Ionization of relativistic K-shell electrons by proton bombardment

K-shell ionization cross sections for Ho, Au, and U by proton impact have been measured over the incident energy range 1-4 MeV. The measured cross sections are larger than those predicted from the plane-wave Born, semiclassical, and binary-encounter approximations including Coulomb-deflection and binding-energy effects by factors from 2 to 20. However, it is shown that these large disagreements can be well explained by the semiclassical theory using relativistic wave functions for the K-shell electrons. The validity of various approximate estimations of electronic relativistic effects is discussed. Some results on He3-ion impact are also discussed. Ratios of x-ray production cross sections for K2, K1, and K2 lines to that of K1 line are shown as a function of proton energy and the results indicate little simultaneous ionization probability in the L, M, and N shells of heavy atoms.