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Publikationen von
Christoph Hahn

Alle Publikationen des HI Jena


P. Pfäfflein, S. Allgeier, S. Bernitt, A. Fleischmann, M. Friedrich, C. Hahn, D. Hengstler, M. O. Herdrich, A. Kalinin, F. M. Kröger, P. Kuntz, M. Lestinsky, B. Loeher, E. B. Menz, T. Over, U. Spillmann, G. Weber, B. Zhu, C. Enss, and T. Stöhlker
Integration of maXs-type microcalorimeter detectors for high-resolution x-ray spectroscopy into the experimental environment at the CRYRING@ESR electron cooler
Physica Scripta 97, 114005 (2022)

Abstract: We report on the first integration of novel magnetic microcalorimeter detectors (MMCs), developed within SPARC (Stored Particles Atomic Physics Research Collaboration), into the experimental environment of storage rings at GSI(6), Darmstadt, namely at the electron cooler of CRYRING@ESR. Two of these detector systems were positioned at the 0 degrees and 180 degrees view ports of the cooler section to obtain high-resolution x-ray spectra originating from a stored beam of hydrogen-like uranium interacting with the cooler electrons. While previous test measurements with microcalorimeters at the accelerator facility of GSI were conducted in the mode of well-established stand-alone operation, for the present experiment we implemented several notable modifications to exploit the full potential of this type of detector for precision x-ray spectroscopy of stored heavy ions. Among these are a new readout system compatible with the multi branch system data acquisition platform of GSI, the synchronization of a quasi-continuous energy calibration with the operation cycle of the accelerator facility, as well as the first exploitation of the maXs detectors time resolution to apply coincidence conditions for the detection of photons and charge-changed ions.

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.


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.

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.

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.

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.

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.


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.


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.

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.


C. Hahn
Energie- und polarisationssensitiver Nachweis harter Röntgenstrahlung an Hochintensitätslasern
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2013)

Abstract: The present thesis details the extensive calibration and characterization of two CdTe-based semiconductor detectors equipped with Timepix-class readout chips, and presents first results of polarimetric measurements conducted with these sensors. The readout's Time-over-Threshold mode provides a means to measure the energy deposited in each of the 65k sensor pixels, opening the way for a compact and versatile Compton polarimeter for the high-energy X-ray regime that is commonly encountered at, e.g., laser-generated plasmas.

Since each pixel features its own dedicated set of conversion electronics, an individual calibration of every pixel is mandatory if the full potential of the energy-sensitive detection mode is to be exploited. Exposures to both gamma and X-ray fluorescence radiation were used to generate the necessary data. In addition, a range of MATLAB programs and classes was created to facilitate the lengthy analyses. The final obtainable energy resolution is on the order of 9%, with higher bias voltages providing some potential for improvement while simultaneously increasing the observed detector noise. The fraction of charge-sharing events, i.e. those that encompass multiple pixels, was found to conform with expectations, increasing with the incident photon's energy while, for a given energy, being somewhat lower at higher bias voltages.

Furthermore, a two-detector Compton polarimeter was constructed where two Timepix detectors are arranged around a passive scattering target of approximately 1 cm diameter, covering azimuthal scattering angles that differ by 90°. This setup was first tested at DESY's PETRA III accelerator. The observed stark contrast between radiation scattered parallel and perpendicular to the incident photon electric field vector confirms the setups' fitness for Compton polarimetry in the energy range of some 100 keV. By adding a Tantalum plate collimator to further restrict the scattering angle of the incident photons, the contrast between both detectors was enhanced by an additional 18%. In this configuration, the setup almost reached the contrast theoretically expected for an ideal Compton polarimeter.


C. Hahn
Charakterisierung ortsauflösender Halbleiterdetektoren für harte Röntgenstrahlung
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2011)

Abstract: The present thesis details the properties of position-sensitive detectors suited for the Compton polarimetry of high-energy X-ray radiation, investigating double-sided Ge(i) and Si(Li) strip detectors as well as a CdTe-based sensor equipped with the novel Timepix detection chip.

In the case of the strip detectors, special concern was dedicated to the so-called charge sharing effect, which denotes an incident photon's charge cloud being distributed across several segments. This obviously hampers the determination of the actual interaction position, a crucial parameter for the Compton-polarimetric analysis. A set of sophisticated software routines to recover such events was developed. Using this modified analysis, the number of multiplicity-1 events reported by the Si(Li) detector was increased by about 30%, while the Ge(i) system, having a much smaller strip width, saw an increase by up to a factor of 30 at energies above 200 keV. Consequently, the number of events usable for the Compton analysis, which has to be restricted to exposures with exactly two discernible interactions, grew in a similar fashion, such that, for the Ge(i) sensor, the computed degree of polarization deviated by as much as 9% from previously obtained values. The wide-stripped Si(Li) detector, on the other hand, proved to be much less susceptible to distortions induced through charge splitting.

In addition, Timepix acquisitions made during an electron acceleration experiment conducted at the JETI laser system were evaluated. To avoid saturation of the sensor, extensive shielding and indirect exposure, utilizing a plastic body for scattering, were necessary. While the employed Time-over-Threshold mode returns a pixelwise measure for the energy deposition, neither an absolute nor inter-pixel calibration was possible. However, a clear correlation between the total output signal and the intensity reported by an electron spectrometer could be observed, confirming the Timepix sensor's general capability of energy-resolved measurements. Furthermore, electron tracks that were visible in the obtained data were used to calculate a rough estimate of the primary photon energy by comparing their extent to the predictions of the Continuous Slowing Down Approximation. This yielded initial electron energies in the MeV range, which are in good agreement with the achieved energies reported for the JETI experiment in question.