-1/2) in the white noise region. Together with a measured shielding factor of about 135 dB this opens up the way for its widespread use in modern accelerator facilities. T. Sieber, H. D. Gersem, J. Golm, D. Haider, N. Marsic, W. Müller, H. Reeg, M. Schmelz, F. Schmidl, M. Schwickert, P. Seidel, T. Stöhlker, R. Stolz, J. Tan, G. Tranquille, V. Tympel, and V. Zakosarenko Next Generation Cryogenic Current Comparator (CCC) for nA Intensity Measurement (2019) Abstract: A Cryogenic Current Comparator (CCC) is an extremely sensitive DC-Beam Transformer based on superconducting SQUID technology. Recently, a CCC without a toroidal core and with an axially oriented magnetic shielding has been developed at the Institute of Photonic Technologies (IPHT) Jena/Germany. It represents a compact and lightweight alternative to the ’classical’ CCC, which was originally developed at PTB Braunschweig and is successfully in operation in accelerators at GSI and CERN. Excellent low-frequency noise performance was demonstrated with a prototype of this new CCC-type. Current measurements and further tests are ongoing, first results are presented together with simulation calculations for the magnetic shielding. The construction from lead as well as simplified manufacturing results in drastically reduced costs compared to formerly used Nb-CCCs. Reduced weight also puts less constraints on the cryostat. Based on highly sensitive SQUIDs, the new prototype device shows a current sensitivity of about 6 pA/Hz1/2 in the white noise region. The measured and calculated shielding factor is ~135 dB. These values, together with a significant cost reduction - resulting also from a compact cryostat design - opens up the way for widespread use of CCCs in modern accelerator facilities. J. Golm, R. Neubert, F. Schmidl, P. Seidel, V. Tympel, T. Stöhlker, F. Kurian, D. M. Haider, M. Schwickert, T. Sieber, M. Schmelz, R. Stolz, V. Zakosarenko, M. F. Fernandes, C. P. Welsch, J. Tan, H. D. Gersem, N. Marsic, and W. F. O. Müller Different Versions of Cryogenic Current Comparators with Magnetic Core for Beam Current Measurements (2019) Abstract: For more than 20 years Cryogenic Current Comparators (CCC) are used to measure the current of charged particle beams with low intensity (nA-range). The device was first established at GSI in Darmstadt and was improved over the past two decades by the cooperation of institutes in Jena, GSI and CERN. The improved versions differ in material parameters and electronics to increase the resolution and in dimensions in order to meet the requirements of the respective application. The device allows non-destructive measurements of the charged particle beam current. The azimuthal magnetic field which is generated by the beam current is detected by low temperature Superconducting Quantum Interference Device (SQUID) current sensors. A complex shaped superconductor cooled down to 4.2 K is used as magnetic shielding and a high permeability core serves as flux concentrator. Three versions of the CCC shall be presented in this work: (1) GSI-Pb-CCC which was running at GSI Darmstadt in a transfer line, (2) CERN-Nb-CCC currently installed in the Antiproton Decelerator at CERN and (3) GSI-Nb-CCC-XD which will be operating in the CRYRING at GSI 2019. Noise, signal and drift measurements were performed in the Cryo-Detector Lab at the University of Jena. 2018 I. A. Maltsev, V. M. Shabaev, R. V. Popov, Y. S. Kozhedub, G. Plunien, X. Ma, and Th. Stöhlker Electron-positron pair production in slow collisions of heavy nuclei beyond the monopole approximation Physical Review A 98, 062709 (2018) Abstract: Electron-positron pair production in low-energy collisions of heavy nuclei is considered beyond the monopole approximation. The calculation method is based on the numerical solving of the time-dependent Dirac equation with the full two-center potential. Bound-free and free-free pair-production probabilities as well as the energy spectra of the emitted positrons are calculated for the collisions of bare uranium nuclei. The calculations are performed for collision energy near the Coulomb barrier for different values of the impact parameter. The obtained results are compared with the corresponding values calculated in the monopole approximation. V. Zakosarenko, M. Schmelz, T. Schönau, S. Anders, J. Kunert, V. Tympel, R. Neubert, F. Schmidl, P. Seidel, T. Stöhlker, D. Haider, M. Schwickert, T. Sieber, and R. Stolz Coreless SQUID-based cryogenic current comparator for non-destructive intensity diagnostics of charged particle beams Superconductor Science and Technology 32, 014002 (2018) Abstract: We report on a novel concept and prototype development of a coreless SQUID-based charged-particle beam monitor as a non-destructive diagnostic tool for accelerator facilities. Omitting the typically used pickup coil with a high magnetic permeability core leads to a significant improvement in low-frequency noise performance. Moreover, a revised shielding geometry allows for very compact and rather lightweight device designs. Based on highly sensitive SQUIDs featuring sub-micron cross-type Josephson tunnel junctions, our prototype device exhibits a current sensitivity of about 6 pA Hz^(−1/2) in the white noise region. Together with a measured shielding factor of about 135 dB this opens up the way for its widespread use in modern accelerator facilities. S. Kraft-Bermuth, D. Hengstler, P. Egelhof, C. Enss, A. Fleischmann, M. Keller, and T. Stöhlker Microcalorimeters for X-Ray Spectroscopy of Highly Charged Ions at Storage Rings Atoms 6, 59 (2018) Abstract: X-ray spectroscopy of highly charged heavy ions is an important tool for the investigation of many topics in atomic physics. Such highly charged ions, in particular hydrogen-like uranium, are investigated at heavy ion storage rings, where high charge states can be produced in large quantities, stored for long times and cooled to low momentum spread of the ion beam. One prominent example is the determination of the 1s Lamb Shift in hydrogen-like heavy ions, which has been investigated at the Experimental Storage Ring (ESR) at the GSI Helmholtz Centre for Heavy Ion Research. Due to the large electron binding energies, the energies of the corresponding photon transitions are located in the X-ray regime. To determine the transition energies with high accuracy, highly resolving X-ray spectrometers are needed. One concept of such spectrometers is the concept of microcalorimeters, which, in contrast to semiconductor detectors, uses the detection of heat rather than charge to detect energy. Such detectors have been developed and successfully applied in experiments at the ESR. For experiments at the Facility for Antiproton and Ion Research (FAIR), the Stored Particles and Atoms Collaboration (SPARC) pursues the development of new microcalorimeter concepts and larger detector arrays. Next to fundamental investigations on quantum electrodynamics such as the 1s Lamb Shift or electron–electron interactions in two- and three-electron systems, X-ray spectroscopy may be extended towards nuclear physics investigations like the determination of nuclear charge radii. V. P. Kosheleva, V. A. Zaytsev, A. Surzhykov, V. M. Shabaev, and Th. Stöhlker Elastic scattering of twisted electrons by an atomic target: Going beyond the Born approximation Physical Review A 98, 022706 (2018) Abstract: The elastic scattering of twisted electrons by neutral atoms is studied within the fully relativistic framework. The electron-atom interaction is taken into account in all orders, thus allowing us to explore high-order effects beyond the first Born approximation. To illustrate these effects, detailed calculations of the total and differential cross sections as well as the degree of polarization of scattered electrons are performed. Together with the analysis of the effects beyond the first Born approximation, we discuss the influence of the kinematic parameters of the incident twisted electrons on the angular and polarization properties of the scattered electrons. I. I. Tupitsyn, N. A. Zubova, V. M. Shabaev, G. Plunien, and Th. Stöhlker Relativistic calculations of x-ray transition energies and isotope shifts in heavy atoms Physical Review A 98, 022517 (2018) Abstract: X-ray transition energies and isotope shifts in heavy atoms are evaluated. The energy levels with vacancies in the inner shells are calculated within the approximation of the average of a nonrelativistic configuration employing the Dirac-Fock-Sturm method. The obtained results are compared with other configuration-interaction theoretical calculations and with experimental data. P. Wustelt, F. Oppermann, L. Yue, M. Möller, T. Stöhlker, M. Lein, S. Gräfe, G. Paulus, and A. Sayler Heteronuclear Limit of Strong-Field Ionization: Fragmentation of HeH⁺ by Intense Ultrashort Laser Pulses Physical Review Letters 121, 073203 (2018) Abstract: The laser-induced fragmentation dynamics of this most fundamental polar molecule HeH+ are measured using an ion beam of helium hydride and an isotopologue at various wavelengths and intensities. In contrast to the prevailing interpretation of strong-field fragmentation, in which stretching of the molecule results primarily from laser-induced electronic excitation, experiment and theory for nonionizing dissociation, single ionization, and double ionization both show that the direct vibrational excitation plays the decisive role here. We are able to reconstruct fragmentation pathways and determine the times at which each ionization step occurs as well as the bond length evolution before the electron removal. The dynamics of this extremely asymmetric molecule contrast the well-known symmetric systems leading to a more general picture of strong-field molecular dynamics and facilitating interpolation to systems between the two extreme cases. T. Gassner, M. Trassinelli, R. Heß, U. Spillmann, D. Banas, K.-H. Blumenhagen, F. Bosch, C. Brandau, W. Chen, C. Dimopoulou, E. Förster, R. Grisenti, A. Gumberidze, S. Hagmann, P.-M. Hillenbrand, P. Indelicato, P. Jagodzinski, T. Kämpfer, Ch. Kozhuharov, M. Lestinsky, D. Liesen, Yu. A. Litvinov, R. Loetzsch, B. Manil, R. Märtin, F. Nolden, N. Petridis, M. S. Sanjari, K. Schulze, M. Schwemlein, A. Simionovici, M. Steck, Th. Stöhlker, C. I. Szabo, S. Trotsenko, I. Uschmann, G. Weber, O. Wehrhan, N. Winckler, D. Winters, N. Winters, E. Ziegler, and H. Beyer Wavelength-dispersive spectroscopy in the hard x-ray regime of a heavy highly-charged ion: the 1s Lamb shift in hydrogen-like gold New Journal of Physics 20, 073033 (2018) Abstract: Accurate spectroscopy of highly-charged high-Z ions in a storage ring is demonstrated to be feasible by the use of specially adapted crystal optics. The method has been applied for the measurement of the is Lamb shift in hydrogen-like gold (Au78+) in a storage ring through spectroscopy of the Lyman x-rays. This measurement represents the first result obtained for a high-Z element using high-resolution wavelength-dispersive spectroscopy in the hard x-ray regime, paving the way for sensitivity to higher-order QED effects. P. Seidel, V. Tympel, R. Neubert, J. Golm, M. Schmelz, R. Stolz, V. Zakosarenko, T. Sieber, M. Schwickert, F. Kurian, F. Schmidl, and T. Stöhlker Cryogenic Current Comparators for Larger Beamlines IEEE Transactions on Applied Superconductivity 28, 1 (2018) Abstract: The non-destructive measurement of charged particle beams with intensities below 1 μA represents still a challenge in current R&D efforts. Beam peak-intensities of modern high power accelerators are in the range of milli-amperes, but for a large number of experiments, the pulse lengths have to be increased by several orders of magnitude (slow extraction process) to avoid saturation in the detectors. At the same time, the intensities of exotic ion- or antiproton-beams – depending on the production yield – might be in the range of nano-amperes or even below. The solution of this measurement problem should moreover include the possibility to calibrate the electrical current with traceability to national standards. M. M. Günther, A. V. Volotka, M. Jentschel, S. Fritzsche, Th. Stöhlker, P. G. Thirolf, and M. Zepf Dispersive refraction of different light to heavy materials at MeV γ-ray energies Physical Review A 97, 063843 (2018) Abstract: The dispersive behavior of materials with atomic charge numbers varying from Z=4 (beryllium, Be) to Z=82 (lead, Pb) was investigated experimentally and theoretically at gamma-ray energies up to 2 MeV. The experiment was performed at the double-crystal gamma spectrometer GAMS6 of the Institut Laue-Langevin in Grenoble. The experimental results were compared with theoretical calculations which account for all major elastic processes involved. Overall, we found a good agreement between theory and experiment. We find that, for the development of refractive optics at$\gamma\$-ray energies beyond those currently in use, high-Z materials become increasingly attractive compared to the beryllium lens-stacks used at x-ray energies.

P. Micke, S. Kühn, L. Buchauer, J. R. Harries, T. M. Bücking, K. Blaum, A. Cieluch, A. Egl, D. Hollain, S. Kraemer, T. Pfeifer, P. O. Schmidt, R. X. Schüssler, Ch. Schweiger, Th. Stöhlker, S. Sturm, R. N. Wolf, S. Bernitt, and J. R. Crespo López-Urrutia
The Heidelberg compact electron beam ion traps
Review of Scientific Instruments 89, 063109 (2018)

Abstract: Electron beam ion traps (EBITs) are ideal tools for both production and study of highly charged ions (HCIs). In order to reduce their construction, maintenance, and operation costs, we have developed a novel, compact, room-temperature design, the Heidelberg Compact EBIT (HC-EBIT). Four already commissioned devices operate at the strongest fields (up to 0.86 T) reported for such EBITs using permanent magnets, run electron beam currents up to 80 mA, and energies up to 10 keV. They demonstrate HCI production, trapping, and extraction of pulsed Ar¹⁶⁺ bunches and continuous 100 pA ion beams of highly charged Xe up to charge state 29+, already with a 4 mA, 2 keV electron beam. Moreover, HC-EBITs offer large solid-angle ports and thus high photon count rates, e.g., in x-ray spectroscopy of dielectronic recombination in HCIs up to Fe²⁴⁺, achieving an electron-energy resolving power of E/ΔE > 1500 at 5 keV. Besides traditional on-axis electron guns, we have also implemented a novel off-axis gun for laser, synchrotron, and free-electron laser applications, offering clear optical access along the trap axis. We report on its first operation at a synchrotron radiation facility demonstrating the resonant photoexcitation of highly charged oxygen.

R. Popov, A. Bondarev, Y. Kozhedub, I. Maltsev, V. Shabaev, I. Tupitsyn, X. Ma, G. Plunien, and T. Stöhlker
One-center calculations of the electron-positron pair creation in low-energy collisions of heavy bare nuclei
The European Physical Journal D 72, 115 (2018)

Abstract: The probabilities of bound-free electron-positron pair creation are calculated for head-on collisions of bare uranium nuclei beyond the monopole approximation. The calculations are based on the numerical solving of the time-dependent Dirac equation in the target reference frame with multipole expansion of the projectile potential. In addition, the energy dependence of the pair-creation cross section is studied in the monopole approximation.

R. Singh Sidhu, J. Glorius, Yu. A. Litvinov, M. K. Pavićević, M. S. Sanjari, and T. Stöhlker
Electroweak Decays of Highly Charged Ions
EPJ Web of Conferences 178, 01003 (2018)

Abstract: In this contribution we review the present status of experimental studies of electroweak decays of highly charged ions. A particular focus will be given on the bound state beta decay measurement of 205Tl.

M. Vockert, G. Weber, U. Spillmann, T. Krings, and Th. Stöhlker
Polarization reconstruction algorithm for a Compton polarimeter
Journal of Physics: Conference Series 1024, 012041 (2018)

Abstract: We present the technique of Compton polarimetry using X-ray detectors based on double-sided segmented semiconductor crystals that were developed within the SPARC collaboration. In addition, we discuss the polarization reconstruction algorithm with particular emphasis on systematic deviations between the observed detector response and our model function for the Compton scattering distribution inside the detector.

V. Shevelko, Yu. A. Litvinov, Th. Stöhlker, and I. Yu. Tolstikhina
Lifetimes of relativistic heavy-ion beams in the High Energy Storage Ring of FAIR
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 421, 45 (2018)

Abstract: The High Energy Storage Ring, HESR, will be constructed at the Facility for Antiproton and Ion Research, FAIR, Darmstadt. For the first time, it will be possible to perform experiments with cooled high-intensity stable and radioactive heavy ions at highly relativistic energies. To design experiments at the HESR, realistic estimations of beam lifetimes are indispensable. Here we report calculated cross sections and lifetimes for typical U88+, U90+, U92+, Sn49+ and Sn50+ ions in the energy range E = 400 MeV/u–5 GeV/u, relevant for the HESR. Interactions with the residual gas and with internal gas-jet targets are also considered.

V. A. Zaytsev, A. S. Surzhykov, V. M. Shabaev, and Th. Stöhlker
Analysis of angular momentum properties of photons emitted in fundamental atomic processes
Physical Review A 97, 043808 (2018)

Abstract: Many atomic processes result in the emission of photons. Analysis of the properties of emitted photons, such as energy and angular distribution as well as polarization, is regarded as a powerful tool for gaining more insight into the physics of corresponding processes. Another characteristic of light is the projection of its angular momentum upon propagation direction. This property has attracted a special attention over the past decades due to studies of twisted (or vortex) light beams. Measurements being sensitive to this projection may provide valuable information about the role of angular momentum in the fundamental atomic processes. Here we describe a simple theoretical method for determination of the angular momentum properties of the photons emitted in various atomic processes. This method is based on the evaluation of expectation value of the total angular momentum projection operator. To illustrate the method, we apply it to the textbook examples of plane-wave, spherical-wave, and Bessel light. Moreover, we investigate the projection of angular momentum for the photons emitted in the process of the radiative recombination with ionic targets. It is found that the recombination photons do carry a nonzero projection of the orbital angular momentum.

O. Novak, R. Kholodov, A. Surzhykov, A. N. Artemyev, and Th. Stöhlker
K-shell ionization of heavy hydrogenlike ions
Physical Review A 97, 032518 (2018)

Abstract: A theoretical study of the K-shell ionization of hydrogenlike ions, colliding with bare nuclei, is performed within the framework of the time-dependent Dirac equation. Special emphasis is placed on the ionization probability that is investigated as a function of impact parameter, collision energy, and nuclear charge. To evaluate this probability in a wide range of collisional parameters we propose a simple analytical expression for the transition amplitude. This expression contains three fitting parameters that are determined from the numerical calculations, based on the adiabatic approximation. In contrast to previous studies, our analytical expression for the transition amplitude and ionization probability accounts for the full multipole expansion of the two-center potential and allows accurate description of nonsymmetric collisions of nuclei with different atomic numbers Z1≠Z2. The calculations performed for both symmetric and asymmetric collisions indicate that the ionization probability is reduced when the difference between the atomic numbers of ions increases.

P. Amaro, U. Loureiro, L. Safari, F. Fratini, P. Indelicato, T. Stöhlker, and J. Santos
Quantum interference in laser spectroscopy of highly charged lithiumlike ions
Physical Review A 97, 022510 (2018)

Abstract: We investigate the quantum interference induced shifts between energetically close states in highly charged ions, with the energy structure being observed by laser spectroscopy. In this work, we focus on hyperfine states of lithiumlike heavy-Z isotopes and quantify how much quantum interference changes the observed transition frequencies. The process of photon excitation and subsequent photon decay for the transition 2s→2p→2s is implemented with fully relativistic and full-multipole frameworks, which are relevant for such relativistic atomic systems. We consider the isotopes 207Pb79+ and 209Bi80+ due to experimental interest, as well as other examples of isotopes with lower Z, namely 141Pr56+ and 165Ho64+. We conclude that quantum interference can induce shifts up to 11% of the linewidth in the measurable resonances of the considered isotopes, if interference between resonances is neglected. The inclusion of relativity decreases the cross section by 35%, mainly due to the complete retardation form of the electric dipole multipole. However, the contribution of the next higher multipoles (e.g., magnetic quadrupole) to the cross section is negligible. This makes the contribution of relativity and higher-order multipoles to the quantum interference induced shifts a minor effect, even for heavy-Z elements.

A. A. Bondarevskaya, D. V. Chubukov, E. A. Mistonova, K. N. Lyashchenko, O. Y. Andreev, A. Surzhykov, L. N. Labzowsky, G. Plunien, D. Liesen, F. Bosch, and Th. Stöhlker
Considerations towards the possibility of the observation of parity nonconservation in highly charged ions in storage rings
Physica Scripta 93, 025401 (2018)

Abstract: The feasibility of an experiment for the observation of parity nonconserving effects using He-like highly charged ions in storage rings is discussed theoretically. The basic idea is the observation of an asymmetry in the emission of the hyperfine quenched transition (1s2s)¹{S₀\to (1s)²¹S₀+γ } with respect to the direction of a beam of ions with polarized nuclei. It will be shown, that for such an experiment 151Eu^61+ ions with nuclear spin I=5/2 in the excited electronic state (1s2s)¹S₀ with zero total electron angular momentum and polarized nuclei are the best available candidates. The nuclei can be polarized if H-like Eu^62+ ions capture an electron from a polarized electron beam which overlays the ion beam on some part of the ring and travels with nearly the same velocity. It is suggested, to monitor steadily the degree of the nuclear polarization by the observation of the selective laser excitation and subsequent decay of the Zeeman sublevels of the excited hyperfine states of the ionic ground state. Estimates for the observation time aiming at an accuracy of PNC measurement of about 0.1 % are given.

2017

I. Maltsev, V. Shabaev, I. Tupitsyn, Y. Kozhedub, G. Plunien, and Th. Stöhlker
Pair production in low-energy collisions of uranium nuclei beyond the monopole approximation
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 408, 97 (2017)

Abstract: A method for calculation of electron-positron pair production in low-energy heavy-ion collisions beyond the monopole approximation is presented. The method is based on the numerical solving of the time-dependent Dirac equation with the full two-center potential. The one-electron wave functions are expanded in the finite basis set constructed on the two-dimensional spatial grid. Employing the developed approach the probabilities of bound-free pair production are calculated for collisions of bare uranium nuclei at the energy near the Coulomb barrier. The obtained results are compared with the corresponding values calculated in the monopole approximation.

Y. Kozhedub, A. Bondarev, X. Cai, A. Gumberidze, S. Hagmann, C. Kozhuharov, I. Maltsev, G. Plunien, V. Shabaev, C. Shao, Th. Stöhlker, I. Tupitsyn, B. Yang, and D. Yu
Intensities of K-X-ray satellite and hypersatellite target radiation in Bi83+-Xe @70MeV/u collisions
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 408, 31 (2017)

Abstract: Non-perturbative calculations of the relativistic quantum dynamics of electrons in the Bi83+-Xe collisions at 70 AMeV are performed. A method of calculation employs an independent particle model with effective single-electron Dirac-Kohn-Sham operator. Solving of the single-electron equations is based on the coupled-channel approach with atomic-like Dirac-Sturm-Fock orbitals, localized at the ions (atoms). Special attention is paid to the inner-shell processes. Intensities of the K satellite and hypersatellite target radiation are evaluated. The role of the relativistic effects is studied.

A. Gumberidze, C. Kozhuharov, R. Zhang, S. Trotsenko, Y. Kozhedub, R. DuBois, H. Beyer, K.-H. Blumenhagen, C. Brandau, A. Bräuning-Demian, W. Chen, O. Forstner, B. Gao, T. Gassner, R. Grisenti, S. Hagmann, P.-M. Hillenbrand, P. Indelicato, A. Kumar, M. Lestinsky, Yu. A. Litvinov, N. Petridis, D. Schury, U. Spillmann, C. Trageser, M. Trassinelli, X. Tu, and Th. Stöhlker
Impact parameter sensitive study of inner-shell atomic processes in the experimental storage ring
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 408, 27 (2017)

Abstract: In this work, we present a pilot experiment in the experimental storage ring (ESR) at GSI devoted to impact parameter sensitive studies of inner shell atomic processes for low-energy (heavy-) ion-atom collisions. The experiment was performed with bare and He-like xenon ions (Xe54+, Xe52+) colliding with neutral xenon gas atoms, resulting in a symmetric collision system. This choice of the projectile charge states was made in order to compare the effect of a filled K-shell with the empty one. The projectile and target X-rays have been measured at different observation angles for all impact parameters as well as for the impact parameter range of ∼35–70 fm.

A. V. Malyshev, D. A. Glazov, A. V. Volotka, I. I. Tupitsyn, V. M. Shabaev, G. Plunien, and Th. Stöhlker
Ground-state ionization energies of boronlike ions
Physical Review A 96, 022512 (2017)

Abstract: High-precision QED calculations of ground-state ionization energies are performed for all boronlike ions with nuclear charge numbers in the range 16≤Z≤96. Rigorous QED calculations are performed within the extended Furry picture and include all many-electron QED effects up to the second order of the perturbation theory. The contributions of third- and higher-order electron-correlation effects are accounted for within the Breit approximation. Nuclear recoil and nuclear polarization effects are taken into account as well. In comparison with previous evaluations of the ground-state ionization energies of boronlike ions the accuracy of the theoretical predictions is improved significantly.

J. Ullmann, Z. Andelkovic, C. Brandau, A. Dax, W. Geithner, C. Geppert, C. Gorges, M. Hammen, V. Hannen, S. Kaufmann, K. König, Y. Litvinov, M. Lochmann, B. Maaß, J. Meisner, T. Murböck, R. Sánchez, M. Schmidt, S. Schmidt, M. Steck, T. Stöhlker, R. Thompson, C. Trageser, J. Vollbrecht, C. Weinheimer, and W. Nörtershäuser
High precision hyperfine measurements in Bismuth challenge bound-state strong-field QED
Nature Communications 8, 15484 (2017)

Abstract: Electrons bound in highly charged heavy ions such as hydrogen-like bismuth 209^Bi^82+ experience electromagnetic fields that are a million times stronger than in light atoms. Measuring the wavelength of light emitted and absorbed by these ions is therefore a sensitive testing ground for quantum electrodynamical (QED) effects and especially the electron–nucleus interaction under such extreme conditions. However, insufficient knowledge of the nuclear structure has prevented a rigorous test of strong-field QED. Here we present a measurement of the so-called specific difference between the hyperfine splittings in hydrogen-like and lithium-like bismuth 209^Bi^82+,80+ with a precision that is improved by more than an order of magnitude. Even though this quantity is believed to be largely insensitive to nuclear structure and therefore the most decisive test of QED in the strong magnetic field regime, we find a 7-σ discrepancy compared with the theoretical prediction.

M. Vockert, G. Weber, U. Spillmann, T. Krings, M. Herdrich, and Th. Stöhlker
Commissioning of a Si(Li) Compton polarimeter with improved energy resolution
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 408, 313 (2017)

Abstract: Abstract On the basis of a double-side segmented Si(Li) crystal a new Compton polarimeter was developed within the SPARC collaboration. The new detector is equipped with a cryogenic first stage of the preamplifiers to improve the energy resolution compared to previous detectors with preamplifiers operating at room temperature. We present first results from a commissioning measurement of the new instrument at the ESR storage ring of GSI in Darmstadt, Germany and contrast it with the performance of an precursor polarimeter system.

M. Herdrich, G. Weber, A. Gumberidze, Z. Wu, and Th. Stöhlker
Fast calculator for X-ray emission due to Radiative Recombination and Radiative Electron Capture in relativistic heavy-ion atom collisions
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 408, 294 (2017)

Abstract: Abstract In experiments with highly charged, fast heavy ions the Radiative Recombination (RR) and Radiative Electron Capture (REC) processes have significant cross sections in an energy range of up to a few GeV / u . They are some of the most important charge changing processes in collisions of heavy ions with atoms and electrons, leading to the emission of a photon along with the formation of the ground and excited atomic states. Hence, for the understanding and planning of experiments, in particular for X-ray spectroscopy studies, at accelerator ring facilities, such as FAIR, it is crucial to have a good knowledge of these cross sections and the associated radiation characteristics. In the frame of this work a fast calculator, named RECAL, for the RR and REC process is presented and its capabilities are demonstrated with the analysis of a recently conducted experiment at the Experimental Storage Ring (ESR) at the GSI Helmholtz Center for Heavy Ion Research in Darmstadt, Germany. A method is presented to determine unknown X-ray emission cross sections via normalization of the recorded spectra to REC cross sections calculated by RECAL.

A. I. Bondarev, Y. S. Kozhedub, I. I. Tupitsyn, V. M. Shabaev, G. Plunien, and Th. Stöhlker
Relativistic calculations of differential ionization cross sections: Application to antiproton-hydrogen collisions
Physical Review A 95, 052709 (2017)

Abstract: A relativistic method based on the Dirac equation for calculating fully differential cross sections for ionization in ion-atom collisions is developed. The method is applied to ionization of the atomic hydrogen by antiproton impact, as a nonrelativistic benchmark. The fully differential, as well as various doubly and singly differential, cross sections for ionization are presented. The role of the interaction between the projectile and the target nucleus is discussed. Several discrepancies in available theoretical predictions are resolved. The relativistic effects are studied for ionization of hydrogenlike xenon ion under the impact of carbon nuclei.

R. Sánchez, M. Lochmann, R. Jöhren, Z. Andelkovic, D. Anielski, B. Botermann, M. Bussmann, A. Dax, N. Frömmgen, C. Geppert, M. Hammen, V. Hannen, T. Kühl, Y. A. Litvinov, R. López-Coto, T. Stöhlker, R. C. Thompson, J. Vollbrecht, W. Wen, C. Weinheimer, E. Will, D. Winters, and W. Nörtershäuser
Laser spectroscopy measurement of the 2s-hyperfine splitting in lithium-like bismuth
Journal of Physics B: Atomic, Molecular and Optical Physics 50, 085004 (2017)

Abstract: We have recently reported on the first direct measurement of the 2s hyperfine transition in lithium-like bismuth (209Bi^80+) at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany. Combined with a new measurement of the 1s hyperfine splitting (HFS) in hydrogen-like (209Bi^82+) the so-called specific difference Δ'E=-61.37(36) meV could be determined and was found to be in good agreement with its prediction from strong-field bound-state quantum electrodynamics. Here we report on additional investigations performed to estimate systematic uncertainties of these results and on details of the experimental setup. We show that the dominating uncertainty arises from insufficient knowledge of the ion beam velocity which is determined by the electron-cooler voltage. Two routes to obtain a cooler-voltage calibration are discussed and it is shown that agreement can be reached either between the experimental Δ'E and the theoretical result, or between the two measurements of the HFS in hydrogen-like bismuth, but not both at the same time.

N. Winckler, A. Rybalchenko, V. Shevelko, M. Al-Turany, T. Kollegger, and Th. Stöhlker
BREIT code: Analytical solution of the balance rate equations for charge-state evolutions of heavy-ion beams in matter
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 392, 67 (2017)

Abstract: A detailed description of a recently developed BREIT computer code (Balance Rate Equations of Ion Transportation) for calculating charge-state fractions of ion beams passing through matter is presented. The code is based on the analytical solutions of the differential balance equations for the charge-state fractions as a function of the target thickness and can be used for calculating the ion evolutions in gaseous, solid and plasma targets. The BREIT code is available on-line and requires the charge-changing cross sections and initial conditions in the input file.

The eigenvalue decomposition method, applied to obtain the analytical solutions of the rate equations, is described in the paper. Calculations of non-equilibrium and equilibrium charge-state fractions, performed by the BREIT code, are compared with experimental data and results of other codes for ion beams in gaseous and solid targets. Ability and limitations of the BREIT code are discussed in detail.

F. Wagner, J. Hornung, C. Schmidt, M. Eckhardt, M. Roth, T. Stöhlker, and V. Bagnoud
Backreflection diagnostics for ultra-intense laser plasma experiments based on frequency resolved optical gating
Review of Scientific Instruments 88, 023503 (2017)

Abstract: We report on the development and implementation of a time resolved backscatter diagnostics for high power laser plasma experiments at the petawatt-class laser facility PHELIX. Pulses that are backscattered or reflected from overcritical plasmas are characterized spectrally and temporally resolved using a specially designed second harmonic generation frequency resolved optical gating system. The diagnostics meets the requirements made by typical experiments, i.e., a spectral bandwidth of more than 30nm with sub-nanometer resolution and a temporal window of 10ps with 50fs temporal resolution. The diagnostics is permanently installed at the PHELIX target area and can be used to study effects such as laser-hole boring or relativistic self-phase-modulation which are important features of laser-driven particle acceleration experiments.

V. Tympel, J. Golm, R. Neubert, P. Seidel, M. Schmelz, R. Stolz, V. Zakosarenko, F. Kurian, M. Schwickert, T. Sieber, and Th. Stöhlker
The Next Generation of Cryogenic Current Comparators for Beam Monitoring
Proceedings of the 5th International Beam Instrumentation Conference, IBIC 2016 (2017)

Abstract: A new Cryogenic Current Comparator with eXtended Dimensions (CCC-XD), compared to earlier versions built for GSI, is currently under development for a non-destructive, highly-sensitive monitoring of nA-intensities of beams for larger beamline diameters planned for the new FAIR accelerator facility at GSI. The CCC consists of a:
1) flux concentrator,
2) superconducting shield against external magnetic field and a
3) superconducting toroidal coil of niobium which is read out by a
4) Superconducting Quantum Interference Device (SQUID).
The new flux concentrator (1) comprises a specially designed highly-permeable core made of nano-crystalline material, in order to assure low-noise operation with high system bandwidth of up to 200 kHz. The superconducting shielding of niobium (2) is extended in its geometric dimensions compared to the predecessor CCC and thus will suppress (better -200 dB) disturbing magnetic fields of the beamline environment more effectively. For the CCD-XD readout, new SQUID sensors (4) with sub-μm Josephson junctions are used which enable the lowest possible noiselimited current resolution in combination with a good suppression of external disturbances. The CCC-XD system, together with a new dedicated cryostat, will be ready for testing in the CRYRING at GSI in spring 2017. For the application of a CCC in the antiproton storage ring at CERN a pulse shape correction has been developed and tested in parallel. Results from electrical measurements of two components (1 and 4) of the new CCC-XD setup will be presented in this work.

2016

A. V. Volotka, A. Surzhykov, S. Trotsenko, G. Plunien, Th. Stöhlker, and S. Fritzsche
Nuclear Excitation by Two-Photon Electron Transition
Physical Review Letters 117, 243001 (2016)

Abstract: A new mechanism of nuclear excitation via two-photon electron transitions (NETP) is proposed and studied theoretically. As a generic example, detailed calculations are performed for the E1E1 1s2sS01→1s2S01 two-photon decay of a He-like Ac87+225 ion with a resonant excitation of the 3/2+ nuclear state with an energy of 40.09(5) keV. The probability for such a two-photon decay via the nuclear excitation is found to be PNETP=3.5×10−9 and, thus, is comparable with other mechanisms, such as nuclear excitation by electron transition and by electron capture. The possibility for the experimental observation of the proposed mechanism is thoroughly discussed.

K.-H. Blumenhagen, S. Fritzsche, T. Gassner, A. Gumberidze, R. Märtin, N. Schell, D. Seipt, U. Spillmann, A. Surzhykov, S. Trotsenko, G. Weber, V. A. Yerokhin, and Th. Stöhlker
Polarization transfer in Rayleigh scattering of hard x-rays
New Journal of Physics 18, 103034 (2016)

Abstract: We report on the first elastic hard x-ray scattering experiment where the linear polarization characteristics of both the incident and the scattered radiation were observed. Rayleigh scattering was investigated in a relativistic regime by using a high- Z target material, namely gold, and a photon energy of 175 keV. Although the incident synchrotron radiation was nearly 100% linearly polarized, at a scattering angle of θ=90° we observed a strong depolarization for the scattered photons with a degree of linear polarization of +27% ± 12% only. This finding agrees with second-order quantum electrodynamics calculations of Rayleigh scattering, when taking into account a small polarization impurity of the incident photon beam which was determined to be close to 98%. The latter value was obtained independently from the elastic scattering by analyzing photons that were Compton-scattered in the target. Moreover, our results indicate that when relying on state-of-the-art theory, Rayleigh scattering could provide a very accurate method to diagnose polarization impurities in a broad region of hard x-ray energies.

M. Fernandes, R. Geithner, J. Golm, R. Neubert, M. Schwickert, Th. Stöhlker, J. Tan, and C. P. Welsch
Non-perturbative measurement of low-intensity charged particle beams
Superconductor Science and Technology 30, 015001 (2016)

Abstract: Non-perturbative measurements of low-intensity charged particle beams are particularly challenging to beam diagnostics due to the low amplitude of the induced electromagnetic fields. In the low-energy antiproton decelerator (AD) and the future extra low energy antiproton rings at CERN, an absolute measurement of the beam intensity is essential to monitor the operation efficiency. Superconducting quantum interference device (SQUID) based cryogenic current comparators (CCC) have been used for measuring slow charged beams in the nA range, showing a very good current resolution. But these were unable to measure fast bunched beams, due to the slew-rate limitation of SQUID devices and presented a strong susceptibility to external perturbations. Here, we present a CCC system developed for the AD machine, which was optimised in terms of its current resolution, system stability, ability to cope with short bunched beams, and immunity to mechanical vibrations. This paper presents the monitor design and the first results from measurements with a low energy antiproton beam obtained in the AD in 2015. These are the first CCC beam current measurements ever performed in a synchrotron machine with both coasting and short bunched beams. It is shown that the system is able to stably measure the AD beam throughout the entire cycle, with a current resolution of 30 nA .

M. Fernandes, R. Geithner, J. Golm, R. Neubert, M. Schwickert, Th. Stöhlker, J. Tan, and C. P. Welsch
Non-perturbative measurement of low-intensity charged particle beams
Superconductor Science and Technology 30, 015001 (2016)

Abstract: Non-perturbative measurements of low-intensity charged particle beams are particularly challenging to beam diagnostics due to the low amplitude of the induced electromagnetic fields. In the low-energy antiproton decelerator (AD) and the future extra low energy antiproton rings at CERN, an absolute measurement of the beam intensity is essential to monitor the operation efficiency. Superconducting quantum interference device (SQUID) based cryogenic current comparators (CCC) have been used for measuring slow charged beams in the nA range, showing a very good current resolution. But these were unable to measure fast bunched beams, due to the slew-rate limitation of SQUID devices and presented a strong susceptibility to external perturbations. Here, we present a CCC system developed for the AD machine, which was optimised in terms of its current resolution, system stability, ability to cope with short bunched beams, and immunity to mechanical vibrations. This paper presents the monitor design and the first results from measurements with a low energy antiproton beam obtained in the AD in 2015. These are the first CCC beam current measurements ever performed in a synchrotron machine with both coasting and short bunched beams. It is shown that the system is able to stably measure the AD beam throughout the entire cycle, with a current resolution of 30 nA .

H. Bernhardt, B. Marx-Glowna, K. Schulze, B. Grabiger, J. Haber, C. Detlefs, R. Lötzsch, T. Kämpfer, R. Röhlsberger, E. Förster, Th. Stöhlker, I. Uschmann, and G. G. Paulus
High purity x-ray polarimetry with single-crystal diamonds
Applied Physics Letters 109, 121106 (2016)

Abstract: We report on the use of synthetic single-crystal diamonds for high purity x-ray polarimetry to improve the polarization purity of present-day x-ray polarimeters. The polarimeter setup consists of a polarizer and an analyzer, each based on two parallel diamond crystals used at a Bragg angle close to 45°. The experiment was performed using one (400) Bragg reflection on each diamond crystal and synchrotron undulator radiation at an x-ray energy of 9838.75 eV. A polarization purity of 8.9 × 10−10 was measured at the European Synchrotron Radiation Facility, which is the best value reported for two-reflection polarizer/analyzer setups. This result is encouraging and is a first step to improve the resolution of x-ray polarimeters further by using diamond crystal polarizers and analyzers with four or six consecutive reflections.

M. Lestinsky, V. Andrianov, B. Aurand, V. Bagnoud, D. Bernhardt, H. Beyer, S. Bishop, K. Blaum, A. Bleile, At. Borovik, F. Bosch, C. Bostock, C. Brandau, A. Bräuning-Demian, I. Bray, T. Davinson, B. Ebinger, A. Echler, P. Egelhof, A. Ehresmann, M. Engström, C. Enss, N. Ferreira, D. Fischer, A. Fleischmann, E. Förster, S. Fritzsche, R. Geithner, S. Geyer, J. Glorius, K. Göbel, O. Gorda, J. Goullon, P. Grabitz, R. Grisenti, A. Gumberidze, S. Hagmann, M. Heil, A. Heinz, F. Herfurth, R. Heß, P.-M. Hillenbrand, R. Hubele, P. Indelicato, A. Källberg, O. Kester, O. Kiselev, A. Knie, C. Kozhuharov, S. Kraft-Bermuth, T. Kühl, G. Lane, Y. Litvinov, D. Liesen, X. Ma, R. Märtin, R. Moshammer, A. Müller, S. Namba, P. Neumayer, T. Nilsson, W. Nörtershäuser, G. G. Paulus, N. Petridis, M. Reed, R. Reifarth, P. Reiß, J. Rothhardt, R. Sanchez, M. Sanjari, S. Schippers, H. Schmidt, D. Schneider, P. Scholz, R. Schuch, M. Schulz, V. Shabaev, A. Simonsson, J. Sjöholm, Ö. Skeppstedt, K. Sonnabend, U. Spillmann, K. Stiebing, M. Steck, T. Stöhlker, A. Surzhykov, S. Torilov, E. Träbert, M. Trassinelli, S. Trotsenko, X. Tu, I. Uschmann, P. Walker, G. Weber, D. Winters, P. Woods, H. Zhao, and Y. Zhang
Physics book: CRYRING@ESR
European Physical Journal Special Topics 225, 797 (2016)

Abstract: The exploration of the unique properties of stored and cooled beams of highly-charged ions as provided by heavy-ion storage rings has opened novel and fascinating research opportunities in the realm of atomic and nuclear physics research. Since the late 1980s, pioneering work has been performed at the CRYRING at Stockholm and at the Test Storage Ring (TSR) at Heidelberg. For the heaviest ions in the highest charge-states, a real quantum jump was achieved in the early 1990s by the commissioning of the Experimental Storage Ring (ESR) at GSI Helmholtzzentrum für Schwerionenforschung (GSI) in Darmstadt where challenging experiments on the electron dynamics in the strong field regime as well as nuclear physics studies on exotic nuclei and at the borderline to atomic physics were performed. Meanwhile also at Lanzhou a heavy-ion storage ring has been taken in operation, exploiting the unique research opportunities in particular for medium-heavy ions and exotic nuclei.

S. Höfer, T. Kämpfer, E. Förster, T. Stöhlker, and I. Uschmann
The formation of rarefaction waves in semiconductors after ultrashort excitation probed by grazing incidence ultrafast time-resolved x-ray diffraction
Structural Dynamics 3, 051101 (2016)

Abstract: We explore the InSb-semiconductor lattice dynamics after excitation of high density electron-hole plasma with an ultrashort and intense laser pulse. By using time resolved x-ray diffraction, a sub-mA ° and sub-ps resolution was achieved. Thus, a strain of 4% was measured in a 3 nm thin surface layer 2 ps after excitation. The lattice strain was observed for the first 5 ps as exponentially decaying, changing rapidly by time and by depth. The observed phenomena can only be understood assuming nonlinear time dependent laser absorption where the absorption depth decreases by a factor of twenty compared to linear absorption.

X. Chen, M. Sanjari, P. Hülsmann, Y. Litvinov, F. Nolden, J. Piotrowski, M. Steck, Th. Stöhlker, and P. Walker
Intensity-sensitive and position-resolving cavity for heavy-ion storage rings
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 826, 39 (2016)

Abstract: Abstract A heavy-ion storage ring can be adapted for use as an isochronous mass spectrometer if the ion velocity matches the transition energy of the ring. Due to the variety of stored ion species, the isochronous condition cannot be fulfilled for all the ions. In order to eliminate the measurement uncertainty stemming from the velocity spread, an intensity-sensitive and position-resolving cavity is proposed. In this paper we first briefly discuss the correction method for the anisochronism effect in the measurement with the cavity. Then we introduce a novel design, which is operated in the monopole mode and offset from the central beam orbit to one side. The geometrical parameters were optimized by analytic and numerical means in accordance with the beam dynamics of the future collector ring at FAIR. Afterwards, the electromagnetic properties of scaled prototypes were measured on a test bench. The results were in good agreement with the predictions.

M. Najafi, I. Dillmann, F. Bosch, T. Faestermann, B. Gao, R. Gernhäuser, C. Kozhuharov, S. Litvinov, Y. Litvinov, L. Maier, F. Nolden, U. Popp, M. Sanjari, U. Spillmann, M. Steck, Th. Stöhlker, and H. Weick
CsI–Silicon Particle detector for Heavy ions Orbiting in Storage rings (CsISiPHOS)
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 836, 1 (2016)

Abstract: Abstract A heavy-ion detector was developed for decay studies in the Experimental Storage Ring (ESR) at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany. This detector serves as a prototype for the in-pocket particle detectors for future experiments with the Collector Ring (CR) at FAIR (Facility for Antiproton and Ion Research). The detector includes a stack of six silicon pad sensors, a double-sided silicon strip detector (DSSD), and a CsI(Tl) scintillation detector. It was used successfully in a recent experiment for the detection of the β+-decay of highly charged 142Pm60+ ions. Based on the Δ E / E technique for particle identification and an energy resolution of 0.9% for Δ E and 0.5% for E (Full Width at Half Maximum (FWHM)), the detector is well-suited to distinguish neighbouring isobars in the region of interest.

D. Bernhardt, C. Brandau, Z. Harman, C. Kozhuharov, S. Böhm, F. Bosch, S. Fritzsche, J. Jacobi, S. Kieslich, H. Knopp, F. Nolden, W. Shi, Z. Stachura, M. Steck, Th. Stöhlker, S. Schippers, and A. Müller
Erratum: Electron-ion collision spectroscopy: Lithium-like xenon ions [Phys. Rev. A 91, 012710 (2015)]
Physical Review A 94, 029903 (2016)

Abstract: *

N. A. Zubova, A. V. Malyshev, I. I. Tupitsyn, V. M. Shabaev, Y. S. Kozhedub, G. Plunien, C. Brandau, and Th. Stöhlker
Isotope shifts of the 2p₃/₂-2p₁/₂ transition in B-like ions
Physical Review A 93, 052502 (2016)

Abstract: Isotope shifts of the 2p3/2−>2p1/2 transition in B-like ions are evaluated for a wide range of the nuclear charge number: Z=8–92. The calculations of the relativistic nuclear recoil and nuclear size effects are performed using a large-scale configuration-interaction Dirac-Fock-Sturm method. The corresponding QED corrections are also taken into account. The results of the calculations are compared with the theoretical values obtained with other methods. The accuracy of the isotope shifts of the 2p3/2−>2p1/2 transition in B-like ions is significantly improved.

F. Wagner, O. Deppert, C. Brabetz, P. Fiala, A. Kleinschmidt, P. Poth, V. A. Schanz, A. Tebartz, B. Zielbauer, M. Roth, T. Stöhlker, and V. Bagnoud
Maximum Proton Energy above 85 MeV from the Relativistic Interaction of Laser Pulses with Micrometer Thick CH₂ Targets
Physical Review Letters 116, 205002 (2016)

Abstract: We present a study of laser-driven ion acceleration with micrometer and submicrometer thick plastic targets. Using laser pulses with high temporal contrast and an intensity of the order of 10^20  W/cm2 we observe proton beams with cutoff energies in excess of 85 MeV and particle numbers of 109 in an energy bin of 1 MeV around this maximum. We show that applying the target normal sheath acceleration mechanism with submicrometer thick targets is a very robust way to achieve such high ion energies and particle fluxes. Our results are backed with 2D particle in cell simulations furthermore predicting cutoff energies above 200 MeV for acceleration based on relativistic transparency. This predicted regime can be probed after a few technically feasible adjustments of the laser and target parameters.

P.-M. Hillenbrand, S. Hagmann, J. M. Monti, R. D. Rivarola, K.-H. Blumenhagen, C. Brandau, W. Chen, R. D. DuBois, A. Gumberidze, D. L. Guo, M. Lestinsky, Yu. A. Litvinov, A. Müller, S. Schippers, U. Spillmann, S. Trotsenko, G. Weber, and Th. Stöhlker
Strong asymmetry of the electron-loss-to-continuum cusp of multielectron U²⁸⁺ projectiles in near-relativistic collisions with gaseous targets
Physical Review A 93, 042709 (2016)

Abstract: The process of electron-loss to the continuum (ELC) has been studied for the collision systems U28+ -> H2 at a collision energy of 50 MeV/u, U28+ -> N2 at 30 MeV/u, and U28+ -> Xe at 50 MeV/u. The energy distributions of cusp electrons emitted at an angle of 0∘ with respect to the projectile beam were measured using a magnetic forward-angle electron spectrometer. For these collision systems far from equilibrium charge state, a significantly asymmetric cusp shape is observed. The experimental results are compared to calculations based on first-order perturbation theory, which predict an almost symmetric cusp shape. Some possible reasons for this discrepancy are discussed.

C. Hahn, G. Weber, R. Märtin, S. Höfer, T. Kämpfer, and Th. Stöhlker
CdTe Timepix detectors for single-photon spectroscopy and linear polarimetry of high-flux hard x-ray radiation
Review of Scientific Instruments 87, 043106 (2016)

Abstract: Single-photon spectroscopy of pulsed, high-intensity sources of hard X-rays — such as laser-generated plasmas — is often hampered by the pileup of several photons absorbed by the unsegmented, large-volume sensors routinely used for the detection of high-energy radiation. Detectors based on the Timepix chip, with a segmentation pitch of 55 μm and the possibility to be equipped with high-Zsensor chips, constitute an attractive alternative to commonly used passive solutions such as image plates. In this report, we present energy calibration and characterization measurements of such devices. The achievable energy resolution is comparable to that of scintillators for γ spectroscopy. Moreover, we also introduce a simple two-detector Compton polarimeter setup with a polarimeter quality of (98 ± 1)%. Finally, a proof-of-principle polarimetry experiment is discussed, where we studied the linear polarization of bremsstrahlung emitted by a laser-driven plasma and found an indication of the X-raypolarization direction depending on the polarization state of the incident laser pulse.

A. V. Volotka, V. A. Yerokhin, A. Surzhykov, Th. Stöhlker, and S. Fritzsche
Many-electron effects on x-ray Rayleigh scattering by highly charged He-like ions
Physical Review A 93, 023418 (2016)

Abstract: The Rayleigh scattering of x rays by many-electron highly charged ions is studied theoretically. The many-electron perturbation theory, based on a rigorous quantum electrodynamics approach, is developed and implemented for the case of the elastic scattering of (high-energetic) photons by heliumlike ions. Using this elaborate approach, we here investigate the many-electron effects beyond the independent-particle approximation (IPA) as conventionally employed for describing the Rayleigh scattering. The total and angle-differential cross sections are evaluated for the x-ray scattering by heliumlike Ni^26+, Xe^52+, and Au^77+ ions in their ground state. The obtained results show that, for high-energetic photons, the effects beyond the IPA do not exceed 2% for the scattering by a closed K shell.

2015

C. Trageser, C. Brandau, C. Kozhuharov, Y. A. Litvinov, A. Müller, F. Nolden, S. Sanjari, and T. Stöhlker
A new data acquisition system for Schottky signals in atomic physics experiments at GSI's and FAIR's storage rings
Physica Scripta 2015, 014062 (2015)

Abstract: A new continuous and broadband data acquisition system for measurements of Schottky-signals of ions revolving in a storage ring has been implemented. This set-up is capable of recording the radio frequency (RF) signal of the ions that circulate in the storage ring with a sustained acquisition rate of more than 3.5×10⁷ IQ-samples per second. This allows several harmonics of the full momentum acceptance of a storage ring to be measured at the same time. The RF signal analyzer modules are complemented by further electronic modules such as counters, precision clocks and synchronization modules that facilitate a seamless integration with main experimental data acquisitions for atomic and nuclear physics. In this contribution, the setup and first results from a test run at the experimental storage ring at GSI, Darmstadt, Germany, are presented.

R. Geithner, F. Kurian, H. Reeg, M. Schwickert, R. Neubert, P. Seidel, and T. Stöhlker
A squid-based beam current monitor for FAIR/CRYRING
Physica Scripta 2015, 014057 (2015)

Abstract: A SQUID-based beam current monitor was developed for the upcoming FAIR-Project, providing a non-destructive online monitoring of the beam currents in the nA-range. The cryogenic current comparator (CCC) was optimized for lowest possible noise-limited current resolution together with a high system bandwidth. This CCC is foreseen to be installed in the CRYRING facility, working as a test bench for FAIR. In this contribution we present results of the completed CCC for FAIR/CRYRING and also arrangements that have been done for the installation of the CCC at CRYRING, regarding the cryostat design.

A. Gumberidze, T. Stöhlker, and Y. Litvinov
Atomic physics at the future facility for antiproton and ion research: status report 2014
Physica Scripta 2015, 014076 (2015)

Abstract: In this contribution, a brief overview of the Stored Particle Atomic physics Research Collaboration scientific program at the upcoming Facility for Antiproton and Ion Research (FAIR) is given. The program comprises a very broad range of research topics addressing atomic structure and dynamics in hitherto unexplored regimes, light–matter interactions, lepton pair production phenomena, precision tests of quantum electrodynamics and standard model in the regime of extreme fields and many more. We also present the current strategy for the realization of the envisioned physics program within the modularized start version (MSV) of FAIR.

T. Stöhlker, and Y. A. Litvinov
Atomic physics experiments at the high energy storage ring
Physica Scripta 2015, 014025 (2015)

Abstract: Facility for Antiproton and Ion Research (FAIR), will offer unprecedented experimental opportunities. The Stored Particles Atomic Research Collaboration (SPARC) at FAIR aims at creating a worldwide unique research program with highly charged ions by utilizing storage ring and trapping facilities. The foreseen experiments will address physics at strong, ultra-short electromagnetic fields including the fundamental interactions between electrons and heavy nuclei as well as the experiments at the border between nuclear and atomic physics. In view of the staged construction of the FAIR facility, SPARC worked out an early realization scheme for experiments with highly-charged heavy-ions at relativistic energies to be conducted in the High-Energy Storage Ring.

S. Tashenov, D. Banas, H. Beyer, C. Brandau, S. Fritzsche, A. Gumberidze, S. Hagmann, P.-M. Hillenbrand, H. Jörg, I. Kojouharov, C. Kozhuharov, M. Lestinsky, Y. A. Litvinov, A. V. Maiorova, H. Schaffner, V. M. Shabaev, U. Spillmann, T. Stöhlker, A. Surzhykov, and S. Trotsenko
Coherent population of magnetic sublevels of 2p₃/₂ state in hydrogenlike uranium by radiative recombination
Physica Scripta 2015, 014027 (2015)

Abstract: The x-rays emitted in the process of radiative recombination (RR) of quasi-free electrons into 2p₃/₂ excited state of hydrogenlike uranium ion were studied experimentally. Both the RR x-ray and the subsequently emitted Lyα₁ x-ray were detected in time-coincidences. The angular distribution of the Lyα₁ x-rays varied as a function of the RR x-ray emission direction. This observation revealed the coherent population of magnetic sublevels of the 2p₃/₂ state in the hydrogenlike uranium ion.

M. S. Sanjari, X. Chen, P. Hülsmann, Y. A. Litvinov, F. Nolden, J. Piotrowski, M. Steck, and T. Stöhlker
Conceptual design of elliptical cavities for intensity and position sensitive beam measurements in storage rings
Physica Scripta 2015, 014060 (2015)

Abstract: Position sensitive beam monitors are indispensable for the beam diagnostics in storage rings. Apart from their applications in the measurements of beam parameters, they can be used in non-destructive in-ring decay studies of radioactive ion beams as well as enhancing precision in the isochronous mass measurement technique. In this work, we introduce a novel approach based on cavities with elliptical cross-section, in order to compensate the limitations of known designs for the application in ion storage rings. The design is aimed primarily for future heavy ion storage rings of the FAIR project. The conceptual design is discussed together with simulation results.

P.-M. Hillenbrand, S. Hagmann, K. E. Stiebing, S. Schippers, Y. A. Litvinov, and T. Stöhlker
Experimental concepts of positron spectroscopy at HESR
Physica Scripta 2015, 014026 (2015)

Abstract: In this paper we discuss the unique possibilities of studying atomic electron–positron pair production processes in high-resolution experiments at the high-energy storage ring (HESR) at the future FAIR accelerator complex. The two pair-production processes bound-free pair production and negative-continuum dielectronic recombination are contrasted. An overview of previous studies is given and requirements for experimental parameters of a magnetic forward-angle positron spectrometer at the HESR are depicted.

S. Trotsenko, A. Gumberidze, Y. Gao, C. Kozhuharov, S. Fritzsche, H. F. Beyer, S. Hagmann, P.-M. Hillenbrand, N. Petridis, U. Spillmann, A. Surzhykov, D. B. Thorn, G. Weber, and T. Stöhlker
Experimental study of the dielectronic recombination into Li-like uranium
Physica Scripta 2015, 014024 (2015)

Abstract: We have measured the x-rays following 116.15 MeV/u U⁸⁹⁺ collisions with H 2 at 35°, 90°, 120° and 150° observation angles with regard to the ion beam direction. From our experimental spectra combined with radiative electron capture calculations, we obtain angular distribution of characteristic x-rays L to K following the resonance transfer and excitation. Our result shows a good qualitative agreement with theoretical predictions.

S. Ringleb, M. Vogel, S. Kumar, W. Quint, G. G. Paulus, and Th. Stöhlker
HILITE—ions in intense photon fields
Physica Scripta 2015, 014067 (2015)

Abstract: We are currently devising the open-endcap Penning trap experiment (high-intensity laser ion-trap experiment) as a tool for ion confinement, manipulation and detection to be used at high-energy and/or high-intensity laser facilities. This instrument will allow studies of laser–ion interactions with well-defined ion targets, and to detect the reaction products non-destructively. The ion target may be controlled concerning the constituent species, the density, shape and position with respect to the laser focus. For commissioning experiments, we optimize the focusing parameters to achieve a high number of ionized particles per shot. The detection electronics is designed to measure all charge states of all nuclei up to xenon. We plan first experiments with argon and xenon irradiated by a titanium:sapphire chirped-pulse-amplification laser system with 10 mJ pulse energy and a pulse duration of 30 fs.

R. Sanchez, J. Ullmann, J. Vollbrecht, Z. Andelkovic, A. Dax, W. Geithner, C. Geppert, C. Gorges, M. Hammen, V. Hannen, S. Kaufmann, K. König, Y. A. Litvinov, M. Lochmann, B. Maaß, J. Meisner, T. Murböck, W. Nörtershäuser, S. Schmidt, M. Schmidt, M. Steck, T. Stöhlker, R. C. Thompson, and C. Weinheimer
Hyperfine transition in ²⁰⁹Bi⁸⁰⁺ -- one step forward
Physica Scripta 2015, 014021 (2015)

Abstract: The hyperfine transitions in lithium-like and hydrogen-like bismuth were remeasured by direct laser spectroscopy at the experimental storage ring. For this we have now employed a voltage divider which enabled us to monitor the electron cooler voltage in situ . This will improve the experimental accuracy by about one order of magnitude with respect to our previous measurement using the same technique.

K.-H. Blumenhagen, U. Spillmann, T. Gaßner, A. Gumberidze, R. Märtin, N. Schell, S. Trotsenko, G. Weber, and T. Stöhlker
Identification and reduction of unwanted stray radiation using an energy- and position-sensitive Compton polarimeter
Physica Scripta 2015, 014032 (2015)

Abstract: In this work, we report on an experiment that investigated the elastic scattering of linearly polarized 175 keV photons on a gold target. A combined measurement of the angular distribution and the linear polarization of the scattered photons was performed using standard germanium detectors and a double-sided Si(Li) strip polarimeter. Since the data analysis is still in progress, we will show results in forthcoming papers and present here how the polarimeter was used to identify a lack of shielding during the experiment.

O. Kovalenko, O. Dolinskii, Y. A. Litvinov, R. Maier, D. Prasuhn, and T. Stöhlker
Investigation of the heavy-ion mode in the FAIR High Energy Storage Ring
Physica Scripta 2015, 014042 (2015)

Abstract: High energy storage ring (HESR) as a part of the future accelerator facility FAIR (Facility for Antiproton and Ion Research) will serve for a variety of internal target experiments with high-energy stored heavy ions (SPARC collaboration). Bare uranium is planned to be used as a primary beam. Since a storage time in some cases may be significant—up to half an hour—it is important to examine the high-order effects in the long-term beam dynamics. A new ion optics specifically for the heavy ion mode of the HESR is developed and is discussed in this paper. The subjects of an optics design, tune working point and a dynamic aperture are addressed. For that purpose nonlinear beam dynamics simulations are carried out. Also a flexibility of the HESR ion optical lattice is verified with regard to various experimental setups. Specifically, due to charge exchange reactions in the internal target, secondary beams, such as hydrogen-like and helium-like uranium ions, will be produced. Thus the possibility of separation of these secondary ions and the primary U⁹²⁺ beam is presented with different internal target locations.

D. Winters, T. Beck, G. Birkl, C. Dimopoulou, V. Hannen, T. Kühl, M. Lochmann, M. Loeser, X. Ma, F. Nolden, W. Nörtershäuser, B. Rein, R. Sánchez, U. Schramm, M. Siebold, P. Spiller, M. Steck, T. Stöhlker, J. Ullmann, T. Walther, W. Wen, J. Yang, D. Zhang, and M. Bussmann
Laser cooling of relativistic heavy-ion beams for FAIR
Physica Scripta 2015, 014048 (2015)

Abstract: Laser cooling is a powerful technique to reduce the longitudinal momentum spread of stored relativistic ion beams. Based on successful experiments at the experimental storage ring at GSI in Darmstadt, of which we show some important results in this paper, we present our plans for laser cooling of relativistic ion beams in the future heavy-ion synchrotron SIS100 at the Facility for Antiproton and Ion Research in Darmstadt.

D. T. Doherty, P. J. Woods, Y. A. Litvinov, M. Ali Najafi, S. Bagchi, S. Bishop, M. Bo, C. Brandau, T. Davinson, I. Dillmann, A. Estrade, P. Egelhof, A. Evdokimov, A. Gumberidze, M. Heil, C. Lederer, S. A. Litvinov, G. Lotay, N. Kalantar-Nayestanaki, O. Kiselev, C. Kozhuharov, T. Kröll, M. Mahjour-Shafei, M. Mutterer, F. Nolden, N. Petridis, U. Popp, R. Reifarth, C. Rigollet, S. Roy, M. Steck, T. Stöhlker, B. Streicher, S. Trotsenko, M. von Schmid, X. L. Yan, and J. C. Zamora
Nuclear transfer reaction measurements at the ESR—for the investigation of the astrophysical ¹⁵O(α,γ)¹⁹Ne reaction
Physica Scripta 2015, 014007 (2015)

Abstract: Astrophysical x-ray bursts are thought to be a result of thermonuclear explosions on the atmosphere of an accreting neutron star. Between these bursts, energy is thought to be generated by the hot CNO cycles. The ¹⁵O(α,γ)¹⁹Ne reaction is one reaction that allows breakout from these CNO cycles and into the rp-process to fuel outbursts. The reaction is expected to be dominated by a single 3/2⁺ resonance at 4.033 MeV in ¹⁹Ne, however, limited information is available on this key state. This work reports on a pioneering study of the ²⁰Ne(p,d)¹⁹Ne reaction, performed in inverse kinematics at the experimental storage ring (ESR) as a means of accessing the astrophysically important 4.033 MeV state in ¹⁹Ne. The unique, background free, high luminosity conditions of the storage ring were utilized for this, the first transfer reaction performed at the ESR. The results of this pioneering test experiment are presented along with suggestions for future measurements at storage ring facilities.

J. Rothhardt, S. Hädrich, S. Demmler, M. Krebs, D. Winters, T. Kuehl, T. Stöhlker, J. Limpert, and A. Tünnermann
Prospects for laser spectroscopy of highly charged ions with high-harmonic XUV and soft x-ray sources
Physica Scripta T166, 14030 (2015)

Abstract: We present novel high photon flux XUV and soft x-ray sources based on high harmonic generation (HHG). The sources employ femtosecond fiber lasers, which can be operated at very high (MHz) repetition rate and average power (>100 W). HHG with such lasers results in ∼1013 photons s−1 within a single harmonic line at ∼40 nm (∼30 eV) wavelength, a photon flux comparable to what is typically available at synchrotron beam lines. In addition, resonant enhancement of HHG can result in narrow-band harmonics with high spectral purity—well suited for precision spectroscopy. These novel light sources will enable seminal studies on electronic transitions in highly-charged ions. For example, at the experimental storage ring 2s1/2– 2p1/2 transitions in Li-like ions can be excited up to Z=47 (∼100 eV transition energy), which provides unique sensitivity to quantum electro-dynamical effects and nuclear corrections. We estimate fluorescence count rates of the order of tens per second, which would enable studies on short-lived isotopes as well. In combination with the Doppler up-shift available in head-on excitation at future heavy-ion storage rings, such as the high energy storage ring, even multi-keV transitions can potentially be excited. Pump–probe experiments with femtosecond resolution could also be feasible and access the lifetime of short-lived excited states, thus providing novel benchmarks for atomic structure theory.

N. Petridis, R. E. Grisenti, Y. A. Litvinov, and T. Stöhlker
Prototype internal target design for storage ring experiments
Physica Scripta 2015, 014051 (2015)

Abstract: The introduction of cryogenically cooled, few micrometer-sized nozzle geometries and an essential modification of the experimental storage ring (ESR) target station allowed for a reliable operation using low- Z gases at target area densities in the range of 10¹³--10¹⁴ cm−². Therefore, a remarkably versatile target source was established, enabling operation over the whole range of desired target gases (from H₂ to Xe) and area densities (~10¹⁰ to ~10¹⁴ cm−²). Moreover, the considerably smaller orifice diameter of the new target source enables a much more compact inlet chamber while, at the same time, maintaining the demanding vacuum requirements of a storage ring. A completely new inlet chamber design is presented here, which, besides the improvements regarding the achievable area densities, will feature a variable beam width down to 1 mm at the ion beam interaction region. This is of paramount importance with respect to the realization of high precision experiments, e.g. by reducing the inaccuracy of the observation angle causing the relativistic Doppler broadening. While being intended for the deployment at the future high energy storage ring within the SPARC collaboration, the new inlet chamber can also replace the current one at the ESR or serve as an internal target for CRYRING.

X. Chen, M. S. Sanjari, J. Piotrowski, P. Hülsmann, Y. A. Litvinov, F. Nolden, M. Steck, and T. Stöhlker
Report on a computer-controlled automatic test platform for precision RF cavity characterizations
Physica Scripta 2015, 014061 (2015)

Abstract: An automated test platform aiming at accurate and efficient cavity characterizations has recently been set up at GSI, Darmstadt. In this proceeding the composition of such a system, on both hardware and software sides, is described in detail. The amount of necessary human work is significantly reduced to the minimum, while the measurement precision is improved considerably.

C. Brandau, C. Kozhuharov, M. Lestinsky, A. Müller, S. Schippers, and T. Stöhlker
Storage-ring experiments on dielectronic recombination at the interface of atomic and nuclear physics
Physica Scripta 2015, 014022 (2015)

Abstract: A brief review about topical developments in the exploitation of the resonant electron–ion collision process of dielectronic recombination (DR) as a sensitive spectroscopic tool is given. The focus will be on DR storage-ring experiments of few-electron highly charged ions. Among others, the questions addressed in these studies cover diverse topics from the areas of strong-field quantum electrodynamics, of lifetime studies using DR resonances, and of nuclear physics. Examples from the storage rings CRYRING in Stockholm, TSR in Heidelberg, and ESR in Darmstadt are given. In addition, an overview is provided about the ongoing developments and future perspectives of DR collision spectroscopy at the upcoming Facility for Antiproton and Ion Research in Darmstadt, Germany.

D. Hengstler, M. Keller, C. Schötz, J. Geist, M. Krantz, S. Kempf, L. Gastaldo, A. Fleischmann, T. Gassner, G. Weber, R. Märtin, Th. Stöhlker, and C. Enss
Towards FAIR: first measurements of metallic magnetic calorimeters for high-resolution x-ray spectroscopy at GSI
Physica Scripta 2015, 014054 (2015)

Abstract: Metallic magnetic calorimeters are energy dispersive particle detectors that are operated at temperatures below 100 mK. Applied to x-ray spectroscopy they combine the high energy resolution of crystal spectrometers with the large energy bandwidth of semiconductor detectors. After the absorption of a photon its energy is converted into heat. A paramagnetic alloy converts the temperature change into a change of magnetization that is read out by a sensitive superconducting quantum interference device magnetometer. With such a metallic magnetic calorimeter we performed two successful measurements at the internal gas target of the experimental storage ring at GSI. In the first beamtime lithium-like Au-ions were targeted on a N2 and a Xe gas target, respectively. In the second beamtime we observed a projectile beam of bare Xe ions interacting with a Xe gas target. In both experiments we achieved an energy resolution below 60 eV from 0 keV to 60 keV . We were able to detect K-lines of Xe ions of different charge states, including the Lyman series up to Ly-η and could resolve the Ly-β-doublet in H-like Xe.

J. Holmberg, A. N. Artemyev, A. Surzhykov, V. A. Yerokhin, and Th. Stöhlker
QED corrections to radiative recombination and radiative decay of heavy hydrogenlike ions
Physical Review A 92, 042510 (2015)

Abstract: One-loop quantum electrodynamic (QED) corrections are studied for two basic atomic processes, radiative recombination of an electron with a bare nucleus and radiative decay of a hydrogenlike ion. The perturbations of the bound-state wave function and the binding energy due to the electron self-energy and the vacuum polarization are computed in the Feynman and Coulomb gauges. QED corrections induced by these perturbations are calculated for the differential cross section and the polarization of the emitted radiation in the radiative recombination of an electron and a bare uranium nuclei, as well as the corresponding corrections to the ratio of the E1 (electric dipole) and M2 (magnetic quadrupole) transition amplitudes in the 2p3/2→1s radiative decay of hydrogenlike uranium. The results obtained indicate the expected magnitude of the QED effects in these processes on a subpercent level.

V. A. Yerokhin, A. N. Artemyev, V. M. Shabaev, Th. Stöhlker, A. Surzhykov, and S. Fritzsche
Target effects in negative-continuum-assisted dielectronic recombination
Physical Review A 92, 042708 (2015)

Abstract: The process of recombination of a quasifree electron into a bound state of an initially bare nucleus with the simultaneous creation of a bound-electron–free-positron pair is investigated. This process is called negative-continuum-assisted dielectronic recombination (NCDR). In a typical experimental setup, the initial electron is not free but bound in a light atomic target. In the present work, we study the effects of the atomic target on the single- and double-differential cross sections of positron production in the NCDR process. Calculations are performed within the relativistic framework based on QED theory, accounting for the electron-electron interaction to first order in perturbation theory. We demonstrate how the momentum distribution of the target electrons removes the nonphysical singularity of the differential cross section which occurs for the initially free and monochromatic electrons.

A. Surzhykov, V. A. Yerokhin, T. Stöhlker, and S. Fritzsche
Corrigendum: Rayleigh x-ray scattering from many-electron atoms and ions
Journal of Physics B: Atomic, Molecular and Optical Physics 48, 189501 (2015)

Abstract: A theoretical analysis is presented for the elastic Rayleigh scattering of x-rays by many-electron atoms and ions. Special emphasis is placed on the angular distribution and linear polarization of the scattered photons for the case when the incident light is completely (linearly) polarized. Based on second-order perturbation theory and the independent particle approximation, we found that the Rayleigh angular distribution is strongly affected by the charge state and shell structure of the target ions or atoms. This effect can be observed experimentally at modern synchrotron facilities and might provide further insight into the structure of heavy atomic systems.

B. Marx-Glowna, K. Schulze, I. Uschmann, T. Kämpfer, G. Weber, C. Hahn, H.-C. Wille, K. Schlage, R. Röhlsberger, E. Förster, Th. Stöhlker, and G. Paulus
Influence of higher harmonics of the undulator in X-ray polarimetry and crystal monochromator design
Journal of Synchrotron Radiation 22, 1151 (2015)

Abstract: The spectrum of the undulator radiation of beamline P01 at Petra III has been measured after passing a multiple reflection channel-cut polarimeter. Odd and even harmonics up to the 15th order, as well as Compton peaks which were produced by the high harmonics in the spectrum, could been measured. These additional contributions can have a tremendous influence on the performance of the polarimeter and have to be taken into account for further polarimeter designs.

Th. Stöhlker, V. Bagnoud, K. Blaum, A. Blazevic, A. Bräuning-Demian, M. Durante, F. Herfurth, M. Lestinsky, Y. Litvinov, S. Neff, R. Pleskac, R. Schuch, S. Schippers, D. Severin, A. Tauschwitz, C. Trautmann, D. Varentsov, and E. Widmann
APPA at FAIR: From fundamental to applied research
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 365, 680 (2015)

Abstract: FAIR with its intense beams of ions and antiprotons provides outstanding and worldwide unique experimental conditions for extreme matter research in atomic and plasma physics and for application oriented research in biophysics, medical physics and materials science. The associated research programs comprise interaction of matter with highest electromagnetic fields, properties of plasmas and of solid matter under extreme pressure, density, and temperature conditions, simulation of galactic cosmic radiation, research in nanoscience and charged particle radiotherapy. A broad variety of APPA-dedicated facilities including experimental stations, storage rings, and traps, equipped with most sophisticated instrumentation will allow the APPA community to tackle new challenges. The worldwide most intense source of slow antiprotons will expand the scope of APPA related research to the exciting field of antimatter.

D. Banaś, M. Pajek, A. Surzhykov, Th. Stöhlker, C. Brandau, A. Gumberidze, C. Kozhuharov, H. F. Beyer, S. Böhm, F. Bosch, M. Czarnota, S. Chatterjee, J.-C. Dousse, S. Fritzsche, S. Hagmann, D. Liesen, P. H. Mokler, A. Müller, A. Kumar, R. Reuschl, D. Sierpowski, U. Spillmann, J. Szlachetko, S. Tashenov, S. Trotsenko, P. Verma, and A. Warczak
Subshell-selective x-ray studies of radiative recombination of U⁹²⁺ ions with electrons for very low relative energies
Physical Review A 92, 032710 (2015)

Abstract: Radiative recombination (RR) into the K shell and L subshells of U92+ ions interacting with cooling electrons has been studied in an x-ray RR experiment at the electron cooler of the Experimental Storage Ring at GSI. The measured radiative recombination rate coefficients for electron-ion relative energies in the range 0–1000 meV demonstrate the importance of relativistic effects. The observed asymmetry of the measured K-RR x-ray emission with respect to the cooling energy, i.e., zero average relative velocity (〈vrel〉=0), are explained by fully relativistic RR calculations. With our new approach, we show that the study of the angular distribution of RR photons for different relative energies opens new perspectives for detailed understanding of the RR of ions with cooling electrons in cold magnetized plasma.

B. Mei, T. Aumann, S. Bishop, K. Blaum, K. Boretzky, F. Bosch, C. Brandau, H. Bräuning, T. Davinson, I. Dillmann, C. Dimopoulou, O. Ershova, Z. Fülöp, H. Geissel, J. Glorius, G. Gyürky, M. Heil, F. Käppeler, A. Kelic-Heil, C. Kozhuharov, C. Langer, T. Le Bleis, Y. Litvinov, G. Lotay, J. Marganiec, G. Münzenberg, F. Nolden, N. Petridis, R. Plag, U. Popp, G. Rastrepina, R. Reifarth, B. Riese, C. Rigollet, C. Scheidenberger, H. Simon, K. Sonnabend, M. Steck, T. Stöhlker, T. Szücs, K. Sümmerer, G. Weber, H. Weick, D. Winters, N. Winters, P. Woods, and Q. Zhong
First measurement of the ⁹⁶Ru(p,γ)⁹⁷Rh cross section for the p process with a storage ring
Physical Review C 92, 035803 (2015)

Abstract: This work presents a direct measurement of the ⁹⁶Ru(p,γ)⁹⁷Rh cross section via a novel technique using a storage ring, which opens opportunities for reaction measurements on unstable nuclei. A proof-of-principle experiment was performed at the storage ring ESR at GSI in Darmstadt, where circulating ⁹⁶Ru ions interacted repeatedly with a hydrogen target. The ⁹⁶Ru(p,γ)⁹⁷Rh cross section between 9 and 11 MeV has been determined using two independent normalization methods. As key ingredients in Hauser-Feshbach calculations, the γ-ray strength function as well as the level density model can be pinned down with the measured (p,γ) cross section. Furthermore, the proton optical potential can be optimized after the uncertainties from the γ-ray strength function and the level density have been removed. As a result, a constrained ⁹⁶Ru(p,γ)⁹⁷Rh reaction rate over a wide temperature range is recommended for p-process network calculations.

V. A. Zaytsev, S. Tashenov, A. V. Maiorova, V. M. Shabaev, and T. Stöhlker
Parity nonconservation effect in the resonance elastic electron scattering on heavy He-like ions
Journal of Physics B: Atomic, Molecular and Optical Physics 48, 165003 (2015)

Abstract: We investigate the parity nonconservation effect in the elastic scattering of polarized electrons on heavy He-like ions, being initially in the ground state. The enhancement of the parity violation is achieved by tuning the energy of the incident electron in resonance with quasidegenerate doubly-excited states of the corresponding Li-like ion. We consider two possible scenarios. In the first one we assume that the polarization of the scattered electron is measured, while in the second one it is not detected. For the second scenario we propose a scheme of a modified electron beam ion source (EBIS) experiment where the detection of a parity violation in the electron scattering seems possible.

D. Atanasov, K. Blaum, F. Bosch, C. Brandau, P. Bühler, X. Chen, I. Dillmann, T. Faestermann, B. Gao, H. Geissel, R. Gernhäuser, S. Hagmann, T. Izumikawa, P.-M. Hillenbrand, C. Kozhuharov, J. Kurcewicz, S. Litvinov, Y. Litvinov, X. Ma, G. Münzenberg, M. Najafi, F. Nolden, T. Ohtsubo, A. Ozawa, F. Ozturk, Z. Patyk, M. Reed, R. Reifarth, M. Sanjari, D. Schneider, M. Steck, T. Stöhlker, B. Sun, F. Suzaki, T. Suzuki, C. Trageser, X. Tu, T. Uesaka, P. Walker, M. Wang, H. Weick, N. Winckler, P. Woods, H. Xu, T. Yamaguchi, X. Yan, and Y. Zhang
Between atomic and nuclear physics: radioactive decays of highly-charged ions
Journal of Physics B: Atomic, Molecular and Optical Physics 48, 144024 (2015)

Abstract: Highly charged radioactive ions can be stored for extended periods of time in storage rings which allows for precision measurements of their decay modes. The straightforward motivation for performing such studies is that fully ionised nuclei or few-electron ions can be viewed as clean quantum-mechanical systems, in which the interactions of the many electrons can be either excluded or treated precisely. Thus, the influence of the electron shell on the decay probability can be investigated. Another important motivation is stellar nucleosynthesis, which proceeds at high temperatures and the involved atoms are therefore highly ionised. Presented here is a compact review of the relevant experiments conducted at heavy-ion storage rings. Furthermore, we outline the perspectives for future experiments at new-generation storage-ring facilities.

M. Guerra, T. Stöhlker, P. Amaro, J. Machado, and J. P. Santos
Electron impact ionization cross-sections for few-electron uranium ions
Journal of Physics B: Atomic, Molecular and Optical Physics 48, 144027 (2015)

Abstract: Electron impact ionization cross sections for the U^88+ , U^89+ , U^90+ and U^91+ ions were calculated with the relativistic binary encounter Bethe model (RBEB), the modified RBEB (MRBEB) and the new MRBEB corrected by the ionic factor (MRBR–IF). Our results were compared with the available three sets of experimental data and the most used theoretical results. The MRBEB–IF results are the ones that better agree with the experimental data of the four analysed ions.

H. Stöcker, T. Stöhlker, and C. Sturm
FAIR - Cosmic Matter in the Laboratory
Journal of Physics: Conference Series 623, 012026 (2015)

G. Weber, H. Bräuning, A. Surzhykov, C. Brandau, S. Fritzsche, S. Geyer, R. E. Grisenti, S. Hagmann, C. Hahn, R. Hess, S. Hess, C. Kozhuharov, M. Kühnel, R. Märtin, N. Petridis, U. Spillmann, S. Trotsenko, D. F. A. Winters, and Th. Stöhlker
Combined linear polarization and angular distribution measurements of x-rays for precise determination of multipole-mixing in characteristic transitions of high- Z systems
Journal of Physics B: Atomic, Molecular and Optical Physics 48, 144031 (2015)

Abstract: By applying novel-type position sensitive x-ray detectors as Compton polarimeters we recently performed a study of the linear polarization of Lyman-α₁ radiation following radiative electron capture into initially bare uranium ions. It was found that a model-independent determination of the ratio of the E1 and M2 transition amplitudes, and consequently of the corresponding transition rates, is feasible by combining the linear polarization data with a measurement of the angular distribution of the emitted radiation. In this work a detailed description of the underlying experimental technique for combined measurements of the linear polarization and the angular distribution of characteristic transitions in high-Z ions is presented. Special emphasis is given to the application of two, two-dimensional position-sensitive x-ray detectors for Compton polarimetry of hard x-rays. Moreover, we demonstrate the polarimeter efficiency of such detector systems can be significantly improved if events, where the charge is spread over neighboring segments, are reconstructed to be used in the polarization analysis.

A. Gumberidze, D. B. Thorn, C. J. Fontes, B. Najjari, H. L. Zhang, A. Surzhykov, A. Voitkiv, S. Fritzsche, D. Banaś, H. Beyer, W. Chen, R. D. DuBois, S. Geyer, R. E. Grisenti, S. Hagmann, M. Hegewald, S. Hess, C. Kozhuharov, R. Märtin, N. Petridis, R. Reuschl, A. Simon, U. Spillmann, M. Trassinelli, S. Trotsenko, G. Weber, D. F. A. Winters, N. Winters, D. Yu, and Th. Stöhlker
Ground-state excitation of heavy highly-charged ions
Journal of Physics B: Atomic, Molecular and Optical Physics 48, 144006 (2015)

Abstract: We have studied the excitation of H-like and He-like uranium (U^91+ and U^90+ ) in relativistic collisions with gaseous targets by observing the subsequent x-ray emission. The experiment was conducted at the ESR storage ring of the GSI accelerator facility in Darmstadt, Germany. The measurements were performed with a newly developed multi-phase target at different collision energies. This enabled us to explore the proton (nucleus) impact excitation as well as the electron impact excitation processes in the relativistic collisions. The large fine-structure splitting in uranium allowed us to unambiguously resolve excitation to different L-shell levels. Moreover, information about the population of different magnetic sublevels has been obtained via an angular differential study of the decay photons associated with the subsequent de-excitation process. The experimental results are compared with calculations performed within the relativistic framework including excitation mechanisms due to both protons (nucleus) and electrons.

H. F. Beyer, T. Gassner, M. Trassinelli, R. Heß, U. Spillmann, D. Banaś, K.-H. Blumenhagen, F. Bosch, C. Brandau, W. Chen, C. Dimopoulou, E. Förster, R. E. Grisenti, A. Gumberidze, S. Hagmann, P.-M. Hillenbrand, P. Indelicato, P. Jagodzinski, T. Kämpfer, C. Kozhuharov, M. Lestinsky, D. Liesen, Y. A. Litvinov, R. Loetzsch, B. Manil, R. Märtin, F. Nolden, N. Petridis, M. S. Sanjari, K. S. Schulze, M. Schwemlein, A. Simionovici, M. Steck, Th. Stöhlker, C. I. Szabo, S. Trotsenko, I. Uschmann, G. Weber, O. Wehrhan, N. Winckler, D. F. A. Winters, N. Winters, and E. Ziegler
Crystal optics for precision x-ray spectroscopy on highly charged ions—conception and proof
Journal of Physics B: Atomic, Molecular and Optical Physics 48, 144010 (2015)

Abstract: The experimental investigation of quantum-electrodydamic contributions to the binding energies of inner shells of highly charged heavy ions requires an accurate spectroscopy in the region of hard x-rays suitable at a limited source strength. For this purpose the focusing compensated asymmetric Laue crystal optics has been developed and a twin-spectrometer assembly has been built and commissioned at the experimental storage ring of the GSI Helmholtzzentrum Darmstadt. We characterize the crystal optics and demonstrate the usefulness of the instrumentation for accurate spectroscopy of both stationary and fast moving x-ray sources. The experimental procedures discussed here may also be applied for other spectroscopic studies where a transition from conventional germanium x-ray detectors to crystal spectrometers seems too demanding because of low source intensity.

J. Vollbrecht, Z. Andelkovic, A. Dax, W. Geithner, C. Geppert, C. Gorges, M. Hammen, V. Hannen, S. Kaufmann, K. König, Y. Litvinov, M. Lochmann, B. Maass, J. Meisner, T. Murboeck, W. Nörtershäuser, R. Sánchez, S. Schmidt, M. Schmidt, M. Steck, T. Stöhlker, R. C. Thompson, J. Ullmann, and C. Weinheimer
Laser spectroscopy of the ground-state hyperfine structure in H-like and Li-like bismuth
Journal of Physics: Conference Series 583, 012002 (2015)

Abstract: The LIBELLE experiment performed at the experimental storage ring (ESR) at the GSI Helmholtz Center in Darmstadt aims for the determination of the ground state hyperfine (HFS) transitions and lifetimes in hydrogen-like ( 209 Bi82+ ) and lithium-like ( 209 Bi 0+ ) bismuth. The study of HFS transitions in highly charged ions enables precision tests of QED in extreme electric and magnetic fields otherwise not attainable in laboratory experiments. While the HFS transition in H-like bismuth was already observed in earlier experiments at the ESR, the LIBELLE experiment succeeded for the first time to measure the HFS transition in Li-like bismuth in a laser spectroscopy experiment.

B. S. Gao, M. A. Najafi, D. R. Atanasov, K. Blaum, F. Bosch, C. Brandau, X. C. Chen, I. Dillmann, C. Dimopoulou, T. Faestermann, H. Geissel, R. Gernhäuser, P.-M. Hillenbrand, O. Kovalenko, C. Kozhuharov, S. A. Litvinov, Y. A. Litvinov, L. Maier, F. Nolden, J. Piotrowski, M. S. Sanjari, C. Scheidenberger, U. Spillmann, M. Steck, T. Stöhlker, C. Trageser, X. L. Tu, H. Weick, N. Winckler, H. S. Xu, T. Yamaguchi, X. L. Yan, Y. H. Zhang, and X. H. Zhou
EPJ Web of Conferences 93, 05003 (2015)

Abstract: Access to stored and cooled highly-charged radionuclides offers unprecedented opportunities to perform high-precision investigations of their decays. Since the few-electron ions, e.g. hydrogen- or helium-like ions, are quantum mechanical systems with clear electronic ground state configurations, the decay studies of such ions are performed under well-defined conditions and allow for addressing fundamental aspects of the decay process. Presented here is a compact review of the relevant experiments conducted at the Experimental Storage Ring ESR of GSI. A particular emphasis is given to the investigations of the two-body beta decay, namely the bound-state β-decay and its time-mirrored counterpart, orbital electron-capture.

D. Bernhardt, C. Brandau, Z. Harman, C. Kozhuharov, S. Böhm, F. Bosch, S. Fritzsche, J. Jacobi, S. Kieslich, H. Knopp, F. Nolden, W. Shi, Z. Stachura, M. Steck, T. Stöhlker, S. Schippers, and A. Müller
Spectroscopy of berylliumlike xenon ions using dielectronic recombination
Journal of Physics B: Atomic, Molecular and Optical Physics 48, 144008 (2015)

Abstract: Be-like ¹³⁶Xe⁵⁰⁺ ions have been investigated employing the resonant electron-ion collision process of dielectronic recombination (DR) as a spectroscopic tool. The experiments were performed at the experimental storage ring in Darmstadt, Germany, using its electron cooler as a target for free electrons. DR Rydberg resonance series 2s²+e⁻ \to 2s 2pj n lj for the associated intra-L-shell transitions 2s² ¹S₀ - 2s 2p₁/₂ ³P₁, 2s² ¹S₀ - 2s 2p₃/₂ ³P₂ and 2s² ¹S₀ - 2s 2p₃/₂ ¹P₁ were observed with high resolution. In addition to these excitations from the ground state we determined resonances associated with excitations 2s 2p₁/₂ ³P₀ to 2p₁/₂ 2p₃/₂ ³P₁ of ions initially in the metastable 2s 2p₁/₂ ³P₀ state. The corresponding excitation energies were determined to be E(¹S₀ \to ³P₁)=127.269(46) eV, E(¹S₀ \to ³P₂)=469.474(81) eV and E(¹S₀ to ¹P₁)=532.801(16) eV and E(³P₀ to 2p₁/₂ 2p₃/₂ ³P₁)=533.733(22) eV. These excitation energies are compared with previous measurements and with recent state-of-the-art atomic structure calculations.

J. Ullmann, Z. Andelkovic, A. Dax, W. Geithner, C. Geppert, C. Gorges, M. Hammen, V. Hannen, S. Kaufmann, K. König, Y. Litvinov, M. Lochmann, B. Maass, J. Meisner, T. Murböck, R. Sánchez, M. Schmidt, S. Schmidt, M. Steck, T. Stöhlker, R. C. Thompson, J. Vollbrecht, C. Weinheimer, and W. Nörtershäuser
An improved value for the hyperfine splitting of hydrogen-like 209 Bi 82+
Journal of Physics B: Atomic, Molecular and Optical Physics 48, 144022 (2015)

Abstract: We report an improved measurement of the hyperfine splitting in hydrogen-like bismuth (209 Bi^82+) at the experimental storage ring ESR at GSI by laser spectroscopy on a coasting beam. Accuracy was improved by about an order of magnitude compared to the first observation in 1994. The most important improvement is an in situ high voltage measurement at the electron cooler (EC) platform with an accuracy at the 10 ppm level. Furthermore, the space charge effect of the EC current on the ion velocity was determined with two independent techniques that provided consistent results. The result of lambda₀=243.821(6) nm provides an important reference value for experiments testing bound-state quantum electrodynamics in the strong magnetic field regime by evaluating the specific difference between the splittings in the hydrogen-like and lithium-like ions.

A. Surzhykov, V. A. Yerokhin, T. Stöhlker, and S. Fritzsche
Rayleigh x-ray scattering from many-electron atoms and ions
Journal of Physics B: Atomic, Molecular and Optical Physics 48, 144015 (2015)

Abstract: A theoretical analysis is presented for the elastic Rayleigh scattering of x-rays by many-electron atoms and ions. Special emphasis is placed on the angular distribution and linear polarization of the scattered photons for the case when the incident light is completely (linearly) polarized. Based on second-order perturbation theory and the independent particle approximation, we found that the Rayleigh angular distribution is strongly affected by the charge state and shell structure of the target ions or atoms. This effect can be observed experimentally at modern synchrotron facilities and might provide further insight into the structure of heavy atomic systems.

M. Vogel, G. Birkl, M. Ebrahimi, D. von Lindenfels, A. Martin, G. Paulus, W. Quint, S. Ringleb, Th. Stöhlker, and M. Wiesel
Extreme-field physics in Penning traps
Hyperfine Interactions 236, 65 (2015)

Abstract: We present two Penning trap experiments concerned with different aspects of the physics of extreme electromagnetic fields, the ARTEMIS experiment designed for bound-electron magnetic moment measurements in the presence of the extremely strong fields close to the nucleus of highly charged ions, and the HILITE experiment, in which well-defined ion targets are to be subjected to high-intensity laser fields.

I. A. Maltsev, V. M. Shabaev, I. I. Tupitsyn, A. I. Bondarev, Y. S. Kozhedub, G. Plunien, and Th. Stöhlker
Electron-positron pair creation in low-energy collisions of heavy bare nuclei
Physical Review A 91, 032708 (2015)

Abstract: A method for calculations of electron-positron pair-creation probabilities in low-energy heavy-ion collisions is developed. The approach is based on the propagation of all one-electron states via the numerical solving of the time-dependent Dirac equation in the monopole approximation. The electron wave functions are represented as finite sums of basis functions constructed from B-splines using the dual-kinetic-balance technique. The calculations of the created particle numbers and the positron energy spectra are performed for the collisions of bare nuclei at the energies near the Coulomb barrier with the Rutherford trajectory and for different values of the nuclear charge and the impact parameter. To examine the role of the spontaneous pair creation, the collisions with a modified velocity and with a time delay are also considered. The obtained results are compared with the previous calculations and the possibility of observation of the spontaneous pair creation is discussed.

P. Wustelt, M. Möller, T. Rathje, A. M. Sayler, T. Stöhlker, and G. G. Paulus
Momentum-resolved study of the saturation intensity in multiple ionization
Physical Review A 91, 031401 (2015)

Abstract: We present a momentum-resolved study of strong field multiple ionization of ionic targets. Using a deconvolution method we are able to reconstruct the electron momenta from the ion momentum distributions after multiple ionization up to four sequential ionization steps. This technique allows an accurate determination of the saturation intensity as well as of the electron release times during the laser pulse. The measured results are discussed in comparison to typically used models of over-the-barrier ionization and tunnel ionization.

A. Akber, M. W. Reed, P. M. Walker, Yu. A. Litvinov, G. J. Lane, T. Kibédi, K. Blaum, F. Bosch, C. Brandau, J. J. Carroll, D. M. Cullen, I. J. Cullen, A. Y. Deo, B. Detwiler, C. Dimopoulou, G. D. Dracoulis, F. Farinon, H. Geissel, E. Haettner, M. Heil, R. S. Kempley, R. Knöbel, C. Kozhuharov, J. Kurcewicz, N. Kuzminchuk, S. Litvinov, Z. Liu, R. Mao, C. Nociforo, F. Nolden, W. R. Plaß, Zs. Podolyak, A. Prochazka, C. Scheidenberger, D. Shubina, M. Steck, Th. Stöhlker, B. Sun, T. P. D. Swan, G. Trees, H. Weick, N. Winckler, M. Winkler, P. J. Woods, and T. Yamaguchi
Increased isomeric lifetime of hydrogen-like ¹⁹²mOs
Physical Review C 91, 031301 (2015)

Abstract: An excited metastable nuclear state of Os192 in a hydrogen-like charge state has been studied for the first time. It was populated in projectile fragmentation of a Au197 beam on a Be9 target with the UNILAC-SIS accelerators at GSI. Fragmentation products in the region of interest were passed through the fragment separator and injected into the experimental storage ring (ESR). Cooling of the injected beam particles enabled Schottky mass spectrometry to be performed. Analysis shows the lifetime of the state to be considerably longer than that of the neutral ion [t_neut=8.5(14)s]; this change is attributed to hindrance of internal conversion in hydrogen-like Os192. Calculations have been performed to estimate the lifetime, and the result has been compared with that measured experimentally. There is good agreement between the expected [t_H-like=13.0(24)s] and measured lifetimes (t_rest=15.1+1.5-1.3 s) from the internal decay of Os192m. This provides a test for the reliability of the values obtained from internal conversion coefficient calculations in highly ionized systems and is the first measurement of its kind to be performed using the ESR setup.

G. Weber, M. Herdrich, R. DuBois, P.-M. Hillenbrand, H. Beyer, L. Bozyk, T. Gassner, R. Grisenti, S. Hagmann, Y. Litvinov, F. Nolden, N. Petridis, M. Sanjari, D. Winters, and Th. Stöhlker
Total projectile electron loss cross sections of U²⁸⁺ ions in collisions with gaseous targets ranging from hydrogen to krypton
Physical Review ST - Accelerators and Beams 18, 034403 (2015)

Abstract: Beam lifetimes of stored U^(28+) ions with kinetic energies of 30 and 50  MeV/u, respectively, were measured in the experimental storage ring of the GSI accelerator facility. By using the internal gas target station of the experimental storage ring, it was possible to obtain total projectile electron loss cross sections for collisions with several gaseous targets ranging from hydrogen to krypton from the beam lifetime data. The resulting experimental cross sections are compared to predictions by two theoretical approaches, namely the CTMC method and a combination of the DEPOSIT code and the RICODE program.

T. Krings, U. Spillmann, D. Protić, C. Roß, Th. Stöhlker, G. Weber, L. Bombelli, R. Alberti, and T. Frizzi
Multi-element readout of structured HPGe-detectors for high-resolution x-ray spectroscopy using CUBE-preamplifiers
Journal of Instrumentation 10, C02043 (2015)

Abstract: Very recently we have shown that CUBE-preamplifiers developed by XGLab s.r.l. can be used for the readout of single elements of thick structured planar HPGe- and Si(Li)-detectors produced by SEMIKON [1]. In this paper we will present the results of a simultaneous multi-element readout of structured detectors using the same preamplifiers for measuring high-energy x-rays (more than 100 keV) with a comparable energy resolution as for the single-element readout. Several high-purity germanium detectors (HPGe-detectors) with different position sensitive structures on one detector contact have been used for the first tests. In addition to that we have modified an existing 16-pixel HPGe-polarimeter from GSI-Darmstadt with the new readout. The detector elements (7 mm × 7 mm each, arranged in a 4 × 4 matrix) are connected to CUBE-preamplifiers used in pulse-reset mode. The technological progress achieved with this detector system resulting in a significant improved energy resolution will contribute a lot to much more precise polarization measurements of x-rays emitted from atom-ion collisions which are part of the physics program of the SPARC collaboration (Stored Particles Atomic Physics Research Collaboration) at GSI and the future FAIR accelerator facility (Facility for Antiproton and Ion Research).

K.-H. Blumenhagen, A. Gumberidze, R. Märtin, N. Schell, U. Spillmann, G. Weber, and Th. Stöhlker
Characterization of a double-sided Si(Li) strip Compton polarimeter
Journal of Physics: Conference Series 583, 012043 (2015)

Abstract: The response of a double-sided segmented Si(Li) detector system has been investigated. The detector has been irradiated with a collimated, highly linearly polarized beam of 53.2 keV photons from the synchrotron radiation source PETRA III at DESY. The detector was mounted on a platform that could be moved with μm precision thus allowing for a defined beam position on the detector surface. In this paper, the effects of the isolation gaps (gap width = 50 μm) between adjacent segments (strips) were studied, in particular with respect to the effect of charge sharing. The fraction of such charge sharing events increases from about 5% (beam hits center of a strip) to over 50% when the beam is focused just on a gap. The fraction of reconstructed Compton scattering events, which is interesting for Compton polarimetry, amounts to about 3% with the beam impinging at a strip center and 2.8% on average. It can therefore be concluded that events related to charge sharing do not critically degrade the performance of the detector as a Compton polarimter.

G. Weber, K.-H. Blumenhagen, H. Bräuning, H. Ding, S. Fritzsche, S. Hess, R. Märtin, U. Spillmann, A. Surzhykov, S. Trotsenko, D. F. A. Winters, V. A. Yerokhin, and T. Stöhlker
Compton polarimetry using double-sided segmented x-ray detectors
Journal of Physics: Conference Series 583, 012041 (2015)

Abstract: Hard x-ray polarimetry of radiation emitted in collisions of heavy ions, electrons or photons with matter provides detailed information on the collision dynamics as well as of the atomic structure in the presence of extreme field strengths. Moreover, it also opens a route for polarization diagnosis of spin-polarized ion and electron beams which, for example, might be useful in future parity non-conservation studies. Owing to recent progress in the development of highly segmented solid-state detectors, a novel type of polarimeter for the hard x-ray regime has become available. Applied as Compton polarimeters, two-dimensional position-sensitive x-ray detectors now allow for precise and efficient measurements of x-ray linear polarization properties. In this report recent polarimetry studies using such detector systems are reviewed.

P.-M. Hillenbrand, S. Hagmann, D. H. Jakubassa-Amundsen, J. M. Monti, D. Banaś, K.-H. Blumenhagen, C. Brandau, W. Chen, P. D. Fainstein, E. De Filippo, A. Gumberidze, D. L. Guo, M. Lestinsky, Yu. A. Litvinov, A. Müller, R. D. Rivarola, H. Rothard, S. Schippers, M. S. Schöffler, U. Spillmann, S. Trotsenko, X. L. Zhu, and Th. Stöhlker
Electron-capture-to-continuum cusp in U⁸⁸⁺+N₂ collisions
Physical Review A 91, 022705 (2015)

Abstract: For the collision system U88+ -> N2 at a collision energy of 90 MeV/u, the energy distribution of electrons being nonradiatively captured from the target into the projectile continuum has been measured under an angle of 0∘ with respect to the projectile beam axis. This measurement of the electron-capture-to-continuum cusp with the highest effective projectile charge Z_eff,p=88 at a near-relativistic collision velocity of β≈0.41 is shown to be characterized by a strong asymmetry in the cusp shape. By comparing the data to measurements of the radiative-electron-capture-to-continuum cusp for the same collision system, the opposite asymmetry of the cusp is traced back to the varying underlying mechanisms. The experimental results are compared with the two theoretical calculations available for this process, one of them in the semirelativistic impulse approximation and the other in the nonrelativistic continuum-distorted-wave approach. A corresponding fully relativistic treatment may be motivated by the presented experimental data.

D. Bernhardt, C. Brandau, Z. Harman, C. Kozhuharov, S. Böhm, F. Bosch, S. Fritzsche, J. Jacobi, S. Kieslich, H. Knopp, F. Nolden, W. Shi, Z. Stachura, M. Steck, Th. Stöhlker, S. Schippers, and A. Müller
Electron-ion collision spectroscopy: Lithium-like xenon ions
Physical Review A 91, 012710 (2015)

Abstract: The resonant process of dielectronic recombination (DR) has been applied as a spectroscopic tool to investigate intra-L-shell excitations 2s−2pj in Li-like 136Xe51+. The experiments were carried out at the electron cooler of the Experimental Storage Ring of the GSI-Helmholtzzentrum für Schwerionenforschung, Darmstadt, Germany. The observed center-of-mass energy range (0–505 eV) covers all resonances associated with the 2s+e−→(2p1/2nlj)J and (2p3/2nlj)J DR processes. Energies and strengths of isolated 2p1/2n and 2p3/2n DR-resonance groups were obtained for principal quantum numbers n up to 43 and 36, respectively. The 2s−2p1/2 and 2s−2p3/2 excitation energies were deduced to be 119.816(42) eV and 492.174(52) eV. The excitation energies are compared with previous measurements of other groups and with recent QED calculations. In addition, the experimental spectra and extracted resonance strengths are compared with multiconfiguration Dirac-Fock calculations. Measurements and theory are found to be in good agreement with each other.

2014

A. Bondarevskaya, E. A. Mistonova, K. N. Lyashchenko, O. Yu. Andreev, A. Surzhykov, L. N. Labzowsky, G. Plunien, D. Liesen, F. Bosch, and Th. Stöhlker
Method for the production of highly charged ions with polarized nuclei and zero total electron angular momentum
Physical Review A 90, 064701 (2014)

Abstract: The capture of polarized electrons by H-like ions provides a method for polarizing the nuclei of He-like heavy ions with zero total electron angular momentum in storage rings for high-energy ions. A detailed analysis for Eu-151 ions with nuclear spin I=5/2 predicts a nuclear polarization degree of about 47% already after one passage through a target containing 100% polarized electrons. Almost 50% of the polarized He-like ions are predicted to be in states with zero total electron angular momentum. Such ions were recently considered as the most promising candidates in experiments at storage rings for the search for violations of the fundamental symmetries and for a nuclear and an electron electric dipole moment.

N. A. Zubova, Y. S. Kozhedub, V. M. Shabaev, I. I. Tupitsyn, A. V. Volotka, G. Plunien, C. Brandau, and Th. Stöhlker
Relativistic calculations of the isotope shifts in highly charged Li-like ions
Physical Review A 90, 062512 (2014)

Abstract: Relativistic calculations of the isotope shifts of energy levels in highly charged Li-like ions are performed. The nuclear recoil (mass shift) contributions are calculated by merging the perturbative and large-scale configuration-interaction Dirac-Fock-Sturm (CI-DFS) methods. The nuclear size (field shift) contributions are evaluated by the CI-DFS method including the electron-correlation, Breit, and QED corrections. The nuclear deformation and nuclear polarization corrections to the isotope shifts in Li-like neodymium, thorium, and uranium are also considered. The results of the calculations are compared with the theoretical values obtained with other methods.

D. Tiedemann, K. E. Stiebing, D. F. A. Winters, W. Quint, V. Varentsov, A. Warczak, A. Malarz, and Th. Stöhlker
A pulsed supersonic gas jet target for precision spectroscopy at the HITRAP facility at GSI
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 764, 387 (2014)

Abstract: A pulsed supersonic gas jet target for experiments at the HITRAP facility at the GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt has been designed and built as a multi-purpose installation for key experiments on fundamental atomic physics in strong fields. This setup is currently installed at the Institut für Kernphysik of Goethe-University, Frankfurt am Main (IKF), in order to explore its operation prior to its installation at the HITRAP facility. Design and performance of the target are described. The measured target densities of 5.9×10^12 atoms/cm3 for helium and 8.1×10^12 atoms/cm³ for argon at the stagnation pressure of 30 bar match the required values. The target-beam diameter of 0.9 mm and the pulsed operation mode (jet built-up-time ≤15 ms) are well suited for the use at HITRAP.

P.-M. Hillenbrand, S. Hagmann, A. B. Voitkiv, B. Najjari, D. Banaś, K.-H. Blumenhagen, C. Brandau, W. Chen, E. De Filippo, A. Gumberidze, D. L. Guo, C. Kozhuharov, M. Lestinsky, Yu. A. Litvinov, A. Müller, H. Rothard, S. Schippers, M. S. Schöffler, U. Spillmann, S. Trotsenko, X. L. Zhu, and Th. Stöhlker
Electron-loss-to-continuum cusp in U⁸⁸⁺ + N₂ collisions
Physical Review A 90, 042713 (2014)

Abstract: The electron loss to the continuum has been studied for the collision system U88+ + N2→U89+ + [N2]∗ + e− at the low-relativistic projectile energy of 90 MeV/u. Using a magnetic electron spectrometer, the energy distribution of cusp electrons emitted at an angle of 0∘ with respect to the projectile beam was measured in coincidence with the up-charged projectile. At the experimental collision energy ionization of the berylliumlike U88+ projectile proceeds predominantly from the L shell, but a contribution from the K shell could also be identified experimentally. The measurement is shown to be in accordance with fully relativistic Dirac calculations applying first-order perturbation theory. Furthermore, the underlying continuum electron distribution in the projectile frame is illustrated.

Y. S. Kozhedub, V. M. Shabaev, I. I. Tupitsyn, A. Gumberidze, S. Hagmann, G. Plunien, and Th. Stöhlker
Relativistic calculations of x-ray emission following a Xe-Bi 83+ collision
Physical Review A 90, 042709 (2014)

Abstract: We study the x-ray emission following the collision of a Bi83+ ion with a neutral Xe atom at the projectile energy 70 MeV/u. The collisional and post-collisional processes are treated separately. The probabilities of various many-electron processes at the collision are calculated within a relativistic independent electron model using the coupled-channel approach with atomiclike Dirac-Fock-Sturm orbitals. The analysis of the post-collisional processes resulting in the x-ray emission is based on the fluorescence yields, the radiation, and Auger decay rates, and allows one to derive intensities of the x-ray emission and compare them with experimental data. A reasonable agreement between the theoretical results and the recent experimental data is observed. The role of the relativistic effects is investigated.

M. Lochmann, R. Jöhren, C. Geppert, Z. Andelkovic, D. Anielski, B. Botermann, M. Bussmann, A. Dax, N. Frömmgen, M. Hammen, V. Hannen, T. Kühl, Y. Litvinov, R. Lopez-Coto, T. Stöhlker, R. Thompson, J. Vollbrecht, A. Volotka, C. Weinheimer, W. Wen, E. Will, D. Winters, R. Sanchez, and W. Nörtershäuser
Observation of the hyperfine transition in lithium-like bismuth 209Bi80+ : Towards a test of QED in strong magnetic fields
Physical Review A 90, 030501 (2014)

Abstract: We performed a laser spectroscopic determination of the 2s hyperfine splitting (HFS) of Li-like 209Bi80+ and repeated the measurement of the 1s HFS of H-like 209Bi82+. Both ion species were subsequently stored in the Experimental Storage Ring at the GSI Helmholtzzentrum für Schwerionenforschung Darmstadt and cooled with an electron cooler at a velocity of ≈0.71c. Pulsed laser excitation of the M1 hyperfine transition was performed in anticollinear and collinear geometry for Bi82+ and Bi80+, respectively, and observed by fluorescence detection. We obtain ΔE(1s)=5086.3(11)meV for Bi82+, different from the literature value, and ΔE(2s)=797.50(18)meV for Bi80+. These values provide experimental evidence that a specific difference between the two splitting energies can be used to test QED calculations in the strongest static magnetic fields available in the laboratory independent of nuclear structure effects. The experimental result is in excellent agreement with the theoretical prediction and confirms the sum of the Dirac term and the relativistic interelectronic-interaction correction at a level of 0.5%, confirming the importance of accounting for the Breit interaction.

S. Tashenov, D. Banaś, H. Beyer, C. Brandau, S. Fritzsche, A. Gumberidze, S. Hagmann, P.-M. Hillenbrand, H. Jörg, I. Kojouharov, Ch. Kozhuharov, M. Lestinsky, Yu. A. Litvinov, A. V. Maiorova, H. Schaffner, V. M. Shabaev, U. Spillmann, Th. Stöhlker, A. Surzhykov, and S. Trotsenko
Observation of Coherence in the Time-Reversed Relativistic Photoelectric Effect
Physical Review Letters 113, 113001 (2014)

Abstract: The photoelectric effect has been studied in the regime of hard x rays and strong Coulomb fields via its time-reversed process of radiative recombination (RR). In the experiment, the relativistic electrons recombined into the 2p3/2 excited state of hydrogenlike uranium ions, and both the RR x rays and the subsequently emitted characteristic x rays were detected in coincidence. This allowed us to observe the coherence between the magnetic substates in a highly charged ion and to identify the contribution of the spin-orbit interaction to the RR process.

B. Botermann, D. Bing, C. Geppert, G. Gwinner, T. Hänsch, G. Huber, S. Karpuk, A. Krieger, T. Kühl, W. Nörtershäuser, C. Novotny, S. Reinhardt, R. Sanchez, D. Schwalm, T. Stöhlker, A. Wolf, and G. Saathoff
Test of Time Dilation Using Stored Li+ Ions as Clocks at Relativistic Speed
Physical Review Letters 113, 120405 (2014)

Abstract: We present the concluding result from an Ives-Stilwell-type time dilation experiment using Li+7 ions confined at a velocity of β=v/c=0.338 in the storage ring ESR at Darmstadt. A Λ-type three-level system within the hyperfine structure of the Li+7S13→P23 line is driven by two laser beams aligned parallel and antiparallel relative to the ion beam. The lasers’ Doppler shifted frequencies required for resonance are measured with an accuracy of <4×10−9 using optical-optical double resonance spectroscopy. This allows us to verify the special relativity relation between the time dilation factor γ and the velocity β, γ√(1- β^2)=1 to within ±2.3×10−9 at this velocity. The result, which is singled out by a high boost velocity β, is also interpreted within Lorentz invariance violating test theories.

F. Wagner, C. João, J. Fils, T. Gottschall, J. Hein, J. Körner, J. Limpert, M. Roth, T. Stöhlker, and V. Bagnoud
Temporal contrast control at the PHELIX petawatt laser facility by means of tunable sub-picosecond optical parametric amplification
Applied Physics B 116, 429 (2014)

Abstract: We report on the development of a preamplifier module for temporal contrast enhancement and control at petawatt-class lasers. The module is based on an ultrafast optical parametric amplifier (uOPA), which produces temporally clean pulses at the 60 μJ level for seeding a chirped pulse amplification (CPA) system, namely the petawatt facility PHELIX. The amplifier module allows for gain reduction in the following amplifiers, resulting in an attenuation of amplified spontaneous emission (ASE) by more than 4 orders of magnitude. Since the ASE of a CPA system linearly depends on the seeding energy, we were able to demonstrate a continuous variation of the temporal contrast by tuning the gain of the uOPA.

P.-M. Hillenbrand, S. Hagmann, D. Atanasov, D. Banaś, K.-H. Blumenhagen, C. Brandau, W. Chen, E. De Filippo, A. Gumberidze, D. L. Guo, D. H. Jakubassa-Amundsen, O. Kovtun, C. Kozhuharov, M. Lestinsky, Yu. A. Litvinov, A. Müller, R. A. Müller, H. Rothard, S. Schippers, M. S. Schöffler, U. Spillmann, A. Surzhykov, S. Trotsenko, N. Winckler, X. L. Yan, V. A. Yerokhin, X. L. Zhu, and Th. Stöhlker
Radiative-electron-capture-to-continuum cusp in U⁸⁸⁺+N₂ collisions and the high-energy endpoint of electron-nucleus bremsstrahlung
Physical Review A 90, 022707 (2014)

Abstract: The radiative electron capture of a target electron into the projectile continuum has been studied for the collision system U88+ + N2 → U88+ + [N+2]∗ + e− + γ at 90 MeV/u. Using a magnetic electron spectrometer, the energy distribution of cusp electrons emitted under an angle of 0∘ with respect to the projectile beam and with a velocity close to the projectile velocity has been measured in coincidence with the emitted photons under various observation angles. The experimental results provide a stringent test for the corresponding process in inverse kinematics, namely, the theory of electron-nucleus bremsstrahlung at the high-energy endpoint. For comparison this process is calculated using fully relativistic Dirac wave functions and using semirelativistic Sommerfeld-Maue wave functions.

B. Marx, K. S. Schulze, I. Uschmann, T. Kämpfer, O. Wehrhan, H. C. Wille, K. Schlage, R. Röhlsberger, E. Weckert, E. Förster, Th. Stöhlker, and G. G. Paulus
High precision measurement of undulator polarization in the regime of hard x-rays
Applied Physics Letters 105, 024103 (2014)

Abstract: We have measured the polarization purity of undulator radiation at 12.9 keV, with hitherto unachievable precision. We could measure a polarization purity of 1.8 × 10−4 by using a silicon channel-cut crystal with six Bragg reflections at 45° as analyzer.

P. Jagodziński, M. Pajek, D. Banaś, H. Beyer, M. Trassinelli, and Th. Stöhlker
Ray-tracing simulations of spherical Johann diffraction spectrometer for in-beam X-ray experiments
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 753, 121 (2014)

Abstract: The results of the Monte-Carlo ray-tracing simulations for a Johann-type Bragg spectrometer with spherically curved-crystal designed to detect the X-rays from a fast-moving source are reported. These calculations were performed to optimize the X-ray spectrometer to be used at the gas-target installed at ion storage ring for high-resolution X-ray experiments. In particular, the two-dimensional distributions of detected photons were studied using the Monte-Carlo method both for the stationary and moving X-ray sources, taking into account a detailed description of X-ray source and X-ray diffraction on the crystal as well as a role of the Doppler effect for in-beam experiments. The origin of the asymmetry of observed X-ray profiles was discussed in detail and the procedure to derive a precise (sub-eV) X-ray transition energy for such asymmetric profiles was proposed. The results are important for the investigations of View the MathML source1s2p P2/3→1s2s S1/3 intrashell transition in excited He-like uranium ions in in-beam X-ray experiments.

D. Banaś, P. Jagodziński, M. Pajek, A. Gumberidze, A. Surzhykov, and Th. Stöhlker
Monte-Carlo simulations of the radiative recombination of ions with electrons in cold magnetized plasma
Physica Scripta 2014, 014001 (2014)

Abstract: We report here on the results of Monte-Carlo simulations of the radiative recombination of highly charged ions with low-energy electrons in the presence of a guiding magnetic field. The simulations are based on a semi-classical geometrical model, recently proposed by our group, which has been developed in order to explain systematic discrepancies, the so-called ‘enhancement effect’, of the radiative recombination rates measured in the guiding magnetic field of electron coolers with respect to theoretical calculations. With the simulations, we demonstrate that the enhancement of radiative recombination rates in the magnetic field could be caused by ‘transverse’ collisions with the impact parameter in the μm range and the impact parameter cut-off value depending on the strength of the guiding B -field in magnetized plasma. In this paper, the methodology of the simulations, the obtained B -field dependence of the radiative recombination enhancement and the observed impact parameter cut-off will be discussed.

W. Chen, G. Vorobyev, D. Guo, F. Herfurth, P.-M. Hillenbrand, U. Spillmann, S. Trotsenko, A. Gumberidze, and Th. Stöhlker
Metal vapor target for precise studies of ion-atom collisions
Review of Scientific Instruments 85, 053513 (2014)

Abstract: Although different ion-atom collisions have been studied in various contexts, precise values of cross-sections for many atomic processes were seldom obtained. One of the main uncertainties originates from the value of target densities. In this paper, we describe a unique method to measure a target density precisely with a combination of physical vapor deposition and inductively coupled plasma optical emission spectrometry. This method is preliminarily applied to a charge transfer cross-section measurement in collisions between highly charged ions and magnesium vapor. The final relative uncertainty of the target density is less than 2.5%. This enables the precise studies of atomic processes in ion-atom collisions, even though in the trial test the deduction of precise capture cross-sections was limited by other systematic errors.

K. S. Schulze, B. Marx, I. Uschmann, E. Förster, Th. Stöhlker, and G. G. Paulus
Determination of the polarization state of x rays with the help of anomalous transmission
Applied Physics Letters 104, 151110 (2014)

Abstract: Besides intensity and direction, the polarization of an electromagnetic wave provides characteristic information on the crossed medium. Here, we present two methods for the determination of the polarization state of x rays by polarizers based on anomalous transmission (Borrmann effect). Using a polarizer-analyzer setup, we have measured a polarization purity of less than 1.5 × 10^−5, three orders of magnitude better than obtained in earlier work. Using the analyzer crystal in multiple-beam case with slightly detuned azimuth, we show how the first three Stokes parameters can be determined with a single angular scan. Thus, polarization analyzers based on anomalous transmission make it possible to detect changes of the polarization in a range from degrees down to arcseconds.

N. Tahir, V. Kim, B. Schlitt, W. Barth, L. Groening, I. Lomonosov, A. Piriz, Th. Stöhlker, and H. Vormann
Three-dimensional thermal simulations of thin solid carbon foils for charge stripping of high current uranium ion beams at a proposed new heavy-ion linac at GSI
Physical Review Special Topics - Accelerators and Beams 17, 041003 (2014)

Abstract: This paper presents an extensive numerical study of heating of thin solid carbon foils by 1.4  MeV/u uranium ion beams to explore the possibility of using such a target as a charge stripper at the proposed new Gesellschaft für Schwerionenforschung high energy heavy–ion linac. These simulations have been carried out using a sophisticated 3D computer code that accounts for physical phenomena that are important in this problem. A variety of beam and target parameters have been considered. The results suggest that within the considered parameter range, the target will be severely damaged by the beam. Thus, a carbon foil stripper does not seem to be a reliable option for the future Gesellschaft für Schwerionenforschung high energy heavy–ion linac, in particular, at FAIR design beam intensities.

Th. Stöhlker, Y. Litvinov, A. Bräuning-Demian, M. Lestinsky, F. Herfurth, R Maier, D. Prasuhn, R. Schuch, and M. Steck
SPARC collaboration: new strategy for storage ring physics at FAIR
Hyperfine Interactions 227, 45 (2014)

Abstract: SPARC collaboration at FAIR pursues the worldwide unique research program by utilizing storage ring and trapping facilities for highly-charged heavy ions. The main focus is laid on the exploration of the physics at strong, ultra-short electromagnetic fields including the fundamental interactions between electrons and heavy nuclei as well as on the experiments at the border between nuclear and atomic physics. Very recently SPARC worked out a realization scheme for experiments with highly-charged heavy-ions at relativistic energies in the High-Energy Storage Ring HESR and at very low-energies at the CRYRING coupled to the present ESR. Both facilities provide unprecedented physics opportunities already at the very early stage of FAIR operation. The installation of CRYRING, dedicated Low-energy Storage Ring (LSR) for FLAIR, may even enable a much earlier realisation of the physics program of FLAIR with slow anti-protons.

S. Busold, A. Almomani, V. Bagnoud, W. Barth, S. Bedacht, A. Blažević, O. Boine-Frankenheim, C. Brabetz, T. Burris-Mog, T. Cowan, O. Deppert, M. Droba, H. Eickhoff, U. Eisenbarth, K. Harres, G. Hoffmeister, I. Hofmann, O. Jäckel, R. Jäger, M. Joost, S. Kraft, F. Kroll, M. Kaluza, O. Kester, Z. Lecz, T. Merz, F. Nürnberg, H. Al-Omari, A. Orzhekhovskaya, G. Paulus, J. Polz, U. Ratzinger, M. Roth, G. Schaumann, P. Schmidt, U. Schramm, G. Schreiber, D. Schumacher, T. Stöhlker, A. Tauschwitz, W. Vinzenz, F. Wagner, S. Yaramyshev, and B. Zielbauer
Shaping laser accelerated ions for future applications – The LIGHT collaboration
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 740, 94 (2014)

Abstract: Abstract The generation of intense ion beams from high-intensity laser-generated plasmas has been the focus of research for the last decade. In the LIGHT collaboration the expertise of heavy ion accelerator scientists and laser and plasma physicists has been combined to investigate the prospect of merging these ion beams with conventional accelerator technology and exploring the possibilities of future applications. We report about the goals and first results of the LIGHT collaboration to generate, handle and transport laser driven ion beams. This effort constitutes an important step in research for next generation accelerator technologies.

V. A. Zaytsev, A. V. Maiorova, V. M. Shabaev, A. V. Volotka, S. Tashenov, G. Plunien, and Th. Stöhlker
Parity-nonconservation effect in the dielectronic recombination of polarized electrons with heavy He-like ions
Physical Review A 89, 032703 (2014)

Abstract: We investigate the parity nonconservation (PNC) effect in the dielectronic recombination of a polarized electron with a heavy He-like ion into doubly excited [(1s2p1/2)_0 nκ]_1/2 and [(1s2s)_0 nκ]_1/2 states of a Li-like ion. We determine the nuclear charge number Z for which these opposite-parity levels are almost crossing and therefore the PNC effect will be significantly enhanced. Calculations are performed for quantum numbers n≥4 and κ=±1.

B. Zielbauer, B. Ecker, P. Neumayer, K. Cassou, S. Daboussi, O. Guilbaud, S. Kazamias, D. Ros, T. Kuehl, U. Eisenbarth, S. Goette, D. Winters, V. Bagnoud, and Th. Stöhlker
Heavy-Ion Spectroscopy with X-Ray Lasers at GSI
X-Ray Lasers 2012 - Proceedings of the 13th International Conference on X-Ray Lasers, Springer Proceedings in Physics (2014)

Abstract: Different pumping schemes for soft X-ray lasers have been investigated at the PHELIX laser facility, including a double-target seeding approach at 18.9 nm. A technical feasibility study of using a Mo XRL beam of several μJ as an excitation source for heavy-ion spectroscopy in a storage ring has been carried out. XRL photon numbers and the beam transport under ultra-high vacuum conditions over almost 30 m are the major challenges.

2013

A. Surzhykov, V. A. Yerokhin, T. Jahrsetz, P. Amaro, Th. Stöhlker, and S. Fritzsche
Polarization correlations in the elastic Rayleigh scattering of photons by hydrogenlike ions
Physical Review A 88, 062515 (2013)

Abstract: The (elastic) Rayleigh scattering of hard x rays by hydrogenlike ions has been investigated within the framework of second-order perturbation theory and Dirac's relativistic equation. The focus of this study was, in particular, on two questions: (i) How is the polarization of scattered photons affected if the incident light is itself (linearly) polarized, and (ii) how do the nondipole contributions to the electron-photon interaction and the relativistic contraction of the wave functions influence such a polarization transfer? Detailed calculations were performed for Ne9+, Xe53+, and U91+ targets and for photon energies up to ten times the 1s ionization threshold of the ions. From the comparison of these fully relativistic computations with the (nonrelativistic) dipole approximation we conclude that relativistic and higher-multipole effects often lead to a significant or even complete depolarization for heavy targets and at high photon energies.

W. Chen, G. Vorobyev, D. Guo, P.-M. Hillenbrand, F. Herfurth, S. Hagmann, U. Spillmann, S. Trotsenko, A. Gumberidze, and Th. Stöhlker
Charge transfer of slow highly charged xenon ions in collisions with magnesium atoms
Physical Review A 88, 052703 (2013)

Abstract: We report an experimental study of the charge-transfer process in collisions of Xe^q+ ions (16≤q≤20) with magnesium atoms at an energy of 5.5q keV. With charge-selective and time-coincidence techniques, we separated the pure capture and capture accompanied by transfer-ionization processes. The experimental data indicate that the magnesium target is around two times more likely to lose two electrons than one in the collision. This finding is very different compared to the calculation based on the extended classic over-the-barrier model. The Xe^q+-Mg collision also behaves very differently from “traditional” collisions between highly charged ions and noble gases. We suggest a one-step dielectronic mechanism for the capture process. The data also show that autoionization dominates the relaxation process after the capture, and fluctuation of the autoionization fraction versus the projectile charge state indicates that for the relaxation processes, the projectile core structure plays a more important role than the detailed characteristics of the projectile states where the target electrons are initially captured.

P. Kienle, F. Bosch, P. Bühler, T. Faestermann, Y. Litvinov, N. Winckler, M. Sanjari, D. Shubina, D. Atanasov, H. Geissel, V. Ivanova, X. Yan, D. Boutin, C. Brandau, I. Dillmann, Ch. Dimopoulou, R. Hess, P.-M. Hillebrand, T. Izumikawa, R. Knöbel, J. Kurcewicz, N. Kuzminchuk, M. Lestinsky, S. Litvinov, X. Ma, L. Maier, M. Mazzocco, I. Mukha, C. Nociforo, F. Nolden, Ch. Scheidenberger, U. Spillmann, M. Steck, Th. Stöhlker, B. Sun, F. Suzaki, T. Suzuki, S. Torilov, M. Trassinelli, X. Tu, M. Wang, H. Weick, D. F. A. Winters, N. Winters, P. Woods, T. Yamaguchi, G. Zhang, and T. Ohtsubo
High-resolution measurement of the time-modulated orbital electron capture and of the β+ decay of hydrogen-like 142Pm^60+ ions
Physics Letters B 726, 638 (2013)

Abstract: The periodic time modulations, found recently in the two-body orbital electron capture (EC) decay of both, hydrogen-like 140Pr58+ and 142Pm60+ ions, with periods near to 7 s7 s and amplitudes of about 20%, were re-investigated for the case of 142Pm60+ by using a 245 MHz resonator cavity with a much improved sensitivity and time resolution. We observed that the exponential EC decay is modulated with a period T=7.11(11) s, in accordance with a modulation period T=7.12(11) s as obtained from simultaneous observations with a capacitive pick-up, employed also in the previous experiments. The modulation amplitudes amount to a_R=0.107(24) and a_P=0.134(27) for the 245 MHz resonator and the capacitive pick-up, respectively. These new results corroborate for both detectors exactly our previous findings of modulation periods near to 7 s, though with distinctly smaller amplitudes. Also the three-body β+ decays have been analyzed. For a supposed modulation period near to 7 s we found an amplitude a=0.027(27), compatible with a=0a=0 and in agreement with the preliminary result a=0.030(30) of our previous experiment. These observations could point at weak interaction as origin of the observed 7 s-modulation of the EC decay. Furthermore, the data suggest that interference terms occur in the two-body EC decay, although the neutrinos are not directly observed.

F. Bosch, Y. A. Litvinov, and T. Stöhlker
Nuclear physics with unstable ions at storage rings
Progress in Particle and Nuclear Physics 73, 84 (2013)

Abstract: During the last two decades, ion storage-cooler rings have been proven as powerful devices for addressing precision experiments in the realm of atomic physics, nuclear physics and nuclear astrophysics. Most important, in particular for stored unstable nuclides, is the unrivalled capability of ion cooler-rings to generate brilliant beams of highest phase–space density owing to sophisticated cooling techniques, and to store them for extended periods of time by preserving their charge state. This report focuses on nuclear physics and nuclear astrophysics experiments with in-flight produced exotic ions that were injected into storage-cooler rings. Those experiments were conducted within the last decade mainly at the only operating facilities that are capable to provide and to store exotic ions, namely the ESR in Darmstadt, Germany and the CSRe-ring in Lanzhou, China. The majority of nuclear physics experiments performed at these equipments concerns ground-state properties of nuclei far from stability, such as masses and lifetimes. The rich harvest of these measurements is presented. In particular it is shown that storage-cooler rings are ideal, if not the only, devices where two-body beta decays of exotic highly-charged ions, such as bound-state beta decay and orbital electron capture, can be studied in every detail, based on “single-ion decay spectroscopy”. Furthermore, experiments at the border between atomic and nuclear physics are discussed which render valuable information on nuclear properties by exploiting one of the most precise tools of atomic spectroscopy on stored ions, the “dielectronic recombination”. Ion storage rings with cooled exotic beams and equipped with thin internal gas targets deliver also unrivalled opportunities for addressing with highest precision key reactions in the fields of nuclear astrophysics and nuclear structure. First very promising experiments exploring the potential of ion cooler-rings in this realm have been already conducted. However, in view of the small nuclear cross sections, many of fervently desired experiments in this field will still suffer from the insufficient number of exotic ions that can be delivered and stored at the time being. The realistic hope on a breakthrough in this field is based on the ion storage rings to come, with their estimated improvements in the intensity of exotic ion beams by many orders of magnitude.

D. Banaś, M. Pajek, A. Gumberidze, A. Surzhykov, and Th. Stöhlker
Differential L-shell radiative recombination rate coefficients for bare uranium ions interacting with low-energy electrons
The European Physical Journal - Special Topics 222, 2317 (2013)

Abstract: Results of the calculations of differential L−shell radiative recombination (RR) rate coefficients for bare uranium ions colliding with free electrons using the nonrelativistic dipole approximation and fully relativistic calculations are reported. The rate coefficients were obtained for very low, in the range of meV, relative electron-ion energies. We demonstrate that even for such low relative ion-electron energies the relativistic effects significantly modify the differential RR rate coefficients for the L−subshells and, as a result, the measurements of the relative electron energy dependence of the L-RR rates could be used for studying of the relativistic effects. These effects are strongest for the L_3-subshell, which is discussed here in more details.

D. F. A. Winters, V. Bagnoud, B. Ecker, U. Eisenbarth, S. Götte, T. Kühl, P. Neumayer, C. Spielmann, Th. Stöhlker, and B. Zielbauer
A beamline for x-ray laser spectroscopy at the experimental storage ring at GSI
Physica Scripta 2013, 014089 (2013)

Abstract: By combining an x-ray laser (XRL) with a heavy-ion storage ring, precision laser spectroscopy of the fine-structure splitting in heavy Li-like ions will be possible. An initial study has been performed to determine the feasibility of a first experiment at the experimental storage ring at GSI in Darmstadt, which also has great potential for the experiments planned for FAIR. We plan to perform a unique, direct and precise measurement of a fine-structure transition in a heavy Li-like ion. Such a measurement will test state-of-the-art atomic structure calculations in strong fields. This endeavour will require that the existing infrastructure is complemented by a dedicated beamline for the XRL. In this paper, we will discuss the details of this project and outline a proof-of-principle experiment.

N. Winters, A. Warczak, J. A. Tanis, T. Gassner, A. Gumberidze, S. Hagmann, P. M. Hillenbrand, C. Kozhuharov, N. Petridis, R. Reuschl, M. Schwemlein, D. Sierpowski, U. Spillmann, R. Strzalka, S. Trotsenko, G. Weber, D. F. A. Winters, Z. Yin, and Th. Stöhlker
A study of radiative double electron capture in bare chromium ions at the ESR
Physica Scripta 2013, 014048 (2013)

Abstract: Radiative double electron capture is a fundamental atomic process which should be observed in collisions of bare ions with atoms, albeit with a much smaller cross-section than single radiative electron capture. A new experiment—to observe this rare process under single-collision conditions—has been performed at the internal gas jet target of the experimental storage ring at GSI in Darmstadt. X-ray spectra associated with single and double charge exchange have been observed in 30 MeV u^(−1) collisions of bare chromium ions (Cr^(24+)) with helium and nitrogen target gases.

F. Bosch, D. R. Atanasov, C. Brandau, I. Dillmann, C. Dimopoulou, T. Faestermann, H. Geissel, S. Hagmann, P.-M. Hillenbrand, P. Kienle, R. Knöbel, C. Kozhuharov, J. Kurcewicz, M. Lestinsky, S. Litvinov, Y. A. Litvinov, X. Ma, F. Nolden, T. Ohtsubo, Z. Patyk, R. Reuschl, M. S. Sanjari, C. Scheidenberger, D. Shubina, U. Spillmann, M. Steck, Th. Stöhlker, B. Sun, M. Trassinelli, S. Trotsenko, X. L. Tu, H. Weick, N. Winckler, M. Winkler, D. Winters, T. Yamaguchi, and X. L. Yan
Beta decay of highly charged ions
Physica Scripta 2013, 014025 (2013)

Abstract: A brief overview is presented in this paper on some experiments conducted at the Experimental Storage Ring (ESR) of GSI which addressed the β decay of stored and cooled highly charged ions. Special emphasis is placed on the two-body beta decay of bare or few-electron ions: bound-state β− decay (β_b) and its time-mirrored counterpart, orbital electron capture. The former decay mode was detected experimentally 20 years ago at the ESR. The latter could be investigated there for the first time in detail for the simplest quantum systems: hydrogen- and helium-like atoms. The main results of these experiments will be presented. Also their impact on stellar nucleosynthesis, in particular the s -process, is discussed.

U. Spillmann, K. H. Blumenhagen, E. Badura, M. Balzer, H. Bräuning, J. Hoffmann, K. Koch, N. Kurz, R. Märtin, S. Minami, W. Ott, Th. Stöhlker, G. Weber, and M. Weber
Employing digital pulse processing electronics for the readout of a Si(Li)—Compton—polarimeter for the SPARC collaboration
Physica Scripta 2013, 014103 (2013)

Abstract: The future x-ray spectroscopy and polarimetry experiment program of the SPARC collaboration at GSI and FAIR relies strongly on the availability of two-dimensional position-sensitive, energy- and time-dispersive thick semiconductor detector systems, including the appropriate signal processing electronics. To meet these demands, the development of a compact and scalable data acquisition system that has higher rate acceptance compared to commercial VME electronics by employing digital pulse processing electronics was started.

S. Hagmann, Th. Stöhlker, Y. Litvinov, C. Kozhuharov, P.-M. Hillenbrand, U. Spillmann, V. Shabaev, K. Stiebing, M. Lestinsky, A. Surzhykov, A. Voitkiv, B. Franzke, D. Fischer, D. Schneider, D. Jakubassa, A. Artiomov, E. DeFilippo, X. Ma, R. Dörner, and H. Rothard
Few-body quantum dynamics of high-Z ions studied at the future relativistic high-energy storage ring
Physica Scripta 2013, 014086 (2013)

Abstract: At the FAIR facility for antiprotons and ion research, the high-energy storage ring will provide highly charged heavy ions with Z all the way to Z = 92 for beam energies ranging from 200 A MeV up to energies of approximately 5 A GeV. This opens up a wealth of opportunities for in-ring atomic physics experiments on few-body quantum dynamics ranging from, for example, the correlated dynamics of various e^(+) –e^(−) pair creation processes to quasi-photoionization of inner shells of the highest- Z ions.

W. Nörtershäuser, M. Lochmann, R. Jöhren, C. Geppert, Z. Andelkovic, D. Anielski, B. Botermann, M. Bussmann, A. Dax, N. Frömmgen, M. Hammen, V. Hannen, T. Kühl, Y. A. Litvinov, J. Volbrecht, Th. Stöhlker, R. C. Thompson, C. Weinheimer, W. Wen, E. Will, D. Winters, and R. M. Sánchez
First observation of the ground-state hyperfine transition in (209)^Bi^(80+)
Physica Scripta 2013, 014016 (2013)

Abstract: The long sought after ground-state hyperfine transition in lithium-like bismuth (209)^Bi^(80+) was observed for the first time using laser spectroscopy on relativistic ions in the experimental storage ring at the GSI Helmholtz Centre in Darmstadt. Combined with the transition in the corresponding hydrogen-like ion (209)^Bi^(82+) , it will allow extraction of the specific difference between the two transitions that is unaffected by the magnetic moment distribution in the nucleus and can therefore provide a better test of bound-state QED in extremely strong magnetic fields.

K. H. Blumenhagen, E. Badura, H. Bräuning, J. Hoffmann, K. Koch, N. Kurz, R. Märtin, S. Minami, W. Ott, U. Spillmann, Th. Stöhlker, G. Weber, and M. Weber
Fully digital readout of segmented solid state detectors
Physica Scripta 2013, 014102 (2013)

Abstract: In this work, the digital readout of semiconductor detectors in combination with digital filters was investigated. Both non-segmented high-purity germanium and segmented planar lithium-drifted silicon detectors were used. In each case, photons from a stationary americium ((241)^Am) gamma source were detected. The resulting preamplifier output pulses were digitized at a fixed sampling frequency and stored entirely. Digital filters were applied to the stored waveforms to extract time and energy information. The performance of different digital filters was compared. The optimum energy resolution obtained was comparable with the value resulting from an analogue readout system based on standard nuclear instrumentation module and versatile module Europe bus electronics.

P. M. Hillenbrand, S. Hagmann, Th. Stöhlker, Y. Litvinov, C. Kozhuharov, U. Spillmann, V. Shabaev, K. Stiebing, M. Lestinsky, A. Surzhykov, A. Voitkiv, B. Franzke, D. Fischer, C. Brandau, S. Schippers, A. Mueller, D. Schneider, D. Jakubassa, A. Artiomov, E. DeFilippo, X. Ma, R. Dörner, and H. Rothard
Future experiments using forward electron spectroscopy to study the quantum dynamics of high- Z ions at the ESR/CRYRING storage rings
Physica Scripta 2013, 014087 (2013)

Abstract: At the FAIR facility for antiproton and ion research, the new ESR + CRYRING combination of storage rings CRYRING@ESR opens up a wealth of opportunities for in-ring atomic physics experiments on few-body quantum dynamics. The low-energy storage ring CRYRING will serve in its new location at FAIR/ESR for experiments with decelerated antiprotons and highly charged ions. We will discuss selected new experiments in the field of quantum dynamics of high- Z ions, for example for adiabatic superheavy quasi-molecules transiently formed with bare and H-like projectiles. Such experiments will be for the first time possible at the future CRYRING at ESR.

D. R. Atanasov, D. Balabanski, L. Batist, K. Blaum, F. Bosch, D. Boutin, C. Brandau, C. Dimopoulou, H. G. Essel, T. Faestermann, H. Geissel, S. Hagmann, R. Hess, P.-M. Hillenbrand, P. Kienle, R. Knöbel, C. Kozhuharov, J. Kurcewicz, M. Lestinsky, S. A. Litvinov, Y. A. Litvinov, X. Ma, R. Märtin, M. Mazzocco, G. Münzenberg, F. Nolden, T. Ohtsubo, Z. Patyk, M. S. Sanjari, C. Scheidenberger, D. Shubina, U. Spillmann, M. Steck, Th. Stöhlker, B. Sun, T. Suzuki, S. Torilov, M. Trassinelli, S. Trotsenko, X. Tu, I. Tupitsyn, H. Weick, N. Winckler, M. Winkler, D. F. A. Winters, N. Winters, H. Xu, T. Yamaguchi, X. Yan, Y. Yuan, and Y. Zhang
Half-life measurements of highly charged radionuclides
Physica Scripta 2013, 014026 (2013)

Abstract: In recent years several measurements of the orbital electron capture half-lives of few-electron ions have been carried out employing the storage ring ESR at GSI. Hydrogen-like and helium-like (140)^Pr and (142)^Pm as well as hydrogen-like (122)^I were studied. Half-lives of the corresponding fully ionized nuclides provide the three-body β^(+) decay constants.

D. Banaś, M. Pajek, A. Gumberidze, A. Surzhykov, and Th. Stöhlker
K-shell differential radiative recombination rates for bare uranium ions interacting with low-energy electrons
Physica Scripta 2013, 014045 (2013)

Abstract: We report on the calculations of the K-shell differential radiative recombination (RR) rate coefficients for very low, in the range of meV, relative electron–ion energies. The rate coefficients were derived for bare uranium ions colliding with free electrons both within the nonrelativistic dipole approximation and using fully relativistic calculations. We show that even for such low relative ion–electron energies, the differential rate coefficient reveals strong relativistic effects. We demonstrate that the measurements of the relative electron energy dependence of the RR rates represent a very sensitive tool for precise studies of the RR process and, in particular, for probing the fine details of the relativistic effects in the RR of ions with electrons. The results are discussed in the context of the first x-ray state-selective RR experiment performed for very low relative electron–ion energies.

R. Reuschl, T. Gassner, U. Spillmann, A. Bräuning-Demian, A. Ananyeva, H. Beyer, K.-H. Blumenhagen, W. Chen, S. Hagmann, M. Hegewald, P. Indelicato, M. Schwemlein, S. Toleikis, M. Trassinelli, S. Trotsenko, D. Winters, N. Winters, and Th. Stöhlker
Lifetime measurement of the 2 3^P_0 state in He-like uranium
Physica Scripta 2013, 014024 (2013)

Abstract: In recent years for the fundamental theory of quantum electrodynamics, considerable progress in the evaluation of higher order corrections has been achieved—not only for hydrogen—but also for helium-like systems—up to very heavy nuclei. We were aiming at a more precise determination of the lifetime of the metastable 2 3^P_0 state in He-like uranium which has a calculated value of 57.3 ps [1, 2]. From the lifetime it is possible to derive the energy of the state. In October 2011 we were able to perform a first test experiment at GSI, Darmstadt to study the feasibility of a new experimental detection technique. This advanced set-up consists of two state-of-the-art energy-, time- and position-sensitive germanium detectors [3] in combination with collimators in a Soller-slit like assembly. A beam of U^(91+) -ions at an energy of 290 MeV u^(−1) is passed through a thin nickel foil in the interaction chamber. From the decrease in intensity as a function of the target distance one may extract a decay curve from which the lifetime can be derived. The advantages of this new set-up, in comparison to former experiments [4, 5] will be discussed and the results of a preliminary data analysis will be presented.

A. Surzhykov, A. V. Maiorova, V. M. Shabaev, Th. Stöhlker, and S. Fritzsche
Parity violation in beryllium-like heavy ions
Physica Scripta 2013, 014027 (2013)

Abstract: A novel scheme is proposed for studying the parity-violating (PV) effects in beryllium-like heavy ions. It is based on the application of circularly polarized ultraviolet light for inducing a single-photon transition between the metastable 1s^(2)2s2p^(3)P_0 and the short-lived 1s^(2)2s2p^(3)P_1 states. We argue that the cross section of such a photoabsorption process is sensitive to the mixing between the allowed magnetic dipole (M1) and the PV electric dipole (E1) excitation channels. Based on relativistic calculations, we find that the PV-mixing may influence the cross section at the level of 10^(−5) % for beryllium-like uranium, U^(88+).

C. Brandau, C. Kozhuharov, A. Müller, D. Bernhardt, D. Banas, F. Bosch, F. J. Currell, C. Dimopoulou, A. Gumberidze, S. Hagmann, P.-M. Hillenbrand, M. Heil, M. Lestinsky, Y. A. Litvinov, R. Märtin, F. Nolden, R. Reuschl, S. Sanjari, S. Schippers, D. Schneider, D. Shubina, H. Simon, U. Spillmann, Z. Stachura, M. Steck, Th. Stöhlker, G. Weber, M. Wiedeking, N. Winckler, and D. F. A. Winters
Probing nuclear properties by resonant atomic collisions between electrons and ions
Physica Scripta 2013, 014050 (2013)

Abstract: The utilization of the resonant atomic electron–ion collision process of dielectronic recombination (DR) as a tool to probe nuclear properties via isotope shifts and hyperfine effects is discussed. Based on DR, this resonance reaction spectroscopy at electron coolers of heavy-ion storage rings denotes a versatile approach to access nuclear parameters such as charge radius, spin, magnetic moment or lifetimes of long-lived excited nuclear states (isomers). The high sensitivity of DR allows for experiments with artificially synthesized rare isotopes and isomers. Recent experimental progress in the preparation of such exotic species at the ESR storage ring in Darmstadt is presented. The DR technique is exemplified for the case of (234)^Pa^(88+) (Z = 91).

A. V. Maiorova, A. Surzhykov, S. Tashenov, V. M. Shabaev, and Th. Stöhlker
Production and diagnostics of spin-polarized heavy ions in the sequential two-electron radiative recombination
Physica Scripta 2013, 014046 (2013)

Abstract: The sequential radiative recombination of initially bare ions, which are collided with two spatially separated electron targets, is studied. It is demonstrated that the magnetic sublevel population of H-like ions, which are formed due to electron capture from the first target, depends on the first radiative recombination (RR) photon emission direction. Such a relative population, which can be parameterized in terms of the polarization parameters, affects then the angular and polarization properties of the second photon emitted in the collision with the second target. The coincidence γ – γ RR measurements may allow us to study, therefore, the process in which (i) H-like ions of some particular polarization are ‘selected out’ of the beam by detecting first recombination photons and (ii) this polarization is ‘measured’ in the second electron capture process. In order to describe the output of the (future) γ – γ correlation measurements, we derive the general expression for angular- and polarization-correlation function. Detailed calculations for the dependence of this function on the experimental setup and collision energy are performed for the RR of bare uranium ions.

I. A. Maltsev, G. B. Deyneka, I. I. Tupitsyn, V. M. Shabaev, Y. S. Kozhedub, G. Plunien, and Th. Stöhlker
Relativistic calculations of charge transfer probabilities in U⁹²⁺–U⁹¹⁺(1s) collisions using the basis set of cubic Hermite splines
Physica Scripta 2013, 014056 (2013)

Abstract: A new approach for solving the time-dependent two-center Dirac equation is presented. The method is based on using the finite basis set of cubic Hermite splines on a two-dimensional lattice. The Dirac equation is treated in the rotating reference frame. The collision of U⁹²⁺ (as a projectile) and U⁹¹⁺ (as a target) is considered at energy E_(lab) = 6 MeV u^(−1) . The charge transfer probabilities are calculated for different values of the impact parameter. The results obtained are compared with previous calculations (Tupitsyn et al 2010 Phys. Rev. A 82 042701), where a method based on atomic-like Dirac–Sturm orbitals was employed. This work can provide a new tool for the investigation of quantum electrodynamics effects in heavy-ion collisions near the supercritical regime.

Y. S. Kozhedub, I. I. Tupitsyn, V. M. Shabaev, S. Hagmann, G. Plunien, and Th. Stöhlker
Relativistic calculations of inner-shell atomic processes in low-energy ion–atom collisions
Physica Scripta 2013, 014053 (2013)

Abstract: Relativistic calculations of inner-shell atomic processes in low-energy Ne–F^(8+)(1s) and Xe–Xe^(53+)(1s) collisions are performed. The method of calculation exploits the active-electron approximation and is based on the coupled-channel approach with atomic-like Dirac–Sturm–Fock orbitals, localized at the ions (atoms). The screening density-functional theory is applied for description of the interaction with passive electrons. The role of relativistic effects is analyzed.

Th. Stöhlker, Y. A. Litvinov, V. Bagnoud, U. Bechstedt, C. Dimopoulou, A. Dolinskii, C. Geppert, S. Hagmann, T. Katayama, T. Kühl, R. Maier, W. Nörtershäuser, D. Prasuhn, R. Schuch, M. Steck, and H. Stockhorst
SPARC experiments at the high-energy storage ring
Physica Scripta 2013, 014085 (2013)

Abstract: The physics program of the SPARC collaboration at the Facility for Antiproton and Ion Research (FAIR) focuses on the study of collision phenomena in strong and even extreme electromagnetic fields and on the fundamental interactions between electrons and heavy nuclei up to bare uranium. Here we give a short overview on the challenging physics opportunities of the high-energy storage ring at FAIR for future experiments with heavy-ion beams at relativistic energies with particular emphasis on the basic beam properties to be expected.

R. Märtin, G. Weber, R. Barday, Y. Fritzsche, J. Enders, U. Spillmann, and Th. Stöhlker
Target-thickness effects in electron–atom bremsstrahlung
Physica Scripta 2013, 014070 (2013)

Abstract: We report on a study of target-thickness effects on the degree of the linear polarization as well as on the emission probability of bremsstrahlung arising in the collision of 100 keV electrons with thin gold targets. For this purpose an experiment at the electron source SPIN at the TU Darmstadt as well as Monte Carlo simulations have been performed. The results indicate that for high- Z targets the degree of linear polarization is significantly altered by straggling of the electrons inside the target.

A. Jochmann, A. Irman, M. Bussmann, J. P. Couperus, T. E. Cowan, A. D. Debus, M. Kuntzsch, K. W. D. Ledingham, U. Lehnert, R. Sauerbrey, H. P. Schlenvoigt, D. Seipt, Th. Stöhlker, D. B. Thorn, S. Trotsenko, A. Wagner, and U. Schramm
High Resolution Energy-Angle Correlation Measurement of Hard X Rays from Laser-Thomson Backscattering
Physical Review Letters 111, 114803 (2013)

Abstract: Thomson backscattering of intense laser pulses from relativistic electrons not only allows for the generation of bright x-ray pulses but also for the investigation of the complex particle dynamics at the interaction point. For this purpose a complete spectral characterization of a Thomson source powered by a compact linear electron accelerator is performed with unprecedented angular and energy resolution. A rigorous statistical analysis comparing experimental data to 3D simulations enables, e.g., the extraction of the angular distribution of electrons with 1.5% accuracy and, in total, provides predictive capability for the future high brightness hard x-ray source PHOENIX (photon electron collider for narrow bandwidth intense x rays) and potential gamma-ray sources.

Y. Litvinov, S. Bishop, K. Blaum, F. Bosch, C. Brandau, L. Chen, I. Dillmann, P. Egelhof, H. Geissel, R. Grisenti, S. Hagmann, M. Heil, A. Heinz, N. Kalantar-Nayestanaki, R. Knöbel, C. Kozhuharov, M. Lestinsky, X. Ma, T. Nilsson, F. Nolden, A. Ozawa, R. Raabe, M. Reed, R. Reifarth, M. Sanjari, D. Schneider, H. Simon, M. Steck, Th. Stöhlker, B. Sun, X. Tu, T. Uesaka, P. Walker, M. Wakasugi, H. Weick, N. Winckler, P. Woods, H. Xu, T. Yamaguchi, Y. Yamaguchi, and Y. Zhang
Nuclear physics experiments with ion storage rings
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 317, 603 (2013)

Abstract: In the last two decades a number of nuclear structure and astrophysics experiments were performed at heavy-ion storage rings employing unique experimental conditions offered by such machines. Furthermore, building on the experience gained at the two facilities presently in operation, several new storage ring projects were launched worldwide. This contribution is intended to provide a brief review of the fast growing field of nuclear structure and astrophysics research at storage rings.

D. Shubina, R. B. Cakirli, Yu. A. Litvinov, K. Blaum, C. Brandau, F. Bosch, J. J. Carroll, R. F. Casten, D. M. Cullen, I. J. Cullen, A. Y. Deo, B. Detwiler, C. Dimopoulou, F. Farinon, H. Geissel, E. Haettner, M. Heil, R. S. Kempley, C. Kozhuharov, R. Knöbel, J. Kurcewicz, N. Kuzminchuk, S. A. Litvinov, Z. Liu, R. Mao, C. Nociforo, F. Nolden, Z. Patyk, W. R. Plass, A. Prochazka, M. W. Reed, M. S. Sanjari, C. Scheidenberger, M. Steck, Th. Stöhlker, B. Sun, T. P. D. Swan, G. Trees, P. M. Walker, H. Weick, N. Winckler, M. Winkler, P. J. Woods, T. Yamaguchi, and C. Zhou
Schottky mass measurements of heavy neutron-rich nuclides in the element range 70 ≤ Z ≤ 79 at the GSI Experimental Storage Ring
Physical Review C 88, 024310 (2013)

Abstract: Storage-ring mass spectrometry was applied to neutron-rich 197Au projectile fragments. Masses of 181,183Lu, 185,186Hf, 187,188Ta, 191W, and 192,193Re nuclei were measured for the first time. The uncertainty of previously known masses of 189,190W and 195Os nuclei was improved. Observed irregularities on the smooth two-neutron separation energies for Hf and W isotopes are linked to the collectivity phenomena in the corresponding nuclei.

S. Salem, Th. Stöhlker, A. Bräuning-Demian, S. Hagmann, C. Kozhuharov, D. Liesen, and A. Gumberidze
Angular distribution of photons for the simultaneous excitation and ionization of He-like uranium ions in relativistic ion-atom collisions
Physical Review A 88, 012701 (2013)

Abstract: Simultaneous ionization and excitation processes are studied for initially He-like uranium ions in collisions with xenon gaseous targets at relativistic energy, 220MeV/u. The virtue of investigating the process of simultaneous excitation and ionization is that one electron ends up in the continuum, while the other electron ends up in a hydrogen-like final state. Experimentally, this process can be identified by observing the radiative decay of the excited levels in coincidence with ions that lost one electron (U91+). In particular, owing to the large fine-structure splitting of H-like U, the angular distribution of photons for the simultaneous ionization and excitation into the different total angular momentum j=1/2 and j=3/2 states of the L shell is determined directly from the obser-ved yields of Lyα1 and Lyα2 radiation at various observation angles. The experimental data show a progress for the dependence of the alignment on the collision impact parameter. It is shown that the current results confirm the theoretical predictions based on the independent-particle approximation and first-order perturbation, for which the simultaneous ionization and excitation processes occur at small impact parameter.

Y. Nakano, Y. Takano, T. Ikeda, Y. Kanai, S. Suda, T. Azuma, H. Bräuning, A. Bräuning-Demian, D. Dauvergne, Th. Stöhlker, and Y. Yamazaki
Resonant coherent excitation of the lithiumlike uranium ion - A scheme for heavy-ion spectroscopy
Physical Review A 87, 060501 (2013)

Abstract: We report our observation of the resonant fluorescence from highly charged uranium ions. Using the resonant coherent excitation (RCE) technique, the 2s-2p_(3/2) transition in 191.68 MeV/u Li-like U^(89+) ions was excited at 4.5 keV with a resonance width of 4.4 eV. The result demonstrated that the RCE can be applied to resonant fluorescence spectroscopy of high-Z ions up to uranium with high efficiency and resolution.

D. Banaś, A. Gumberidze, S. Trotsenko, A. V. Volotka, A. Surzhykov, H. F. Beyer, F. Bosch, A. Bräuning-Demian, S. Fritzsche, S. Hagmann, C. Kozhuharov, A. Kumar, X. Ma, R. Mann, P. H. Mokler, D. Sierpowski, U. Spillmann, S. Tashenov, Z. Stachura, A. Warczak, and Th. Stöhlker
Two-photon energy distribution from the decay of the 2 ¹S₀ state in He-like uranium
Physical Review A 87, 062510 (2013)

Abstract: We have performed a measurement of the spectral shape from the two-photon decay of the 1s2s 1S0 state in He-like uranium. The two-photon emission followed the ionization of initially Li-like uranium ions in collisions with a N2 gas-jet target. The measured shape of the two-photon energy distribution shows good agreement with results of the relativistic calculations that take into account the electron-electron interaction rigorously up to the first order in quantum electrodynamic perturbation expansion. From the full width at half maximum of the measured two-photon energy distribution, we confirm the theoretically predicted modification of the shape due to the relativistic effects.

B. Marx, K. S. Schulze, I. Uschmann, T. Kämpfer, R. Lötzsch, O. Wehrhan, W. Wagner, C. Detlefs, T. Roth, J. Härtwig, E. Förster, Th. Stöhlker, and G. G. Paulus
High-Precision X-Ray Polarimetry
Physical Review Letters 110, 254801 (2013)

Abstract: The polarization purity of 6.457- and 12.914-keV x rays has been improved to the level of 2.4×10-10 and 5.7×10-10. The polarizers are channel-cut silicon crystals using six 90° reflections. Their performance and possible applications are demonstrated in the measurement of the optical activity of a sucrose solution.

N. A. Tahir, A. Shutov, A. P. Zharkov, P. Spiller, A. R. Piriz, G. Rodriguez Prietoc, C. Deutsch, and Th. Stöhlker
Ion Beam Driven High Energy Density Physics Studies at FAIR at Darmstadt: The HEDgeHOB Collaboration
Contributions to Plasma Physics 53, 292 (2013)

Abstract: High Energy Density (HED) physics spans over numerous areas of basic and applied physics, for example, astrophysics, planetary physics, geophysics, inertial fusion and many others. Due to this reason, it has been a subject of very active research over the past many decades. Static as well as dynamic methods have been applied to generate samples of HED matter in the laboratory. The most commonly used tool in the static techniques is the diamond anvil cell while the dynamic methods involve shock compression of matter. During the past fifteen years, great progress has been made on the development of bunched intense particle beams that have emerged as an additional new tool for studying HED physics. In this paper we present two experiment designs that have been worked out for HED physics studies at the Facility for Antiprotons and Ion Research (FAIR) at Darmstadt. This facility has entered into construction phase and will provide one of the largest and most powerful particle accelerators in the world. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

W. Q. Wen, M. Lochmann, X. Ma, M. Bussmann, D. F. A. Winters, W. Nörtershäuser, B. Botermann, C. Geppert, N. Frömmgen, M. Hammen, V. Hannen, R. Jöhren, Th. Kühl, Yu. A. Litvinov, R. Sanchez, Th. Stöhlker, J. Vollbrecht, C. Weinheimer, C. Dimopoulou, F. Nolden, and M. Steck
Optical measurement of the longitudinal ion distribution of bunched ion beams in the ESR
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 90, 711 (2013)

Abstract: An optical technique to study the longitudinal distribution of ions in a bunched ion beam circulating in a storage ring is presented. It is based on the arrival-time analysis of photons emitted after collisional excitation of residual gas molecules. The beam-induced fluorescence was investigated in the ultraviolet regime with a channeltron and in the visible region using a photomultiplier tube. Both were applied to investigate the longitudinal shape of bunched and electron-cooled (209)^Bi^(80+) ion beams at about 400 MeV/u in the experimental storage ring (ESR) at GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt, Germany. Bunch lengths were determined with an uncertainty of about 0.5 m using the UV-sensitive channeltron and with slightly lower accuracy from the photomultiplier data due to the slower transitions in the red region of the spectrum. The Gaussian shape of the longitudinal distribution of ions inside the bunch was confirmed. With the information of the transverse beam size that can be measured simultaneously by a newly installed ionization profile monitor (IPM) at the ESR, an accurate determination of the ion density in the bunched beam will be allowed.

A. Surzhykov, Y. Litvinov, Th. Stöhlker, and S. Fritzsche
Hyperfine-induced effects on the linear polarization of Kα₁ emission from heliumlike ions
Physical Review A 87, 052507 (2013)

Abstract: The linear polarization of the characteristic photon emission from few-electron ions is studied for its sensitivity with regard to the nuclear spin and magnetic moment of the ions. Special attention is paid, in particular, to the Kα1 (1s2p3/2 1,3P1,2→1s2 1S0) decay of selected heliumlike ions following the radiative electron capture into initially hydrogenlike species. Based on the density matrix theory, a unified description is developed that includes both the many-electron and hyperfine interactions as well as the multipole-mixing effects arising from the expansion of the radiation field. It is shown that the polarization of the Kα1 line can be significantly affected by the mutipole mixing between the leading M2 and hyperfine-induced E1 components of 1s2p 3P2,Fi→1s2 1S0,Ff transitions. This E1-M2 mixing strongly depends on the nuclear properties of the considered isotopes and can be addressed experimentally at existing heavy-ion storage rings.

A. Gumberidze, D. Thorn, C. Fontes, B. Najjari, H. Zhang, A. Surzhykov, A. Voitkiv, S. Fritzsche, D. Banaś, H. Beyer, W. Chen, R. DuBois, S. Geyer, R. Grisenti, S. Hagmann, M. Hegewald, S. Hess, C. Kozhuharov, R. Märtin, I. Orban, N. Petridis, R. Reuschl, A. Simon, U. Spillmann, M. Trassinelli, S. Trotsenko, G. Weber, D. Winters, N. Winters, D. Yu, and Th. Stöhlker
Electron- and Proton-Impact Excitation of Hydrogenlike Uranium in Relativistic Collisions
Physical Review Letters 110, 213201 (2013)

Abstract: The K shell excitation of H-like uranium (U91+) in relativistic collisions with different gaseous targets has been studied at the experimental storage ring at GSI Darmstadt. By performing measurements with different targets as well as with different collision energies, we were able to observe for the first time the effect of electron-impact excitation (EIE) process in the heaviest hydrogenlike ion. The large fine-structure splitting in H-like uranium allowed us to unambiguously resolve excitation into different L shell levels. State-of-the-art calculations performed within the relativistic framework which include excitation mechanisms due to both protons (nucleus) and electrons are in good agreement with the experimental findings. Moreover, our experimental data clearly demonstrate the importance of including the generalized Breit interaction in the treatment of the EIE process.

J. Gunst, A. Surzhykov, A. Artemyev, S. Fritzsche, S. Tashenov, A. Maiorova, V. M. Shabaev, and Th. Stöhlker
Parity-nonconservation effects on the radiative recombination of heavy hydrogenlike ions
Physical Review A 87, 032714 (2013)

Abstract: Based on the theoretical analysis of the radiative recombination of heavy hydrogen-like ions with unpolarized electrons, a scheme is proposed for observing atomic parity nonconservation (PNC). The scheme employs the sensitivity of the polarization properties of recombination photons on the PNC-induced mixing of opposite-parity ionic levels. For the electron capture into the 1s2p(3)^P_(0) state of helium-like ions, in particular, the PNC leads to a rotation of the photon linear polarization on the angle, directly proportional to the 1s2p (3)^P_(0)–1s2s (1)^S_(0) mixing parameter. Owing to the recent advances in the development of x-ray polarimeters, the observation of such a rotation angle and, hence, the corresponding parity mixing is likely to become feasible in the future.

2012

P. Neumayer, B. Aurand, R. Fraga, B. Ecker, R. E. Grisenti, A. Gumberidze, D. C. Hochhaus, A. Kalinin, M. C. Kaluza, T. Kühl, J. Polz, R. Reuschl, Th. Stöhlker, D. Winters, N. Winters, and Z. Yin
Evidence for ultra-fast heating in intense-laser irradiated reduced-mass targets
Physics of Plasmas 19, 122708 (2012)

Abstract: We report on an experiment irradiating individual argon droplets of 20 μm diameter with laser pulses of several Joule energy at intensities of 10^{19} W/cm^{2}. K-shell emission spectroscopy was employed to determine the hot electron energy fraction and the time-integrated charge-state distribution. Spectral fitting indicates that bulk temperatures up to 160 eV are reached. Modelling of the hot-electron relaxation and generation of K-shell emission with collisional hot-electron stopping only is incompatible with the experimental results, and the data suggest an additional ultra-fast (sub-ps) heating contribution. For example, including resistive heating in the modelling yields a much better agreement with the observed final bulk temperature and qualitatively reproduces the observed charge state distribution.

A. V. Maiorova, A. Surzhykov, S. Tashenov, V. M. Shabaev, and Th. Stöhlker
Production and diagnostics of spin-polarized heavy ions in sequential two-electron radiative recombination
Physical Review A 86, 032701 (2012)

Abstract: In the present work we investigate the sequential radiative recombination (RR) of initially bare ions colliding with two spatially separated electron targets. It is shown that magnetic sublevel population of the hydrogenlike ions, produced by the electron capture from the first target, depends on the emission direction of the (first) RR photon. This population, which can be expressed in terms of the polarization parameters, affects then the angular and polarization properties of the radiation emitted in the collision with the second target. The coincidence measurements of two subsequent RR photons may allow one to understand, therefore, the production and diagnostics of the ion spin polarization. In order to describe this polarization production and diagnostic scheme we derive the general expression for the γ-γ correlation function. Detailed calculations for the dependence of this function on the geometry of photon emission and collision energy are performed for the radiative recombination of bare uranium ions.

S. Fritzsche, A. Surzhykov, A. Gumberidze, and Th. Stöhlker
Electron emission from highly charged ions - signatures of magnetic interactions and retardation in strong fields
New Journal of Physics 14, 083018 (2012)

Abstract: The electron emission of highly charged ions has been reanalyzed with the goal of separating the magnetic and retardation contributions to the electron-electron (e-e) interaction from the static Coulomb repulsion in strong fields. A remarkable change in the electron angular distribution due to the relativistic terms in the e-e interaction is found, especially for the autoionization of beryllium-like projectiles, following a 1s → 2p_(3/2) Coulomb excitation in collision with some target nuclei. For low-energetic, high-Z projectiles with Tp ≤ 10 MeV u^(−1), a diminished (electron) emission in the forward direction as well as oscillations in the electron angular distribution due to the magnetic and retarded interactions are predicted for the autoionization of the 1s 2s^(2) 2p_(3/2) 3^P_2 resonance into the 1s^2 2s^2 S_(1/2) ground and the 1s^2 2p^2P_(1/2) excited levels of the finally lithium-like ions, and in contrast to a pure Coulomb repulsion between the bound and emitted electrons. The proposed excitation-autoionization process can be observed at existing storage rings and will provide a novel insight into the dynamics of electrons in strong fields.

M. Vogel, W. Quint, G.G. Paulus, and Th. Stöhlker
A Penning trap for advanced studies with particles in extreme laser fields
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 285, 65 (2012)

Abstract: We present a Penning trap as a tool for advanced studies of particles in extreme laser fields. Particularly, trap-specific manipulation techniques allow control over the confined particles’ localization and spatial density by use of trap electrodes as ‘electrostatic tweezers’ and by application of a ‘rotating wall’, respectively. It is thereby possible to select and prepare well-defined ion ensembles and to optimize the laser–particle interaction. Non-destructive detection of reaction educts and products with up to single-ion sensitivity supports advanced studies by maintaining the products for further studies at extended confinement times of minutes and above. The trap features endcaps with conical openings for applications with strongly focused lasers. We show that such a modification of a cylindrical trap is possible while harmonicity and tunability are maintained.

G. R. Plateau, C. G. R. Geddes, D. B. Thorn, M. Chen, C. Benedetti, E. Esarey, A. J. Gonsalves, N. H. Matlis, K. Nakamura, C. B. Schroeder, S. Shiraishi, T. Sokollik, J. van Tilborg, C. Toth, S. Trotsenko, T. S. Kim, M. Battaglia, Th. Stöhlker, and W. P. Leemans
Low-Emittance Electron Bunches from a Laser-Plasma Accelerator Measured using Single-Shot X-Ray Spectroscopy
Physical Review Letters 109, 064802 (2012)

Abstract: X-ray spectroscopy is used to obtain single-shot information on electron beam emittance in a low-energy-spread 0.5 GeV-class laser-plasma accelerator. Measurements of betatron radiation from 2 to 20 keV used a CCD and single-photon counting techniques. By matching x-ray spectra to betatron radiation models, the electron bunch radius inside the plasma is estimated to be ~0.1  μm. Combining this with simultaneous electron spectra, normalized transverse emittance is estimated to be as low as 0.1 mm mrad, consistent with three-dimensional particle-in-cell simulations. Correlations of the bunch radius with electron beam parameters are presented.

R. Märtin, G. Weber, R. Barday, Y. Fritzsche, U. Spillmann, W. Chen, R. D. DuBois, J. Enders, M. Hegewald, S. Hess, A. Surzhykov, D. B. Thorn, S. Trotsenko, M. Wagner, D. F. A. Winters, V. A. Yerokhin, and Th. Stöhlker
Polarization Transfer of Bremsstrahlung Arising from Spin-Polarized Electrons
Physical Review Letters 108, 26480 (2012)

Abstract: We report on a study of the polarization transfer between transversely polarized incident electrons and the emitted x rays for electron-atom bremsstrahlung. By means of Compton polarimetry we performed for the first time an energy-differential measurement of the complete properties of bremsstrahlung emission related to linear polarization, i.e., the degree of linear polarization as well as the orientation of the polarization axis. For the high-energy end of the bremsstrahlung continuum the experimental results for both observables show a high sensitivity on the initial electron spin polarization and prove that the polarization orientation is virtually independent of the photon energy.

G. Weber, H. Bräuning, S. Fritzsche, A. Gumberidze, R. Märtin, R. Reuschl, M. Schwemlein, U. Spillmann, A. Surzhykov, D. F. A. Winters, and Th. Stöhlker
Compton polarimeters for the study of hard X-rays arising from energetic collisions of electrons and ions with matter
AIP Conference Proceedings 1438, 73 (2012)

Abstract: Novel position-sensitive x-ray detectors are presented that, when applied as Compton polarimeters, enable precise and efficient linear polarization studies of hard x-rays up to several 100 keV. We give an analytical formula which yields a rough estimate of the polarimeter efficiencies of such detector systems. Moreover, we briefly summarize a recent linear polarization measurement of the Lyman-α1 radiation in a H-like high-Z system, namely U91+.

M. Grieser, Yu. A. Litvinov, R. Raabe, K. Blaum, Y. Blumenfeld, P. A. Butler, F. Wenander, P. J. Woods, M. Aliotta, A. Andreyev, A. Artemyev, D. Atanasov, T. Aumann, D. Balabanski, A. Barzakh, L. Batist, A.-P. Bernardes, D. Bernhardt, J. Billowes, S. Bishop, M. Borge, I. Borzov, F. Bosch, A. J. Boston, C. Brandau, W. Catford, R. Catherall, J. Cederkall, D. Cullen, T. Davinson, I. Dillmann, C. Dimopoulou, G. Dracoulis, Ch. E. Düllmann, P. Egelhof, A. Estrade, D. Fischer, K. Flanagan, L. Fraile, M. A. Fraser, S. J. Freeman, H. Geissel, J. Gerl, P. Greenlees, R. E. Grisenti, D. Habs, R. von Hahn, S. Hagmann, M. Hausmann, J. J. He, M. Heil, M. Huyse, D. Jenkins, A. Jokinen, B. Jonson, D. T. Joss, Y. Kadi, N. Kalantar-Nayestanaki, B. P. Kay, O. Kiselev, H.-J. Kluge, M. Kowalska, C. Kozhuharov, S. Kreim, T. Kröll, J. Kurcewicz, M. Labiche, R. C. Lemmon, M. Lestinsky, G. Lotay, X. W. Ma, M. Marta, J. Meng, D. Mücher, I. Mukha, A. Müller, A. Murphy, G. Neyens, T. Nilsson, C. Nociforo, W. Nörtershäuser, R. D. Page, M. Pasini, N. Petridis, N. Pietralla, M. Pfützner, Z. Podolyak, P. Regan, M. W. Reed, R. Reifarth, P. Reiter, R. Repnow, K. Riisager, B. Rubio, M. S. Sanjari, D. W. Savin, C. Scheidenberger, S. Schippers, D. Schneider, R. Schuch, D. Schwalm, L. Schweikhard, D. Shubina, E. Siesling, H. Simon, J. Simpson, J. Smith, K. Sonnabend, M. Steck, T. Stora, Th. Stöhlker, B. Sun, A. Surzhykov, F. Suzaki, O. Tarasov, S. Trotsenko, X. L. Tu, P. Van Duppen, C. Volpe, D. Voulot, P. M. Walker, E. Wildner, N. Winckler, D. F. A. Winters, A. Wolf, H. S. Xu, A. Yakushev, T. Yamaguchi, Y. J. Yuan, Y. H. Zhang, and K. Zuber
Storage ring at HIE-ISOLDE Technical design report
European Journal of Physics - Special Topics 207, 1 (2012)

Abstract: We propose to install a storage ring at an ISOL-type radioactive beam facility for the first time. Specifically, we intend to setup the heavy-ion, low-energy ring TSR at the HIE-ISOLDE facility in CERN, Geneva. Such a facility will provide a capability for experiments with stored secondary beams that is unique in the world. The envisaged physics programme is rich and varied, spanning from investigations of nuclear ground-state properties and reaction studies of astrophysical relevance, to investigations with highly-charged ions and pure isomeric beams. The TSR might also be employed for removal of isobaric contaminants from stored ion beams and for systematic studies within the neutrino beam programme. In addition to experiments performed using beams recirculating within the ring, cooled beams can also be extracted and exploited by external spectrometers for high-precision measurements. The existing TSR, which is presently in operation at the Max-Planck Institute for Nuclear Physics in Heidelberg, is well-suited and can be employed for this purpose. The physics cases as well as technical details of the existing ring facility and of the beam and infrastructure requirements at HIE-ISOLDE are discussed in the present technical design report.

G. Weber, R. Märtin, A. Surzhykov, M. Yasuda, V. A. Yerokhin, and Th. Stöhlker
PEBSI – A Monte Carlo simulator for bremsstrahlung arising from electrons colliding with thin solid-state targets
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 279, 155 (2012)

Abstract: We present a Monte Carlo code dedicated to the simulation of bremsstrahlung arising in collisions of polarized electrons with thin target foils. The program consists of an electron transport algorithm taking into account elastic electron-nucleus scattering and inelastic collisions with target electrons as well as a treatment of polarized-electron bremsstrahlung emission. Good agreement is found between the predictions of the electron transport code and data stemming from other simulation programs and experiments. In addition, we present first results from the bremsstrahlung simulation which indicate a significant decrease in the degree of linear polarization of bremsstrahlung even for the thinnest gold targets considered.

I. I. Tupitsyn, Y. S. Kozhedub, V. M. Shabaev, A. I. Bondarev, G. B. Deyneka, I. A. Maltsev, S. Hagmann, G. Plunien, and Th. Stöhlker
Relativistic calculations of the K-K charge transfer and K-vacancy production probabilities in low-energy ion-atom collisions
Physical Review A 85, 032712 (2012)

Abstract: The previously developed technique for evaluation of charge transfer and electron-excitation processes in low-energy heavy-ion collisions [ Tupitsyn et al. Phys. Rev. A 82 042701 (2010)] is extended to collisions of ions with neutral atoms. The method employs the active-electron approximation, in which only the active-electron participates in the charge transfer and excitation processes while the passive electrons provide the screening density-functional theory (DFT) potential. The time-dependent Dirac wave function of the active electron is represented as a linear combination of atomic-like Dirac-Fock-Sturm orbitals, localized at the ions (atoms). The screening DFT potential is calculated using the overlapping densities of each ion (atom), derived from the atomic orbitals of the passive electrons. The atomic orbitals are generated by solving numerically the one-center Dirac-Fock and Dirac-Fock-Sturm equations by means of a finite-difference approach with the potential taken as the sum of the exact reference ion (atom) Dirac-Fock potential and of the Coulomb potential from the other ion within the monopole approximation. The method developed is used to calculate the K-K charge transfer and K-vacancy production probabilties for the Ne(1s^(2) 2s^(2) 2p^(6))-F^(8+)(1s) collisions at the F^(8+)(1s) projectile energies 130 and 230 keV/u. The obtained results are compared with experimental data and other theoretical calculations. The K-K charge transfer and K-vacancy production probabilities are also calculated for the Xe-Xe^(53+)(1s) collision.

D. R. Atanasov, N. Winckler, D. Balabanski, L. Batist, F. Bosch, D. Boutin, C. Brandau, C. Dimopoulou, H. G. Essel, T. Fästermann, H. Geissel, I. Hachiuma, S. Hess, T. Izumikawa, P. Kienle, R. Knöbel, C. Kozhuharov, J. Kurcewicz, N. Kuzminchuk, S. A. Litvinov, Yu. A. Litvinov, R. S. Mao, R. Märtin, M. Mazzocco, G. Münzenberg, K. Namihira, F. Nolden, T. Ohtsubo, Z. Patyk, R. Reuschl, M. S. Sanjari, C. Scheidenberger, D. Shubina, U. Spillmann, M. Steck, Th. Stöhlker, B. Sun, T. Suzuki, M. Trassinelli, I. I. Tupitsyn, H. Weick, M. Winkler, D. F. A. Winters, and T. Yamaguchi
Half-life measurements of stored fully ionized and hydrogen-like I-122 ions
European Physical Journal A 48, 22 (2012)

Abstract: The half-lives of fully ionized and hydrogen-like (H-like) I-122 ions have been measured in a heavy-ion storage ring. The β^(+)-decay constants for both charge states and the electron capture (EC) decay constant of H-like ions have been determined. The EC-decay constant in H-like I-122 ions λ^(H-like)_(EC) = 7.35(33) · 10^(−4) s^(−1) is, within the uncertainty, the same as the one in neutral atoms. This result is in agreement with the estimates of recent theoretical considerations on the EC-decay of few-electron ions that explicitly take into account the conservation of the total angular momentum of the nucleus plus lepton(s) system and its projections. No firm confirmation could be concluded from our results on the predicted effect that allowed Gamow-Teller transitions become forbidden if the initial and final total angular momenta are not equal.

V. P. Shevelko, M. S. Litsarev, Th. Stöhlker, H. Tawara, I. Y. Tolstikhina, and G. Weber
Electron Loss and Capture Processes in Collisions of Heavy Many-Electron Ions with Neutral Atoms
in: V. Shevelko and H. Tawara (ed.): Atomic Processes in Basic and Applied Physics (Springer Berlin Heidelberg) (2012)

Abstract: The present status and properties of charge-changing processes—electron capture and electron loss—are considered for heavy many-electron ions colliding with neutral atoms over a wide energy range E = 10 keV/u–100 GeV/u. The role of single- and multiple-electron charge-changing processes is discussed, and a brief description of available computer codes for calculation of the corresponding cross sections is presented. Experimental data for electron-loss and capture cross sections for germanium, xenon, lead, and uranium ions colliding with H, N, Ne, Ar, and Xe targets are given in comparison with numerical calculations applying different theoretical models as well as semiempirical formulae.

2011

A. V. Maiorova, V. M. Shabaev, A. V. Volotka, V. A. Zaytsev, G. Plunien, and Th. Stöhlker
Parity nonconservation effect in resonance recombination of polarized electrons with heavy hydrogen-like ions
Journal of Physics B 44, 225003 (2011)

Abstract: Parity nonconservation (PNC) effect in recombination of a polarized electron with a heavy H-like ion in the case of resonance with a doubly excited state of the corresponding He-like ion is studied. It is assumed that photons of the energy corresponding to the one-photon decay of the doubly excited state into the 2 1S0 or 2 3P0 state are detected at a given angle with respect to the incident electron momentum. Calculations are performed for helium-like thorium (Z = 90) and gadolinium (Z = 64), where the 2 1^S_0 and 2 3^P_0 levels are near to cross and, therefore, the PNC effect is strongly enhanced.

N. Petridis, A. Kalinin, U. Popp, V. Gostishchev, Y. Litvinov, C. Dimopoulou, F. Nolden, M. Steck, C. Kozhuharov, D. B. Thorn, A. Gumberidze, S. Trotsenko, S. Hagmann, U. Spillmann, D. F. A. Winters, R. Dörner, Th. Stöhlker, and R. E. Grisenti
Energy loss and cooling of relativistic highly charged uranium ions interacting with an internal hydrogen droplet target beam
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 656, 1 (2011)

Abstract: We present measurements of the energy loss of relativistic highly charged uranium ions interacting with a target beam of near-liquid density hydrogen droplets at the experimental storage ring (ESR) at GSI. Our results reveal that a liquid droplet target beam virtually behaves like a homogeneous gas jet target with respect to both energy loss and ion beam cooling. We also provide first results on ion beam cooling efficiency at high hydrogen area target densities, which are consistent with numerical estimations based on a simple model of the cooling force.

A. Gumberidze, S. Fritzsche, S. Hagmann, C. Kozhuharov, X. Ma, M. Steck, A. Surzhykov, A. Warczak, and Th. Stöhlker
Magnetic-sublevel population and alignment for the excitation of H- and He-like uranium in relativistic collisions
Physical Review A 84, 042710 (2011)

Abstract: We have measured the alignment of the L-shell magnetic substates following the K-shell excitation of hydrogen- and helium-like uranium in relativistic collisions with a low-Z gaseous target. Within this experiment, the population distribution for the L-shell magnetic sublevels has been obtained via an angular differential study of the decay photons associated with the subsequent deexcitation process. The results show a very distinctive behavior for the H- and He-like heavy systems. In particular, for K → L excitation of He-like uranium, a considerable alignment of the L-shell levels was observed. A comparison of our experimental findings with recent rigorous relativistic predictions provides a good qualitative and a reasonable quantitative agreement, emphasizing the importance of the magnetic-interaction and many-body effects in the strong-field domain of high-Z ions.

A. Gumberidze, Th. Stöhlker, D. Banaś, H. F. Beyer, C. Brandau, H. Bräuning, S. Geyer, S. Hagmann, S. Hess, P. Indelicato, P. Jagodziński, C. Kozhuharov, A. Kumar, D. Liesen, R. Märtin, R. Reuschl, S. Salem, A. Simon, U. Spillmann, M. Trassinelli, S. Trotsenko, G. Weber, and D. F. A. Winters
Precision studies of fundamental atomic structure with heaviest few-electron ions
Hyperfine Interactions 199, 59 (2011)

Abstract: One- and few-electron ions traditionally serve as an important testing ground for fundamental atomic structure theories and for the effects of QED, relativity and electron correlation. In the domain of high nuclear charges, new opportunities open up for precise testing and consolidating of the present understanding of the atomic structure in the regime of extreme electromagnetic fields. In this review, the current progress in experimental investigations of the heaviest H- and He-like systems at GSI Darmstadt is presented together with the planned future developments.

R. Märtin, R. Barday, D. Jakubassa-Amundsen, J. Enders, Y. Poltoratska, U. Spillmann, A. Surzhykov, G. Weber, V. A. Yerokhin, and Th. Stöhlker
Polarization Of The High-Energy End Of The Electron-Nucleus Bremsstrahlung In Electron-Atom Collisions
AIP Conference Proceedings 1336, 94 (2011)

Abstract: The linear polarization of bremsstrahlung radiation emitted in collisions of spin‐polarized and unpolarized electrons with carbon and gold targets has been measured for an incident kinetic energy of 100 keV. We present preliminary results for the degree of linear polarization for incident unpolarized electrons as a function of bremsstrahlung photon energy.

Th. Stöhlker, H. F. Beyer, A. Bräuning-Demian, C. Brandau, A. Gumberidze, R. E. Grisenti, S. Hagmann, F. Herfurth, Ch. Kozhuharov, Th. Kühl, D. Liesen, Yu. Litvinov, R. Märtin, W. Nörtershäuser, O. Kester, N. Petridis, W. Quint, U. Schramm, R. Schuch, U. Spillmann, S. Trotsenko, and G. Weber
SPARC: The Stored Particle Atomic Research Collaboration At FAIR
AIP Conference Proceedings 1336, 132 (2011)

Abstract: The future international accelerator Facility for Antiproton and Ion Research (FAIR) encompasses 4 scientific pillars containing at this time 14 approved technical proposals worked out by more than 2000 scientists from all over the world. They offer a wide range of new and challenging opportunities for atomic physics research in the realm of highly‐charged heavy ions and exotic nuclei. As one of the backbones of the Atomic, Plasma Physics and Applications (APPA) pillar, the Stored Particle Atomic Physics Research Collaboration (SPARC) has organized tasks and activities in various working groups for which we will present a concise survey on their current status.

M. Trassinelli, A. Kumar, H. F. Beyer, P. Indelicato, R. Märtin, R. Reuschl, Y. S. Kozhedub, C. Brandau, H. Bräuning, S. Geyer, A. Gumberidze, S. Hess, P. Jagodzinski, C. Kozhuharov, D. Liesen, U. Spillmann, S. Trotsenko, G. Weber, D. F. A. Winters, and Th. Stöhlker
Differential energy measurement between He- and Li-like uranium intra-shell transitions
Physica Scripta T144, 014003 (2011)

Abstract: We present the first clear identification and highly accurate measurement of the intra-shell transition 1s2p^3P_2 → 1s2s^3S_1 of He-like uranium performed via x-ray spectroscopy. The present experiment was conducted at the gas-jet target of the ESR storage ring in GSI (Darmstadt, Germany), where a Bragg spectrometer, with a bent germanium crystal, and a Ge(i) detector were mounted. Using the ESR deceleration capabilities, we performed a differential measurement between the 1s2p^3P_2→1s2s^3S_1 He-like U transition energy, at 4510 eV, and the 1s^2 2p^2P_(3/2) → 1s^2 2s^2S_(1/2) Li-like U transition energy, at 4460 eV. By a proper choice of the ion velocities, the x-ray energies from the He- and Li-like ions could be measured, in the laboratory frame, at the same photon energy. This allowed for a drastic reduction of experimental systematic uncertainties, principally due to the Doppler effect, and for a comparison with theory without the uncertainties arising from one-photon quantum electrodynamics predictions and nuclear size corrections.

D. F. A. Winters, T. Kühl, D. H. Schneider, P. Indelicato, R. Reuschl, R. Schuch, E. Lindroth, and Th. Stöhlker
Laser spectroscopy of the (1s2 2s2p) 3P0–3P1 level splitting in Be-like krypton
Physica Scripta T144, 014013 (2011)

Abstract: Heavy few-electron ions, such as He-, Li- and Be-like ions, are ideal atomic systems to study the effects of correlation, relativity and quantum electrodynamics. Very recently, theoretical and experimental studies of these species achieved a considerable improvement in accuracy. Be-like ions are interesting because their first excited state, i.e. (1s2 2s2p)3P0, has an almost infinite lifetime (τ0) in the absence of nuclear spin (I), as it can only decay by a two-photon E1M1 transition to the (1s2 2s2)1S0 ground state. In addition, the energy difference between the 3P0 and the next higher-lying 3P1 state is expected to remain almost completely unaffected by QED effects, and should thus be dominated by the effects of correlation and relativity. Therefore, we want to determine the (1s2 2s2p) 3P0–3P1 level splitting in Be-like krypton (84Kr32+), which has I=0, by means of laser spectroscopy at the experimental storage ring at GSI. In such an experiment, the energy splitting can be obtained with very good accuracy and can be compared with recent calculations.

G. Weber, H. Bräuning, R. Märtin, U. Spillmann, and Th. Stöhlker
Monte Carlo simulations for the characterization of position-sensitive x-ray detectors dedicated to Compton polarimetry
Physica Scripta T144, 014034 (2011)

Abstract: We present here a Monte Carlo program based on the EGS5 package for modeling the detector response of position-sensitive x-ray detectors. The program is used to estimate the polarimeter quality of two novel detector systems applied in Compton polarimetry. The validity of the underlying physical models is verified by comparing the simulation output to experimental data obtained at the experimental storage ring, ESR.

Y. Nakano, Y. Takano, T. Shindo, T. Ikeda, Y. Kanai, S. Suda, T. Azuma, H. Bräuning, A. Bräuning-Demian, Th. Stöhlker, D. Dauvergne, and Y. Yamazaki
Observation of intrashell radiative decay of Li-like uranium (2p_(3/2) - 2s_(1/2)) using silicon drift detectors
Physica Scripta T144, 014010 (2011)

Abstract: We observed the x-ray emission of 191.7 MeV u^(−1) Li-like uranium associated with resonant coherent excitation from 1s^(2) 2s to 1s^(2) 2p_(3/2) (4459 eV) in a thin silicon crystal target. De-excitation x-rays were observed by using large-area silicon drift detectors installed inside the target vacuum chamber together with their preamplifiers. We found that the x-ray yield under the resonance condition was clearly enhanced by a factor of three compared to that under the random incidence condition.

D. B. Thorn, A. Gumberidze, S. Trotsenko, D. Banas, H. Beyer, C. J. Bostock, I. Bray, W. Chen, R. DuBois, C. J. Fontes, S. Fritzsche, D. V. Fursa, R. Grisenti, S. Geyer, S. Hagmann, S. Hess, M. Hegewald, C. Kozhuharov, R. Märtin, I. Orban, N. Petridis, R. Reuschl, A. Simon, U. Spillmann, A. Surzhykov, M. Trassinelli, G. Weber, D. F. A. Winters, N. Winters, H. L. Zhang, and Th. Stöhlker
Polarization and anisotropic emission of K-shell radiation from heavy few electron ions
Canadian Journal of Physics 89, 513 (2011)

Abstract: The population of magnetic sublevels in hydrogen-like uranium ions has been investigated in relativistic ion–atom collisions by observing the subsequent X-ray emission. Using the gas target at the experimental storage ring facility we observed the angular emission of Lyman-α radiation from hydrogen-like uranium ions. The alignment parameter for three different interaction energies was measured and found to agree well with theory. In addition, the use of different gas targets allowed for the electron-impact excitation process to be observed.

F. Ferro, A. Surzhykov, and T. Stöhlker
Hyperfine transitions in He-like ions as a tool for nuclear-spin-dependent parity-nonconservation studies
Physical Review A 83, 052518 (2011)

Abstract: In this paper a scheme is proposed for measuring nuclear-spin-dependent parity-nonconservation effects in highly charged ions. The idea is to employ circularly polarized laser light for inducing the transition between the level (1s2s)^1S_0 and the hyperfine sublevels of (1s2s)^3S_1 in He-like ions with nonzero nuclear spin. We argue that an interference between the allowed magnetic dipole M1 and the parity-violating electric dipole E1 decay channel leads to an observable asymmetry of order 10^(-7) in the transition cross section, in the atomic range 28 ⩽ Z ⩽ 35. Experimental requirements for asymmetry measurements are discussed in the case of He-like (34)_Se^(77).

F. Fratini, S. Trotsenko, S. Tashenov, Th. Stöhlker, and A. Surzhykov
Photon-photon polarization correlations as a tool for studying parity nonconservation in heliumlike uranium
Physical Review A 83, 052505 (2011)

Abstract: Due to electron-nucleus weak interaction, atomic bound states with different parities turn out to be mixed. We discuss a prospective method for measuring the mixing parameter between the nearly degenerate metastable states 1s_(1/2) 2s_(1/2):J=0 and 1s_(1/2) 2p_(1/2):J=0 in heliumlike uranium. Our analysis is based on the polarization properties of the photons emitted in the two-photon decays of such states.

G. Saathoff, S. Reinhardt, R. Holzwarth, T. Hänsch, Th. Udem, D. Bing, D. Schwalm, A. Wolf, S. Karpuk, G. Huber, C. Novotny, B. Botermann, C. Geppert, W. Nörtershäuser, T. Kühl, T. Stöhlker, and G. Gwinner
Comment on: “Lorentz violation in high-energy ions” by Santosh Devasia
The European Physical Journal C 71, 1 (2011)

Abstract: In an article “Lorentz violation in high-energy ions” by S. Devasia published in this Journal [EPJ C 69, 343 (2010)], our recent Doppler shift experiments on fast ion beams are reanalyzed. Contrary to our analysis, Devasia concludes that our results provide an “indication of Lorentz violation”. We argue that this conclusion is based on a fundamental misunderstanding of our experimental scheme and reiterate that our results are in excellent agreement with Special Relativity.

B. Marx, I. Uschmann, S. Höfer, R. Lötzsch, O. Wehrhan, E. Förster, M. C. Kaluza, T. Stöhlker, H. Gies, C. Detlefs, T. Roth, J. Hartwig, and G. G. Paulus
Determination of high-purity polarization state of X-rays
Optics Communications 284, 915 (2011)

Abstract: We report on the measurement of the highest purity of polarization of X-rays to date. The measurements are performed by combining a brilliant undulator source with an X-ray polarimeter. The polarimeter is composed of a polarizer and an analyzer, each based on four reflections at channel-cut crystals with a Bragg angle very close to 45°. Experiments were performed at three different X-ray energies, using different Bragg reflections: Si(400) at 6457.0 eV, Si(444) at 11,183.8 eV, and Si(800) at 12,914.0 eV. At 6 keV a polarization purity of 1.5 × 10^{-9} is achieved. This is an improvement by more than two orders of magnitude as compared to previously reported values. The polarization purity decreases slightly for shorter X-ray wavelengths. The sensitivity of the polarimeter is discussed with respect to a proposed experiment that aims at the detection of the birefringence of vacuum induced by super-strong laser fields.

A. Kumar, S. Trotsenko, A. V. Volotka, D. Banas, H. F. Beyer, H. Bräuning, S. Fritzsche, A. Gumberidze, S. Hagmann, S. Hess, C. Kozhuharov, R. Reuschl, U. Spillmann, M. Trassinelli, G. Weber, and Th. Stöhlker
Spectral distribution of the 2S → 1S two-photon transition in atoms and few-electron ions
Pramana - Journal of Physics 76, 331 (2011)

Abstract: The two-photon decay of the 2S state to the ground state in dressed atoms and one- or two-electron ions has been studied for several decades. Relativistic calculations have shown an Z-dependence of the spectral shape of this two-photon transition in one- or two-electron ions. We have measured the spectral distribution of the 1s2s 1^S_0 → 1s2 1^S_0 two-photon transition in He-like tin at the ESR storage ring using a new approach for such experiments. In this method, relativistic collisions of initially Li-like projectiles with a gaseous target were used to populate exclusively the first excited state, 1s2s, of He-like tin, which provided a clean two-photon spectrum. The measured two-photon spectral distribution was compared with fully relativistic calculations. The obtained results show very good agreement with the calculations for He-like tin.

F. Ferro, A. Artemyev, A. Surzhykov, and T. Stöhlker
Hyperfine interaction effects on the (1s2p)P-3(1)-(1s2s)S-1(0) energy splitting in He-like ions for parity nonconservation studies
Canadian Journal of Physics 89, 73 (2011)

Abstract: Accurate theoretical knowledge of the (1s2p)^3P_1–(1s2s)^1S_0 splitting in He-like ions is demanded for future experimental studies of the nuclear spin-dependent part of the weak interaction. In this paper we perform a calculation of the hyperfine structure of (1s2p)^3P_1–(1s2s)^1S_0 within 3 ≤ Z ≤ 35, Z being the atomic number. In this Z range parity nonconservation (PNC) effects are amplified by the close energy proximity of the opposite parity levels (1s2p)^3P_1 and (1s2s)^1S_0. We find that the hyperfine structure is relevant for Z > 12, and produces splitting among the hyperfine sublevels as large as 150 meV for medium Z He-like ions (Z  ~ 35).

B. Botermann, C. Novotny, D. Bing, C. Geppert, G. Gwinner, T. W. Hänsch, G. Huber, S. Karpuk, T. Kühl, W. Nörtershäuser, S. Reinhardt, G. Saathoff, D. Schwalm, Th. Stöhlker, and A. Wolf
Preparatory measurements for a test of time dilation in the ESR
Canadian Journal of Physics 89, 85 (2011)

Abstract: We present preparatory measurements for an improved test of time dilation at the experimental storage ring (ESR) at GSI in Darmstadt. A unique combination of particle accelerator experiments and laser spectroscopy is used to perform this test with the highest precision. {7}^Li+ ions are accelerated to 34% of the speed of light at the GSI Helmholtzzentrum für Schwerionenforschung and stored in the experimental storage ring. The forward and backward Doppler shifts of an electric dipole transition of these ions are measured with laser spectroscopy techniques. From these Doppler shifts, both the ion velocity β = ν/c and the time dilation factor can be derived for testing Special Relativity. Two laser systems have been developed to drive the {3}^S_{1} → {3}^P_{2} transition in {7}^Li+. Moreover, a detector system composed of photomultipliers, both to monitor the exact laser ion beam overlap as well as to optimize fluorescence detection, has been set up and tested. We investigate optical-optical double-resonance spectroscopy on a closed Λ-type three-level system to overcome Doppler broadening. A residual, broadened fluorescence background caused by velocity-changing processes in the ion beam is identified, and a background subtraction scheme implemented. At the present stage the experimental sensitivity, although already comparable with previous measurements on slower ion beams at the TSR storage ring that led to  < 8.4 × 10^{–8}, suffer from a poor signal-to-noise ratio. Modifications of the ion source as well as the detection system are discussed that promise to improve the sensitivity by one order of magnitude.

2010

A. N. Artemyev, A. Surzhykov, P. Indelicato, G. Plunien, and Th. Stöhlker
Finite basis set approach to the two-centre Dirac problem in Cassini coordinates
Journal of Physics B 43, 235207 (2010)

Abstract: We lay out a new approach to solving the two-centre Dirac eigenvalue problem. It is based on the application of B-spline basis sets constructed in Cassini coordinates. The approach provides a very promising way for the theoretical description of various atomic processes such as charge transfer, excitation, ionization and electron-positron pair production that accompany slow collisions of heavy ions. Moreover, yielding a (quasi-) complete set of eigensolutions, the finite basis set method allows a systematic analysis of the quantum electrodynamics (QED) corrections to the energy levels of heavy quasi-molecules formed in these collisions.

G. Weber, H. Bräuning, A. Surzhykov, C. Brandau, S. Fritzsche, S. Geyer, S. Hagmann, S. Hess, C. Kozhuharov, R. Märtin, N. Petridis, R. Reuschl, U. Spillmann, S. Trotsenko, D. F. A. Winters, and Th. Stöhlker
Direct Determination of the Magnetic Quadrupole Contribution to the Lyman-alpha(1) Transition in a Hydrogenlike Ion
Physical Review Letters 105, 243002 (2010)

Abstract: We report the observation of an interference between the electric dipole (E1) and the magnetic quadrupole (M2) amplitudes for the linear polarization of the Ly-alpha(1) (2p(3/2) -> 1s(1/2)) radiation of hydrogenlike uranium. This multipole mixing arises from the coupling of the ion to different multipole components of the radiation field. Our observation indicates a significant depolarization of the Ly-alpha(1) radiation due to the E1-M2 amplitude mixing. It proves that a combined measurement of the linear polarization and of the angular distribution enables a very precise determination of the ratio of the E1 and the M2 transition amplitudes and the corresponding transition rates without any assumptions concerning the population mechanism for the 2p(3/2) state.

I. I. Tupitsyn, Y. S. Kozhedub, V. M. Shabaev, G. B. Deyneka, S. Hagmann, C. Kozhuharov, G. Plunien, and Th. Stöhlker
Relativistic calculations of the charge-transfer probabilities and cross sections for low-energy collisions of H-like ions with bare nuclei
Physical Review A 82, 042701 (2010)

Abstract: A method for solving the time-dependent two-center Dirac equation is developed. The time-dependent Dirac wave function is represented as a sum of atomiclike Dirac-Sturm orbitals, localized at the ions. The atomic orbitals are generated by solving numerically the one-center Dirac and Dirac-Sturm equations by means of a finite-difference approach with the Coulomb potential taken as the sum of the exact reference-nucleus potential and of the other nucleus within the monopole approximation. An original procedure for calculating the two-center integrals with these orbitals is proposed. As a first test of the approach developed here, calculations of the charge-transfer and ionization cross sections for the H(1s)-proton collisions at proton energies from 1 to 100 keV are performed. The obtained results are compared with related experimental and other theoretical data. To investigate the role of the relativistic effects, the charge-transfer cross sections in collisions of Ne9+(1s)-Ne10+ (at energies from 0.1 to 10 MeV/u) and U91+(1s)-U92+ (at energies from 6 to 10 MeV/u) are calculated for both relativistic and nonrelativistic cases.

D. B. Thorn, C. G. R. Geddes, N. H. Matlis, G. R. Plateau, E. H. Esarey, M. Battaglia, C. B. Schröder, S. Shiraishi, Th. Stöhlker, C. Toth, and W. P. Leemans
Spectroscopy of betatron radiation emitted from laser-produced wakefield accelerated electrons
Review of Scientific Instruments 81, 10E325 (2010)

Abstract: X-ray betatron radiation is produced by oscillations of electrons in the intense focusing field of a laser-plasma accelerator. These hard x-rays show promise for use in femtosecond-scale time-resolved radiography of ultrafast processes. However, the spectral characteristics of betatron radiation have only been inferred from filter pack measurements. In order to achieve higher resolution spectral information about the betatron emission, we used an x-ray charge-coupled device to record the spectrum of betatron radiation, with a full width at half maximum resolution of 225 eV. In addition, we have recorded simultaneous electron and x-ray spectra along with x-ray images that allow for a determination of the betatron emission source size, as well as differences in the x-ray spectra as a function of the energy spectrum of accelerated electrons.

V. P. Shevelko, Th. Stöhlker, H. Tawara, I. Yu. Tolstikhina, and G. Weber
Electron capture in intermediate-to-fast heavy ion collisions with neutral atoms
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 268, 2611 (2010)

Abstract: Electron capture processes of heavy ions, like Ge(q+), Xe(q+). Pb(q+), and U(q+), respectively, with the charge q approximate to 10-40, occurring in collisions with gaseous targets are considered in the E = 0.1-100 MeV/u projectile energy range. Calculations of single-electron capture cross sections are performed using the CDW and the CAPTURE computer codes These are compared with available experimental data and CTMC (Classical-Trajectory Monte Carlo) calculations. Although the overall agreement is found to be within a factor of two, In some cases of heavy many-electron projectiles, e g. U(28+) + N(2), Ar collisions, experimental cross sections at high energies are far smaller than theoretical predictions Moreover, for these collision systems the observed energy dependencies are quite different from each other. Possible reasons for this behavior and how the theoretical models can be improved are discussed.

F. Ferro, A. Artemyev, Th. Stöhlker, and A. Surzhykov
Isotopic tuning of the 2 3P0−2 1S0 and 2 3P1−2 1S0 transition energies in He-like ions for future parity-nonconservation experiments
Journal of Instrumentation 5, C08006 (2010)

Abstract: Crossings of opposite-parity levels in He-like ions for particular values of the atomic number Z offer excellent conditions for precise experimental tests of parity-nonconservation (PNC). Such tests may be performed by inducing, through intense lasers, transitions that are only allowed in presence of PNC. Several spectroscopy experiments have been proposed in the past, but only now, with the advent of high-intensity laser facilities (e.g. PHELIX at GSI or POLARIS at Jena), their feasibility is within reach. In this paper, we use relativistic many-body perturbation theory (RMBPT) to calculate accurately the crossing conditions of the excited levels (1s2s) 1S0 and (1s2p) 3P0 and of (1s2s) 1S0 and (1s2p) 3P1 in intermediate and heavy He-like ions. Moreover, we show the dependence of the energy splittings upon the choice of the nuclear isotope.

V. Bagnoud, B. Aurand, A. Blazevic, S. Borneis, C. Bruske, B. Ecker, U. Eisenbarth, J. Fils, A. Frank, E. Gaul, S. Götte, C. Häfner, T. Hahn, K. Harres, H.-M. Heuck, D. Hochhaus, D. H. H. Hoffmann, D. Javorkova, H.-J. Kluge, T. Kühl, S. Kunzer, M. Kreutz, T. Merz-Mantwill, P. Neumayer, E. Onkels, D. Reemts, O. Rosmej, M. Roth, Th. Stöhlker, A. Tauschwitz, B. Zielbauer, D. Zimmer, and K. Witte
Commissioning and early experiments of the PHELIX facility
Applied Physics B 100, 137 (2010)

Abstract: At the Helmholtz center GSI, PHELIX (Petawatt High Energy Laser for heavy Ion eXperiments) has been commissioned for operation in stand-alone mode and, in combination with ions accelerated up to an energy of 13 MeV/u by the heavy ion accelerator UNILAC. The combination of PHELIX with the heavy-ion beams available at GSI enables a large variety of unique experiments. Novel research opportunities are spanning from the study of ion-matter interaction, through challenging new experiments in atomic physics, nuclear physics, and astrophysics, into the field of relativistic plasma physics.

G. Weber, H. Bräuning, S. Hess, R. Märtin, U. Spillmann, and Th. Stöhlker
Performance of a position sensitive Si(Li) x-ray detector dedicated to Compton polarimetry of stored and trapped highly-charged ions
Journal of Instrumentation 5, C07010 (2010)

Abstract: We report on a novel two-dimensional position sensitive Si(Li) detector dedicated to Compton polarimetry of x-ray radiation arising from highly-charged ions. The performance of the detector system was evaluated in ion-atom collision experiments at the ESR storage ringe at GSI, Darmstadt. Based on the data obtained, the polarimeter efficiency is estimated in this work.

F. Ferro, A. Artemyev, Th. Stöhlker, and A. Surzhykov
Isotope shift of the 1s2p P-3(0)-1s2s S-1(0) level splitting in heavy He-like ions: Implications for atomic parity-nonconservation studies
Physical Review A 81, 062503 (2010)

Abstract: Heavy He-like ions are considered to be promising candidates for atomic parity-nonconservation (PNC) studies, thanks to their relatively simple atomic structure and the significant mixing between the almost degenerate (for the atomic numbers Z~64 and Z~91) opposite-parity levels 1s2s 1S0 and 1s2p 3P0. A number of experiments exploiting this level mixing have been proposed, and their implementation requires a precise knowledge of the 2 3P0–2 1S0 energy splitting for different nuclear charges and isotopes. In this paper we performed a theoretical analysis of the level splitting, employing the relativistic many-body perturbation theory and including QED corrections for all isotopes in the intervals 54⩽Z⩽71 and 86⩽Z⩽93. Possible candidates for future experimental PNC studies are discussed.

V. M. Shabaev, A. V. Volotka, C. Kozhuharov, G. Plunien, and Th. Stöhlker
Parity-nonconservation effect with the laser-induced 2 3S1 - 2 1S0 transition in heavy heliumlike ions
Physical Review A 81, 052102 (2010)

Abstract: The parity-nonconservation (PNC) effect on the laser-induced 2 3S1–2 1S0 transition in heavy heliumlike ions is considered. A simple analytical formula for the PNC correction to the cross section is derived for the case, when the opposite-parity 2 1S0 and 2 3P0 states are almost degenerate and, therefore, the PNC effect is strongly enhanced. Numerical results are presented for heliumlike gadolinium and thorium, which seem the most promising candidates for such experiments. In both Gd and Th cases the photon energy required will be anticipated with a high-energy laser built at GSI. Alternatively, it can be gained with ultraviolet lasers utilizing relativistic Doppler tuning at FAIR facilities in Darmstadt.

V. Bagnoud, J. Fils, J. Hein, M. C. Kaluza, G. G. Paulus, Th. Stöhlker, and M. Wolf
High‐Contrast Ultrafast OPA Module For The PHELIX Facility
AIP Conference Proceedings 1228, 217 (2010)

Abstract: In this paper, a new system for improvement of the pulse contrast in CPA laser systems by use of an ultrafast OPA is reviewed together with a scheme to create sub‐picosecond synchronized OPA pump pulses. The scheme is being implemented at the PHELIX facility at GSI‐Darmstadt, Germany.

Q. Zhong, T. Aumann, S. Bishop, K. Blaum, K. Boretzky, F. Bosch, H. Bräuning, C. Brandau, T. Davinson, I. Dillmann, O. Ershova, H. Geissel, G. Gyürky, M. Heil, F. Käppeler, A. Kelić, C. Kozhuharov, C. Langer, T. L. Bleis, Y. A. Litvinov, G. Lotay, J. Marganiec, N. Petridis, R. Plag, U. Popp, R. Reifarth, B. Riese, C. Rigollet, C. Scheidenberger, H. Simon, Th. Stöhlker, T. Szücs, G. Weber, H. Weick, D. F. A. Winters, N. Winters, and P. J. Woods
96^Ru(p,γ)97^Rh measurement at the GSI storage ring
Journal of Physics: Conference Series 202, 012011 (2010)

Abstract: A pioneering experiment was recently performed at the Experimental Storage Ring (ESR) at GSI. Fully stripped ions of 96 Ru were injected into the storage ring and slowed down to a few MeV per nucleon. The 97 Rh ions from the 96 Ru(p,γ) reaction at a newly developed hydrogen jet target were detected with Double Sided Silicon Strip Detectors (DSSSD) mounted inside a pocket. The experiment and the status of the analysis at a beam energy of 11 MeV per nucleon will be presented.

S. Trotsenko, A. Kumar, A. V. Volotka, D. Banas, H. F. Beyer, H. Bräuning, S. Fritzsche, A. Gumberidze, S. Hagmann, S. Hess, P. Jagodzinski, C. Kozhuharov, R. Reuschl, S. Salem, A. Simon, U. Spillmann, M. Trassinelli, L. C. Tribedi, G. Weber, D. Winters, and Th. Stöhlker
Spectral Shape of the Two-Photon Decay of the 2 1S0 State in He-Like Tin
Physical Review Letters 104, 033001 (2010)

Abstract: The spectral distribution of the 1s2s 1S0→1s2 1S0 two-photon decay of He-like tin was measured using a novel approach at the gas-jet target of the ESR storage ring. Relativistic collisions of Li-like projectiles with low-density gaseous matter have been exploited to selectively populate the desired 1s2s state. Compared to conventional techniques, this approach results in a substantial gain in statistical and systematic accuracy, which allowed us to achieve for the first time a sensitivity to relativistic effects on the two-photon decay spectral shape as well as to discriminate the measured spectrum for Sn from theoretical shapes for different elements along the He-isoelectronic sequence.

2009

S. Hess, H. Bräuning, U. Spillmann, S. Geyer, S. Hagmann, C. Kozhuharov, T. Krings, A. Kumar, R. Märtin, B. E. O'Rourke, R. Reuschl, S. Trotsenko, G. Weber, D. Winters, and Th. Stöhlker
Polarized tunable monoenergetic x-rays produced by radiative electron capture into the K-shell of Xe^(54+)
Journal of Physics: Conference Series 194, 012025 (2009)

Abstract: Many physical processes such as bremsstrahlung, synchrotron radiaton and radiative recombination produce polarized X-rays. However, only a few facilities are able to deliver tunable monoenergetic high quality beams of almost completely polarized X-rays to the user. We used radiative electron capture (REC) into the K-shell of bare xenon to produce tunable and highly linearly polarized X-rays in a storage ring environment (Fig. 1) which we confirmed by a dedicated compton polarimeter.

M. Trassinelli, A. Kumar, H. F. Beyer, P. Indelicato, R. Märtin, R. Reuschl, Y. S. Kozhedub, C. Brandau, H. Bräuning, S. Geyer, A. Gumberidze, S. Hess, P. Jagodzinski, C. Kozhuharov, D. Liesen, U. Spillmann, S. Trotsenko, G. Weber, D. F. A. Winters, and Th. Stöhlker
Observation of the 2p_(3/2) → 2s_(1/2) intra-shell transition in He-like uranium
Europhysics Letters 87, 63001 (2009)

Abstract: We present the first observation of the 1s 2p^3P_2 → 1s 2s^3S_1 transition in He-like uranium. The experiment was performed at the internal gas-jet target of the ESR storage ring at GSI exploiting a Bragg crystal spectrometer and a germanium solid-state detector. Using the 1s^(2) 2p^2P_(3/2) → 1s^(2) 2s^2S_(1/2) transition in Li-like uranium as reference and the deceleration capabilities of the ESR storage ring, we obtained the first evaluation of the energy of an intra-shell transition for a He-like heavy ion.

G. Weber, C. Omet, R. D. DuBois, O. de Lucio, Th. Stöhlker, C. Brandau, A. Gumberidze, S. Hagmann, S. Hess, C. Kozhuharov, R. Reuschl, P. Spiller, U. Spillmann, M. Steck, M. Thomason, and S. Trotsenko
Beam lifetimes and ionization cross sections of U^(28+)
Physical Review Special Topics - Accelerators and Beams 12, 084201 (2009)

Abstract: Beam lifetimes of stored U^(28+) ions with energies between 10 and 180  MeV/u were measured in the heavy ion synchrotron SIS18 and in the experimental storage ring (ESR) of the GSI accelerator facility. By using the internal gas jet target of the ESR, it was possible to obtain projectile ionization cross sections for collisions with H_2 and N_2 from the lifetime data. The experimental cross sections are compared to theoretical data predicted by the n-body classical-trajectory Monte Carlo (CTMC) method of Olson et al. and to calculations of Shevelko et al. using the LOSS-R code. In addition, both theoretical approaches are probed by using the resulting cross sections as input parameters for the STRAHLSIM code, which models the beam losses and, consequently, the lifetimes in the heavy ion synchrotron SIS18. Both the cross section measurement and the SIS18 lifetime study indicate that the LOSS-R code cross sections are in better agreement with the experimental results than the n-body CTMC calculations.

S. Hess, H. Bräuning, U. Spillmann, C. Brandau, S. Geyer, S. Hagmann, M. Hegewald, C. Kozhuharov, T. Krings, A. Kumar, R. Märtin, D. Protic, B. O'Rourke, R. Reuschl, M. Trassinelli, S. Trotsenko, G. Weber, D. F. A. Winters, and Th. Stöhlker
Polarization studies of radiative electron capture into highly-charged uranium ions
Journal of Physics: Conference Series 163, 012072 (2009)

Abstract: Recent advances in the development of 2D microstrip detectors open up new possibilities for hard x-ray spectroscopy, in particular for polarization studies. These detectors make ideal Compton polarimeters, which enable us to study precisely the polarization of hard x-rays. Here, we present recent results from measurements of Radiative Electron Capture into the K-shell of highly-charged uranium ions. The experiments were performed with a novel 2D Si(Li) Compton polarimeter at the Experimental Storage Ring at GSI. Stored and cooled beams of U^(91+) and U^(92+) ions, with kinetic energies of 43 MeV/u and 96 MeV/u respectively, were crossed with a hydrogen gasjet. The preliminary data analysis shows x-rays from the K-REC process, emitted perpendicularly to the ion beam, to be strongly linearly polarized.

H. Bräuning, S. Hess, S. Geyer, U. Spillmann, Ch. Kozhuharov, Th. Krings, A. Kumar, R. Märtin, D. Protic, R. Reuschl, M. Trassinelli, S. Trotsenko, G. Weber, D. Winters, and Th. Stöhlker
Polarization Measurements of Radiative Electron Capture Transitions in Highly Charged Ions
AIP Conference Proceedings 1099, 117 (2009)

Abstract: A dedicated Si(Li) Compton polarimeter combining energy and time resolution with a large detection area of 64 × 64 mm^2 and a two dimensional position resolution of 2 mm has been used for the first time to study the polarization of x‐rays emitted via radiative electron capture (REC) into the K and L‐shell of heavy highly charged ions. First data for the collision system 96.6 MeV/u U^(92+) → H_2 are presented. The angular distribution of the Compton scattered photons inside the detector indicates that both K‐ and L‐REC processes lead to the emission of strongly linearly polarized light.

Th. Stöhlker, D. Banaś, H. Bräuning, S. Fritzsche, S. Geyer, A. Gumberidze, S. Hagmann, S. Hess, C. Kozhuharov, A. Kumar, R. Märtin, B. E. O’Rourke, R. Reuschl, U. Spillmann, A. Surzhykov, S. Tashenov, S. Trotsenko, G. Weber, and D. F. A. Winters
Polarization and angular correlation studies of X-rays emitted in relativistic ion-atom collisions
The European Physical Journal - Special Topics 169, 5 (2009)

Abstract: Particle and photon polarization phenomena occurring in collisions of relativistic ions with matter have recently attracted particular interest. Investigations of the emitted characteristic x-ray and radiative electron capture radiation has been found to be a versatile tool for probing our present understanding of the dynamics of particles in extreme electromagnetic fields. Owing to the progress in x-ray detector technology, in addition, accurate measurements of the linear polarization for hard x-ray photons as well as the determination of the polarization plane became possible. This new diagnostic tool enables one today to derive information about the polarization of the ion beams from the photon polarization features of the radiative electron capture process.

A. Kumar, S. Trotsenko, A. V. Volotka, D. Banaś, H. F. Beyer, H. Bräuning, A. Gumberidze, S. Hagmann, S. Hess, C. Kozhuharov, R. Reuschl, U. Spillmann, M. Trassinelli, G. Weber, and Th. Stöhlker
Spectral shape of the 2E1 decay from 2s state in He-like tin
The European Physical Journal - Special Topics 169, 19 (2009)

Abstract: The two-photon 2E1 transition from the singlet 2s state of heliumlike tin was measured. An alternative experimental approach was adopted in the present investigation where the two-photon emission results from the decay of the 2s state by selective K-shell ionization of Li-like tin ions in relativistic collisions with a low-Z gaseous target, which allowed for a measurement of the undistorted two-photon spectral shape. The measured 2E1 energy distribution was compared with fully relativistic calculations, which predict a Z dependence of the distribution. The preliminary results indicate the best agreement with the relativistic many body calculations for tin, and thus confirm the sensitivity on Z. Detailed data analysis is still in progress.

2008

S. N. Andreev, I. L. Beigman, I. Y. Tolstikhina, V. P. Shevelko, G. Baur, and Th. Stöhlker
Ionization of Heavy Ions in Collisions with Neutral Atoms at Relativistic Energies
Bulletin of the Lebedev Physics Institute 35, 89 (2008)

Abstract: Ionization processes of heavy ions colliding with atoms and ions at relativistic energies are considered. Formulaes for calculating ionization cross sections in the Born approximation using momentum-transfer representation without regard to magnetic interactions are given as well as those in dipole and impulse approximations. Using the LOSS-R [25] and HERION codes, calculations of relativistic cross sections are performed for H-like multiply changed ions with the nuclear charge Z approximate to 80-90, colliding with neutral atoms and for multiply changed uranium ions colliding with protons and carbon atoms. The results of calculations are compared with available experimental data and calculations performed by other authors.

2007

R. DuBois, O. d. Lucio, M. Thomason, G. Weber, Th. Stöhlker, K. Beckert, P. Beller, F. Bosch, C. Brandau, A. Gumberidze, S. Hagmann, C. Kozhuharov, F. Nolden, R. Reuschl, J. Rzadkjewicz, P. Spiller, U. Spillmann, M. Steck, and S. Trotsenko
Beam lifetimes for low-charge-state heavy ions in the GSI storage rings
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 261, 230 (2007)

Abstract: Lifetimes for 10 – 50 MeV/u U28+ ions were measured for base vacuum conditions in the ESR storage ring at GSI-Darmstadt. The lifetimes are due to total electron loss from U28+ resulting from interactions with background gases in the ring. Lifetimes were also measured for interactions with H2 and N2 targets. These data provide information about the relative magnitudes and energy dependences of the stripping cross-sections resulting from interactions with H2 and N2, gases which represent the primary constituents in high and ultra-high vacuum environments.

S. Trotsenko, T. Stöhlker, D. Banas, C. Z. Dong, S. Fritzsche, A. Gumberidze, S. Hagmann, S. Hess, P. Indelicato, C. Kozhuharov, M. Nofal, R. Reuschl, J. Rzadkiewicz, U. Spillmann, A. Surzhykov, M. Trassinelli, and G. Weber
Investigation of the Decay Properties of the 1s(2s) 2 State in Li-Like Uranium
Journal of Physics: Conference Series 58, 141 (2007)

Abstract: We report on an experiment aiming for a study of the radiative decay modes of the 1s (2s)2 level in Li-like uranium. The experiment was performed of initially Be-like uranium colliding with N2 molecules at an energy of 90 MeV/u. By measuring the x-ray production associated with K-shell ionization of the projectile, a high selectivity for the production of the 1s (2s)2 level is observed.

G. Weber, T. Stöhlker, D. Banas, S. Fritzsche, A. Gumberidze, S. Hagmann, S. Hess, C. Kozhuharov, M. Nofal, U. Popp, R. Reuschl, U. Spillmann, A. Surzhykov, and S. Trotsenko
Radiative processes studied for bare uranium ions in collisions with H_2
Journal of Physics: Conference Series 58, 243 (2007)

Abstract: Radiative processes occurring in collision of decelerated bare uranium ions and molecular hydrogen are studied at the heavy-ion storage ring ESR. The combination of the deceleration technique and the narrow Compton profile of molecular hydrogen allowed us to resolve a multitude of REC transitions into the bound states of the projectile and to resolve unambiguously the tip region of primary bremsstahlung. For this purpose, a supersonic molecular hydrogen jet-target, precooled with liquid nitrogen and optimized for long-term stability, was applied.

R. Reuschl, D. Banas, H. F. Beyer, S. Chatterjee, A. Gumberidze, S. Hess, T. Krings, D. Liesen, D. Protić, U. Spillmann, T. Stöhlker, M. Trassinelli, S. Trotsenko, and G. Weber
Recent experimental developments for the Lamb shift investigation in heavy ions
Journal of Physics: Conference Series 58, 407 (2007)

Abstract: The latest commissioning experiment of a two arm transmission crystal x-ray spectrometer along with high-performance position-sensitive microstrip germanium detectors is presented. The goal of the experiment was to observe with high resolution the Ly-α-transitions of H-like Pb 81+ produced in collisions with Kr atoms. Due to a photon efficiency of only 10^−8 the position sensitivity as well as the energy and time resolution of segmented solid state Germanium detectors are absolutely essential for experiments using crystal x-ray spectrometers dealing with beams of heavy ions. A detector system with the desired properties has become available through a collaboration with the Forschungszentrum Jülich.

1996

P. H. Mokler, and Th. Stöhlker
The physics of highly charged heavy ions revealed by storage/cooler rings
Advances in Atomic, Molecular, and Optical Physics 37, 297 (1996)

Abstract: This chapter presents an overview of the current advances in the rapidly developing field of heavy ion accelerator technology. Now, essential aspects in this area are accessible. In the meantime, great progress already has been made in the fundamental physics in this field. This is particularly true for achievements in the atomic physics of highly charged heavy ions. There are two general domains to be considered in the atomic physics of highly charged heavy ions: the fields of collisions and of atomic structure. Both aspects have to be explored equally because they are strongly interconnected. The interaction processes has to be investigated to know, for instance, the population of excited states to help answer questions on the atomic structure; conversely, the structure has to be known to understand the interactions. In both the fields, fundamental principles can be studied uniquely. This is in particular true for the heaviest ion species with only a few- or even zero-electrons left.

C. Scheidenberger, H. Geissel, H. H. Mikkelsen, F. Nickel, S. Czajkowski, H. Folger, H. Irnich, G. Munzenberg, W. Schwab, Th. Stöhlker, and B. Voss
Energy-loss-straggling experiments with relativistic heavy ions in solids
Physical Review Letters 77, 3987 (1996)

Abstract: Energy-loss straggling measurements performed at the high-momentum-resolution magnetic spectrometer FRS with bare and highly charged O-8, Xe-54, (79)AU, and U-92 ions with specific kinetic energies (700-1000) MeV/u are reported. The results are in good agreement with rigorous calculations recently reported by Lindhard and Sorensen and reveal systematic deviations from the well-known relativistic Bohr formula, which was obtained within the framework of the first-order Born approximation.

P. Beiersdorfer, S. R. Elliott, A. Osterheld, Th. Stöhlker, J. Autrey, G. V. Brown, A. J. Smith, and K. Widmann
Search for 1s2s S-3(1)-1s2p P-3(2) decay in U90+
Physical Review A 53, 4000 (1996)

Abstract: We present high-resolution crystal-spectrometer measurements that cover the wavelength range 3.0-3.2 Angstrom containing the electric-dipole-allowed 2s(1/2)-2p(3/2) transitions in highly charged uranium ions. Strong features from 2s(1/2)-2p(3/2) transitions in lithiumlike, berylliumlike, and boronlike uranium were observed. In addition, a weak feature with intensity just above the level of background fluctuations was observed at the predicted location of the 1s2s S-3(1)-1s2p P-3(2) transition in heliumlike U90+. The feature was observed in spectra where the intensity of the 2s(1/2)-2p(3/2) transition in lithiumlike uranium, and thus the ionization balance, was optimized; it appeared absent in spectra where the average ionization balance was lower. The measured energy of the feature is 4510.05+/-0.24 eV, which agrees closely with the values predicted for the 1s2s S-3(1)-1s2p P-3(2) transition by recent theories. Detailed spectral modeling calculations indicate the possibility that a weak transition in berylliumlike U88+ is situated within 4 eV of the-location of the heliumlike S-3(1)-P-3(2) transition. This transition connects the level 1s (2)2p(1/2)2p(3/2) (3)p(1) with the 1s (2)2s(1/2)2p(1/2) P-3(0) metastable ground level in berylliumlike uranium. The accuracy with which the energy of the berylliumlike transition can be predicted is insufficient to rule out a blend with the heliumlike S-3(1)-P-3(2) transition.

J. Ruzicka, S. Saro, V. P. Zrelov, P. V. Zrelov, E. D. Lapchik, H. Geissel, H. Irnich, C. Kozhuharov, A. Magel, G. Munzenberg, F. Nickel, C. Scheidenberger, H. J. Schott, W. Schwab, Th. Stöhlker, and B. Voss
Investigation of possible applications of Cherenkov technique to measure average energy of beams of relativistic Au-197(79) nuclei in energy range 0.64-0.99 GeV/u
Nuclear Instruments & Methods in Physics Research Section a-Accelerators Spectrometers Detectors and Associated Equipment 369, 23 (1996)

Abstract: A possibility of using the Cherenkov technique to measure the average energy of collimated relativistic nuclear Au-79 beams is investigated in the article for specific beam-radiator interaction (strong deceleration, fragmentation) produces a considerable effect of the spreading of the Cherenkov one. Two Cherenkov methods - achromatic (E, alpha) and combined (''combi'')- were employed for this purpose. At an energy about 1 GeV/u the accuracy of measurement by the ''combi'' method was +/-1.4MeV/u. The accuracy of measurement by the (E,alpha) method was +/-0.9MeV/u at the energy E(Au) approximate to 640MeV/u. These results turned out to be in good agreement with initial E(Au)(0) data from the SIS accelerator with allowances made for energy losses over the beam line (the maximum discrepancy was less than or equal to 0.4%). With all the set of these data, the direction condition was for the first time checked for VChR from heavy nuclei. It is shown to be fulfilled to an accuracy above +/-5' in the angle range 27 degrees < Theta(Ch) < 38.5 degrees.

1995

R. E. Marrs, S. R. Elliott, and Th. Stöhlker
Measurement of 2-Electron Contributions to the Ground-State Energy of Helium-Like Ions
Physical Review A 52, 3577 (1995)

Abstract: We report direct measurements of two-electron contributions to the ground-state energy of high-Z heliumlike ions. The difference in radiative-recombination x-ray energy (i.e., ionization potential of the recombined ion) was measured for bare and hydrogenlike target ions trapped in an electron-beam ion trap for six elements ranging from Z = 32 to Z = 83. The achieved uncertainties (as small as 1.6 eV) test the second-order many-body contributions to the energy and approach the size of the two-electron (screened) Lamb shift: We also report a measurement of the ground-state ionization potential of heliumlike bismuth relative to heliumlike xenon. All results are in agreement with available theories.

T. Kandler, P. H. Mokler, Th. Stöhlker, H. Geissel, H. Irnich, C. Kozhuharov, A. Kriessbach, M. Kucharski, G. Munzenberg, F. Nickel, P. Rymuza, C. Scheidenberger, Z. Stachura, T. Suzuki, A. Warczak, D. Dauvergne, and R. W. Dunford
Transition Selective Investigation of the Resonant Transfer and Excitation in U90+-]C Collisions
Physics Letters A 204, 274 (1995)

Abstract: Resonant transfer and excitation in highly charged uranium ions has been investigated by applying the X-ray/particle coincidence technique. This complete detection method for the RTE process in combination with the large L-shell fine structure splitting in uranium allows, for the first time, an L-subshell differential as well as angular differential study of the excitation and radiative stabilization of different intermediate electronic states. Measured L-subshell differential cross sections for the KLL-RTE in U90+-->C collisions compare well with full relativistic calculations. For the radiative stabilization of the KL(1/2)L(3/2) intermediate states a significant anisotropy was found.

P. H. Mokler, Th. Stöhlker, R. W. Dunford,