L-subshell ionization of Au, Tl, Pb, Bi, Th, and U by protons

Abstract

$L$-subshell ionization cross sections for Au, Tl, Pb, Bi, Th, and U by proton impact have been determined over the projectile range 0.5-3.5 MeV. The measured x-ray production cross sections of the total $L$ shell and of some well resolved lines or groups of lines are consistent with those obtained by different authors in the same regions of bombarding energies and atomic numbers. Ionization cross sections were obtained by using the above results and experimental values for the relative radiative transition probabilities, fluorescence yields, and Coster-Kronig factors. The branching ratios of radiative transitions were measured and are in good agreement with the widely used theoretical calculations of Scofield, Rosner, and Bhalla. The values of fluorescence and Coster-Kronig yields were taken from previously published experiments performed in our laboratories. The influence of these experimental data on the shape of cross section versus proton energy curves is discussed. Comparisons of the experimentally determined $L$-subshell ionization cross sections are made with calculations in the plane-wave Born (PWBA), semiclassical, and binary-encounter approximations. The large effect of binding-energy, trajectory, and relativistic corrections on the PWBA calculations invalidates quantitative conclusions regarding agreement between experimental and theoretical values. Semiclassical arguments are presented, however, to explain some general aspects of the ionization cross-section curves.