L-shell ionizations of Au, Bi, and U in proton and helium-ion impact

The x-ray production cross sections of the total $L$ shell and of the $L\alpha$, $L\beta$, $L\gamma$, $\mathrm{Ll}$, and $L\eta$ transitions have been measured for Au, Bi, and U for proton impact over the energy range from 1.0 to 4.5 MeV and for ${}_{}{}^3{}_{}{}^{}\mathrm{He}_{}^{+}{}_{}{}^{}$ impact from 3.0 to 9.0 MeV. The measured x-ray production cross sections and the ratios of the x-ray production for various transitions are compared with calculations based on the plane-wave Born approximation (PWBA) and the binary-encounter approximation (BEA). Generally, the PWBA is in better agreement with the observed results. Various correction factors to these simplified theories are also estimated and discussed. The ratios of the x-ray production for protoń impact to that for helium-ion impact, at equal velocities, deviate systematically from the theoretical ${\mathrm{Z}}_1^2$ dependence. This behavior is discussed in terms of the nuclear repulsion for the projectile, the change in binding energy of the target electrons, and the polarization of electron orbit. It is shown that the observed total $L$-shell ionization cross sections for various elements taken from the present work and other works can be represented by a single smooth curve, which at higher energies is in good agreement with the BEA curve of Garcia derived for the $K$-shell ionization, though the experimental values at lower energies are considerably higher than the theoretical predictions.