Photorecombination studies of highly charged ions at the storage ring ESR: a progress report

In this contribution, we report on photorecombination measurements of highly charged ions (HCI) that were performed using the electron cooler of the heavy-ion storage ring ESR of the Gesellschaft für Schwerionenforschung (GSI) as a target of free electrons. The experimental technique of resonance reaction spectroscopy by means of dielectronic recombination (DR) was employed in two new ways: The determination of isotope shifts in the DR resonance spectrum of few-electron heavy ions is a novel method with unique properties for investigations at the interface between electrons and the atomic nucleus, e.g., charge radii and hyperfine effects. First results from a pilot experiment with the two isotopes A = 142 and A = 150 of Li-like neodymium ANd57+ are discussed. Until very recently, electron-ion collision experiments at high relative energies of more than a few keV were not accessible at the electron coolers of storage rings. Such high energies are needed to access DR processes that involve the K-shell electrons of very heavy ions. The novel approach of DR measurements with a stochastically (pre-)cooled ion beam resolves this problem. Results for the photorecombination of hydrogen-like U91+ in the energy range (63 keV - 74 keV) of the KLL-DR process are presented.