High resolution for x-ray spectroscopy studies with highly charged heavy ions at the CRYRING@ESR electron cooler

In this work, we report on the first x-ray spectroscopy study associated with the RR processes for bare lead ions at the electron cooler of the CRYRING@ESR, as storage rings are currently the only facilities routinely delivering hydrogen-like ions at high-Z in large quantities. With ultra-cold electron beam temperatures and near zero electron-ion collision energies, the effective production of characteristic projectile x-rays was well demonstrated at 0 deg and 180 deg observation geometries in our experiment by decelerated 10 MeV/u hydrogen-like lead ions. To reveal the role of radiative feeding transitions in the formation of observed intense Lyman and Balmer lines, an elaborate theoretical model describing the radiative decay dynamics and each (n, l, j)-state population varying over time is put to a test. As a result, the presented rigorous treatment reproduces observed x-ray spectroscopy really well in terms of the RR transitions and characteristic x-ray lines. In addition, we found a strong enhancement for l = n − 1 states in inner shells due to radiative Yrast-cascades from high Rydberg states, that finally contribute strikingly to the observed intensities of characteristic x-ray lines. Further on the current thesis lays the basis for a successful effort to push the experimental resolution of x-ray spectroscopy for L → K ground-state transitions at high-Z of below 80 eV at about 100 keV. This was done in an RR experiment of free electrons into the bound states of initially hydrogen-like uranium ions by adopting low temperature x-ray detectors, namely metallic magnetic calorimeters. Such an experiment allowed us for the first time to resolve the substructure of the Kα2 line and partially the Kα1 line in helium-like uranium ions. The preliminary data again prove the unique potential of the experimental method based on x-ray spectroscopy at the electron cooler of the CRYRING@ESR.