Multiple Ionization of Atomic Targets by Proton Impact

We have combined experimental measurements of multiply charged ion production, doubly differential electron emission cross sections and Auger electron spectra produced in fast-proton noble-gas collisions with theoretical inner and outer shell ionization cross sections in an effort to provide a unified description of the ionization process. A detailed analysis is provided for 1-4 MeV proton ionization of krypton. Our measured relative yields of Kr+2 and Kr+3 ions are in approximate agreement with the photoionization results of Krause and Carlson. If we assume nearly half of the 3d vacancies created in Kr are filled through multiple (3dNNN) Auger transitions, the M-subshell ionization cross sections that we derive from our Auger electron spectra are in good agreement with theoretical calculations. The measured absolute yields of Kr+2 and Kr+3 are, however, a factor of 2 smaller than those derived from the decay of inner shell vacancies. This discrepancy leads us to believe that multiple ionization in the initial interaction is a significant contribution to the observed yields.