Future Arrangement at the ESR Storage Ring

The experimental technique established at the Super-EBIT can also be applied at the ESR storage ring at GSI Darmstadt. A prerequisite for such studies is the simultaneous storage of two beams of highly charged heavy ions with same atomic number Z but different charge states Q. Such an operation mode of the storage ring has already been exploited in some atomic physics experiments by using the charge state breeding technique. The experimental situation is depicted in the Figure.

ESR Bare ions are injected into the ring and accumulated up to a certain number of stored ions. In the electron cooler section of the ring the initially bare ions recombine with cooler electrons via radiative recombination thus producing in addition a beam of H-like ions. In the cooler section the beams are merged together, a situation which is closely related to the one at the Super-EBIT. The x-rays emitted via RR of the cooler electrons into the ground state of the bare- and H-like ions can now be detected simultaneously by a solid state detector viewing the interaction region at an observation angle close to 0 deg. Such a detector set-up has already been used in various experiments conducted at the electron cooler. In these experiments the x-ray emission has always been detected in coincidence with the down charged ions in order to suppress possible background radiation. The charge state breeding technique requires the sacrifice of this coincidence option for x-ray events associated with capture into Depending on the background the bare ions (compare Figure). this may constitute a serious experimental restriction. One can overcome this drawback by shifting the particle detector into a position where capture into the bare ions can be measured, which will of course also destroy the beam of H-like ions. On the other hand, one can still measure sequentially the RR peaks for bare and H-like ions within one storage cycle, i.e.: breeding of the H-like charge state and measuring RR into H-like ions and, subsequently, detection of RR into the bare ions. Up to now, the precision in the Doppler shift corrections of Δ E/E ≈ 5x10^-5 at the ESR is determined by the uncertainty in the determination of the absolute beam velocity. It constitutes the most serious limitation for precision x-ray experiments involving ground state transitions in H-like ions. In the proposed relative measurement, where energy differences of 2.2 keV or less are to be determined, this uncertainty would introduce an error of less than ± 0.1 eV.