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mitted along a ^110& axis of a 120-mm-thick Si crystal. The emerging charge state distribution FC(qout) for well-channeled ions is governed mainly by electron impact ionization ~EII!. The corresponding EII cross sections were obtained by fitting the experimental FC(qout) with Monte Carlo simulations. For M shell ionization, they were found to be twice larger than those given by the binary encounter dipole approximation. The measured energy loss spectra were also compared to Monte Carlo simulations. The mean values and widths of these spectra increase with qout , reflecting the increase of the stopping power S with increasing transverse energy E’ . The measured stopping for channeled ions with frozen charge state 731 and for nonchanneled ions with charge state close to 901 is in good agreement with theoretical estimates. Owing to the very high ion velocity, there is a significant contribution (25%) to the stopping from Si-L shell excitation even for the best channeled ions. The width and the asymmetrical shape ~skewness m) of the energy-loss spectra depend strongly on qout(m.0 for very well-channeled ions, m,0 for poorly channeled ions!. For well-channeled ions, energy-loss spectra were reproduced by Monte Carlo simulations with the S(E’) curve extracted from fitting the mean energy losses. @S1050-2947~99!01604-2#

The deceleration capabilities of the ESR have been used for the first time in a dedicated ground state QED experiment conducted at the gasjet target of the ring. By decelerating bare uranium ions from 358 MeV/u down to energies of as low as 49 MeV/u, X-ray spectra have been obtained which provide an abundant yield of characteristic X-ray transitions. The experiment demonstrates that, by choosing the appropriate beam energy and gasjet target, almost all excited projectile states can be selectively populated. Moreover, the experiment provides the first data for beam lifetimes of stored decelerated high-Z ions. Such data are essential for the design of future experiments dealing with decelerated ion beams far below 50 MeV/u.

Utilizing a multi-charge state mode for the operation of the heavy ion storage ring ESR, heavy ions in the charge state q can be bred from stored ions with an original charge state q + 1 via radiative recombination in the electron cooler. Using additionally the internal gas target of the ESR, also ionization and electron capture have to be taken into account. This method was tested with Pb ions, injected as bare species (Pb82+) at 277 MeV/u into the ring. The observed evolution of the intensity for the coasting beams with different charge states is described by simple rate equations applying proper rates for the charge changing processes in the cooler and in the gas jet. Using this model, a comparison of the efficiencies of the two different methods is made, i.e. the direct accumulation of the desired charge state and the breeding technique. In the latter case, projectiles ionized in the gas target are transferred back to the Pb81+ charge state via radiative recombination in the cooler and can thus efficiently be reused.

We present full scale ab initio relativistic calculations for the L-K vacancy transfer in collisions of 208 MeV Ni 23+ on a Ge solid target. Our Dirac-Fock-Slater method allows us to achieve a very accurate quantitative explanation for the experimental impact parameter-dependent vacancy probabilities in terms of dynamic creation and annihilation of Ni n = 2 shell vacancies.

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 U 90+ →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.

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 2·107 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.

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 U 90+ → C collisions are presented.

Using a particle-photon coincidence technique, we have measured the influence of the solid target preparation and thickness on the target K-vacancy production probability (PK(b)) in collisions of 3.6 MeV/u Ni23+ with Ge. The probabilities were measured as a function of the impact parameter b which is calculated from the projectile scattering angle using a screened Bohr-type potential. For target thicknesses varying by more than a factor of 100, no change in the shape of the PK(b) curve was found. Even the data obtained with implanted Ge targets (≈ 1 × 1016atoms cm−2), used for the first time in an L-K vacancy-transfer experiment, showed the typical solid target behaviour with the adiabatic peak being shifted towards smaller impact parameters.

Total cross sections for radiative double electron capture (RDEC) in collisions of fast highly charged ions with light target atoms were estimated. The applied method is based on the principle of detailed balance using the cross sections for the double photoionization process. Preliminary results of a dedicated RDEC experiment aiming at the observation of photons with twice the single REC photon energy, associated with double charge exchange, are discussed. Additionally, results of complementary measurements of double capture (without photon registration) by fully stripped U ions in collisions with C atoms at 295 MeV/u are presented. Here, a surprisingly enhanced cross section for double electron capture is found in accordance with observations reported in the literature.

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.

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 energy loss of low energy ( Si crystal is calculated using a Monte Carlo (MC) and a Molecular Dynamics (MD) computer code, respectively. The MC simulation is based on a modified binary collision model where large range interactions are treated separately within the momentum approximation. Because of the small thickness of the Si crystal the MD calculations are performed for the whole crystal considered, using the Stillinger-Weber many-body potential for silicon. For the B-Si interatomic interaction in both codes the same numerically calculated potential is used. By comparison of the MC and MD results the validity of the binary collision model is discussed.

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.

With heavy, highly-charged ions the electron-electron correlation is probed in the relativistic domain by Resonant Transfer and Excitation, RTE. For the studied case of initially He-like U90+ ions the KLL-RTE splits into three resonance groups according to the j values of the electrons involved in the doubly-excited intermediate states. For a K to L excitation into a j = ½ state with a simultaneous capture also to a j' = ½ state, relativistic effects yield a strong cross section enhancement. The doubly excited states with jj' = 1/2-3/2 show a strong anistropy for the radiative ground state transition of the j' = 3/2 electron manifesting a pronounced non-isotropic population of the magnetic substates.

The photon angular distributions for radiative electron capture (REC) into the $j=\frac{1}{2}$ and $j=\frac{3}{2}$ $L$ subshell levels were measured and calculated for ${\mathrm{U}}^{90+}$ \ensuremath{\rightarrow} C collisions at 89 MeV/u. The experiment provides the first study of the photon angular distribution of REC into a projectile $p$ state ($j=\frac{3}{2}$) which was found to exhibit a slight backward peaking in the laboratory frame. For radiative capture to the $j=\frac{1}{2}$ states, the measured angular distribution deviates considerably from symmetry around 90\ifmmode^\circ\else\textdegree\fi{}. The results demonstrate that the usual ${sin}^{2}{\ensuremath{\theta}}_{\mathrm{lab}}$ distribution is not valid in the high-$Z$ regime.