Limitation of EBIS/T Ion Yield by Radiative Recombination

The operation of an electron beam ion source and trap (EBIS/T) at an electron beam energy, which prevents ionization to the first electron of the next shell can provide an increased abundance of the highest charge state. For Pb54+ with empty n-shell this increase amounts to a factor of 5, which otherwise could be reached in the yield of ions only by an increase of the electron beam power by a factor of 56. This is true, however, only, if radiative recombination of Pb54+ is being neglected. We present a comparison of computed ionization and recombination cross sections in the energy range of interest for empty n-and m-shells of lead as a representative heavy element, which shows that at the threshold energy for ionizing the first electron of the next shell, RR cross sections are about 2 times smaller than those for ionization and that the use of this shell effect can be beneficial to obtain an ion yield, which otherwise needs substantial increase of the beam energy and power.