Relativistic effects in the impact-parameter dependence of K-shell ionization of heavy elements

The impact-parameter dependence of K-vacancy production in Ta, Au, Bi and U by 4 MeV protons has been measured using a magnetic spectrometer and fast coincidence techniques. The ionization probabilities are greater than non-relativistic SCA predictions by as much as a factor of four. The inclusion of relativistic electronic wavefunctions in the SCA calculations substantially reduces the discrepancies.