Peer-Review Publications

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.

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.

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.

We report on an ytterbium-doped photonic crystal fiber with a core diameter of 60 microm and mode-field-area of ~2000 µm^2 of the emitted fundamental mode. Together with the short absorption length of 0.5 m this fiber possesses a record low nonlinearity which makes this fiber predestinated for the amplification of short laser pulses to very high peak powers. In a first continuous-wave experiment a power of 320 W has been extracted corresponding to 550 W per meter. To our knowledge this represents the highest power per unit length ever reported for fiber lasers. Furthermore, the robust single-transverse-mode propagation in a passive 100 microm core fiber with a similar design reveals the potential of extended large-mode-area photonic crystal fibers.

We report on an ytterbium-doped photonic-crystal-fiber-based chirped-pulse amplification system delivering 131 W average power 220 fs pulses at 1040 nm center wavelength in a diffraction-limited beam. The pulse repetition rate is 73 MHz, corresponding to a pulse energy of 1.8 µJ and a peak power as high as 8.2 MW.

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.

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.

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.

By decelerating highly-charged, very heavy ions to low energies in the storage and cooler ring, ESR, a new brilliant source for projectile x-rays has been obtained. In a first precision spectroscopy experiment up to 2*10^7 stored bare U92+ ions have been decelerated down to 49 MeV/u before detecting the characteristic projectile x-rays associated with one-electron capture at the ESR gas target. The Lyman, Balmer and Paschen series are distinctly observed for hydrogenic U91+ ions.

X-rays are emitted with the radiative recombination of free electrons in an electron cooler of a heavy-ion storage ring. Due to a small width of the X-ray lines, an observation angle close to 0 degrees and an accurate determination of the ion velocity, the ground-state Lambshift of hydrogenlike uranium (470 +/- 16) eV could be measured to an accuracy of 3.4%. A re-evaluation of a measurement of the 1S(1/2) Lambshift in hydrogenlike gold gave a new value of (202.3 +/- 7.9) eV as compared to the former value of (212 +/- 15) eV. The results are in excellent agreement with QED calculations and are more precise than any other measurements previously reported for a high-Z, hydrogenlike ion.

Measurements of the photon angular distribution of Radiative Electron Capture into the M shell have been performed with He-like uranium ions in the range 110-140 MeV/u. In addition, L REC was studied at a projectile energy of 140 MeV/u. In both cases, the experimental data show an asymmetry around 90° and agree well with a fully relativistic theory.

The process of radiative electron capture (REC) in relativistic collisions of high-Z ions with low-Z gaseous and solid targets is studied experimentally and theoretically. The observed x-ray spectra are analyzed with respect to photon angular distributions as well as to total K-REC cross sections. The experimental results for angle-differential cross sections are well reproduced by exact relativistic calculations which yield significant deviations from standard sin^2(θ) distributions. Total cross sections for K-REC are shown to follow a simple scaling rule obtained from exact relativistic calculations as well as from a nonrelativistic dipole approximation. The agreement between these different theoretical approaches must be regarded as fortuitous, but it lends support to the use of the nonrelativistic approach for practical purposes.

Projectile K-shell ionization cross sections were measured for 80-200 MeV/u H- and He-like Bi ions incident on thin C, Al and Ni targets. The results are compared with predictions of first-order perturbation theories, RSCA and PWBA, as well as with experimental results published for Xe and U ions at similar ratios eta of projectile velocity to K-shell electron velocity. All experimental data lie a factor of 1.5 above theory for 0.6 < eta < 1.5.

Experimental cross sections for non-radiative single and double electron capture from Ne target into decelerated H-like Ge ions at collision energies of (4-12) MeV u-1 are presented. The results are compared with theoretical calculations and an empirical scaling rule. Information concerning the impact parameter dependence of electron capture is extracted using classical trajectory Monte Carlo calculations.

By using segmented solid state X-ray detectors and applying X-ray/ charge-state selective particle coincidences, the ionic structures of Bi-83(82+) and Bi-83(81+) have been studied separately at 82 MeV/u under single collision conditions. The high granularity of the Ge(i) X-my detectors used allowed a partial Doppler correction for the X-ray events while maintaining a large total solid angle. An absolute precision of K-transition energies of 10(-3) is feasible; the relative accuracy is better than 30 eV. The experimental values compare very well with the theoretical transition energies in H- and He-like Bi ions.

Radiative Electron Capture (REC) in 4 to 12 MeV/u Ge^31+ —> H2 collisions has been studied using an X-ray/particle coincidence technique. This technique allowed a systematic investigation of K-shell REC as well as a separation of REC into the projectile L- and M-shells. The cross sections are discussed within a general scaling picture based on the reduced projectile velocity.

Equilibrium charge-state distributions have been measured for 617.2 MeV/u Au and 758.2 MeV/u Xe projectiles traversing C, Al, and Pb targets. In all cases bare ions dominate the charge-state distributions as expected for such relativistic velocities. For both projectiles used, the Al target produced the largest fraction of bare ions. This result is in agreement with calculations and with measurements for uranium ions given in the literature. These first atomic physics measurements at the new SIS accelerator have been performed during commissioning of the fragment separator (FRS).

Radiative Electron Capture (REC) in collisions of hydrogenic germanium ions with hydrogen is measured for projectile energies between 4 and 12 MeV/u. Extrapolating the resulting centroid energies of the K-REC radiation to zero collision velocity the K-binding energy in helium-like germanium-ions is determined. The value compares well with the theoretical prediction. REC in atomic collisions is advertised as a spectroscopic tool for structure investigations of very heavy few-electron projectiles.

In collisions between highly charged, heavy ions and light target atoms at adiabaticity parameters of 0.5 less-than-or-equal-to v2/u2 less-than-or-equal-to 0.75, resonant transfer and excitation (RTE) is an important charge-exchange process. Experiments for medium Z-ions are reviewed. Special emphasis is given to the competing radiative electron capture (REC) process. The general dependences of REC are discussed. In particular, we elucidate results obtained by the triple X-ray/X-ray/charge-changed particle coincidence technique for hydrogenic projectiles up to 36Kr35+ ions. With this method we could isolate a single RTE resonance, the 2s2p1P1 state, which stabilizes radiatively to the metastable 1s2s1S0 state. This state can only decay via two photon emission (2E1). Applying the triple coincidence technique we observed additional strong photon coincidences induced by cascades following REC into high projectile states.

The doubly excited 2s2p 1P1 level of Kr34+ populated via resonant transfer and excitation (RTE) feeds selectively the metastable 1s2s 1S0 state which can only decay via simultaneous emission of two photons to the ground state 1s2 1S0. X-ray/X-ray coincidence measurements in heavy ion-atom collisions enable the direct measurement of the spectral distribution of the two-photon decay in He-like ions. In addition, we observe strong photon cascades induced by radiative electron capture.

Evidence for resonant two-electron capture and excitation, i.e. the time-recersed double Auger process with radiative stabilitization, was found by measuring coincidences between two K-X-rays of a heavy ion associated with its double charge exchange. Hydrogen-like Ge31+ ions with energies of 4.5-11.5 MeV/u were directed into a neon gas target. Contributions from the double non-resonant transfer and excitation process are also discussed.

For non-naked highly-charged heavy ions, dielectronic recombination (DR) can be the dominant electron capture process. Depending on the kind of a DR resonance involved, cross sections may reach values up to 10^2-18 cm^2 at the peak maximum. The widths of the DR resonances are typically <1 eV. Tuning a merged electron beam with an internal temperature typically - 0.1 eV to the DR resonance, a well defined narrow part can be cut out of the original "hot" primary ion beam by this charge-exchange process. The charge-changed new beam is pretty "cool" (DE/E=10^-5). Acase study for Ge^31+ +e=Ge^30+** (2P^2^1D_2) is given. Technical applications of DR beam tailoring techniques at heavy-ion storage rings will be discussed.