Sonstige Publikationen

Based on systematic experimental studies and on exact relativistic calculations of the REC process in relativistic heavy-ion atom collisions, the limits of applicability of the nonrelativistic dipole approximation are discussed. In particular, direct comparison between a general REC scaling law and all available total electron-capture cross sections related to REC into bare and H-like ions is given. Similar to K-REC, a fortuitous agreement between the nonrelativistic approach and the experimental data is found. Moreover, the results of a dedicated L-REC experiment are presented, where this process was investigated for energy-resolved subshells. The measured angular distributions for REC into the j = 1/2 and j = 3/2 levels are in excellent agreement with exact relativistic calculations and manifest a complete breakdown of the nonrelativistic theory.

Resonant transfer and excitation (RTE) followed by X-ray emission in highly charged uranium ions colliding with carbon atoms has been investigated by applying the X-ray-particle-coincidence technique. This renders possible a subshell differential investigation of the population and radiative stabilization of different excited intermediate states. Preliminary results for the measured RTE excitation function for U90+ —> C collisions are presented.

A 200-keV super electron beam ion trap is being used to study the few electron ions of heavy elements. This device can produce and trap any highly charged ion at rest in the laboratory, including bare U92+ ions. Measurements of the ground state binding energies of hydrogenlike and heliumlike ions, and the 2s-2p transition energies in lithiumlike through fluorinelike ions are being used to determine the QED energies in high-Z ions. The ionization cross sections for hydrogenlike and heliumlike high-Z ions are being directly measured for the first time. There are future opportunities for several new kinds of experiments.

The photon angular distributions for radiative electron capture (REC) into the j=1/2 and j=3/2 L subshell levels were measured and calculated for U90+ —> C collisions at 89 MeV/u. The experiment provides first study of the photon angular distribution of REC into projectile p state (j=3/2) which was found to be exhibit a slight backward peaking in the laboratory frame. For radiative capture to the j=1/2 states, the measured angular distribution deviates considerably from symmetry around 90°. The results demonstrate that the usual sin^2(θlab) distribution is not valid in the high-Z regime.

The SIS accelerator facilities at GSI in combination with a magnetic spectrometer allow the direct measurement of stopping powers for relativistic heavy ions up to uranium with high precision for the first time. Here, we report on representative results obtained for projectiles up to xenon in the energy range from 700 to 1000 MeV/u. Systematic deviations from the Bethe stopping-power theory are observed, whereas the data are in good agreement with theory once Mott and Bloch corrections and the Fermi density effect are included.

Relativistic heavy-ion collisions of few-electron projectiles ranging from argon up to uranium have been investigated in solid and gaseous media. Electron-loss and electron-capture cross sections, charge-state distributions, as well as energy loss and energy deposition have been measured and are compared with theoretical predictions. Especially fully-ionized heavy projectiles represent a unique possibility to test atomic-collision theories.

Projectile X-ray emission and charge-exchange processes are studied for bare and one-electron Bi, Pb, and U ions in collisions with gaseous and solid targets at relativistic energies. The experimental data for both the total and subshell differential cross sections are discussed and compared with theoretical predictions. Moreover, the experiments were extended to atomic structure studies. The result for the groundstate Lamb-shift for hydrogenic uranium is presented, which compares very well with theoretical prediction.

The heavy ion synchrotron/storage ring facility at GSI, SIS/ESR, provides intense beams of cooled, highly-charged ions up to naked uranium (U92+). By electron capture during ion-atom collisions in the gas target of the ESR or by recombination at ion-electron encounters in the 'electron cooler" excited states are populated. The detailed structure of very heavy one-, two- and three-electron ions is studied. The different mechanisms leading to the excited states are described, as well as the new experimental tools now available for a detailed spectroscopy of these interesting systems. Special emphasis is given to X-ray transitions to the groundstates in H- and He-like systems. For the heaviest species the groundstate Lambshift can now be probed on an accuracy level of better than 10% using solid-state X-ray detectors. Applying dispersive X-ray analysing techniques, this accuracy will certainly be improved in future. However, utilizing the dielectronic recombination resonances for a spectroscopy, the structure in Li-like heavy ions can already be probed now on the sub eV level.

Charge state distributions for 3-16 MeV/u Ni ions penetrating C-stripper foils measured during UNILAC operation over the past decade are summarized. The results agree reasonably well with calculations including excitation, stabilization and charge changing processes. Special emphasis is given to the fraction of emerging few-electron ions, i.e. from naked to Li-like ions. The results are applied to a beam foil spectroscopy experiment investigating intra-L shell and Rydberg transitions in He- and Li-like Ni ions.

The Lyman a transitions of hydrogenlike uranium associated with electron capture were measured in collisions of stored bare U92+ ions with gaseous targets at the storage ring ESR. By applying x-ray-particle coincidence techniques the ground-state transition energies in the rest frame could be determined with a precision of about 600 ppm. The experimental result is in excellent agreement with the theoretical prediction for the ground-state Lamb shift in hydrogenlike uranium and is already sensitive to the 2s Lamb shift.

Bound-state beta- decay was observed for the first time by storing bare Dy-163(66)66+ ions in a heavy-ion storage ring. From the number of Ho-163(67)66+ daughter ions, measured as a function of the storage time, a half-life of 47+/-5/4 d was derived.