M-shell ionization of Au, Bi, and U by protons and helium ions in the MeV region

The M-shell x-ray production cross sections in Au, Bi, and U were measured with a Si(Li) detector for proton and ³He-ion bombardments over the energy range 1.0-4.5 MeV and 3.0-9.0 MeV, respectively. Absolute ionization cross sections are derived and compared to the predictions of the binary-encounter approximation (BEA) and the plane-wave Born approximation (PWBA). Systematic deviation of the experimental results from the scaled universal curve of the BEA is found. It is shown that this deviation can be understood by taking into account the velocity distribution function for the $3d$ electrons. Good agreement between the PWBA calculation of Choi and the experimental results is obtained for the M-shell ionization of Au. The ratios of the ionization cross sections for proton bombardment to those for helium-ion bombardment, at equal velocities, deviate substantially from the theoretical Z₁² dependence and show so-called crossover behavior, which has been found for the K- and -shell ionization.