Recombination of U28+ ions in a dense, cold electron target

Abstract Recombination of highly charged ions with free electrons is studied in single-pass merged-beams experiments at the UNILAC accelerator of GSI. Specific features of the experimental setup are the high electron density (up to 10 10 cm −3 ) and the high magnetic field (up to 1 T) in the electron target, which exceed the corresponding numbers for typical electron coolers in storage rings by nearly two orders of magnitude. This provides unique possibilities to study new effects in electron-ion recombination. A previous experiment with U 28+ ions has shown a large enhancement of the recombination rate at low center-of-mass energies, where the dominating contribution is expected from radiative recombination. The observed rates exceed theoretical calculations by more than an order of magnitude. For the clarification of this observation we extended the previous measurement up to a center-of-mass energy E = 450 eV, checking the possibility of fortuitous resonances of dielectronic recombination near zero center-of-mass energy. The analysis of Rydberg sequences in the observed spectrum of resonances does not suggest the presence of DR at E = 0.