Projectile-charge dependence of inner-shell-ionization cross sections in a high-velocity region

Abstract

Ratios of ionization cross sections for proton impact to those for $^{3}\mathrm{H}\mathrm{e}\ensuremath{-}\mathrm{i}\mathrm{o}\mathrm{n}$ impact have been measured on $\mathrm{Al}K$ and $\mathrm{Y}L$ shells at projectile energies of 7, 9, 12, 16, and 21 MeV /amu. The ratio tends to unity with an increase in the projectile energy for both the $\mathrm{Al}K$ and $\mathrm{Y}L$ ionizations. This behavior can well be explained in terms of the Binstock-Reading theory combined with the distant-collision theory of Hill and Merzbacher: The polarization effect in distant collisions and the decreasing electron-density effect in close collisions cancel out each other in this high-energy region.