K-shell ionization of light elements by proton and helium-3-ion impact

$K$-shell ionization cross sections for Al and Si by proton and ${}_{}{}^3{}_{}{}^{}\mathrm{H}\mathrm{e}-\mathrm{i}\mathrm{o}\mathrm{n}$ impacts have been measured with thin targets and a proportional counter over the projectile energy range 0.75-4 MeV/amu. The results, together with others on light elements, are compared with theoretical predictions based on the plane-wave Born approximation and the binary-encounter approximation, and it is found that the scaling law, taking into account the effects of the Coulomb deflection of the projectile and the increase of binding energy of $K$-shell electrons, gives a good fit to the experimental results over the entire energy range measured. Ratios of the $K$-shell ionization cross sections for helium-ion impact to those for proton impact show deviations from the $Z_1^2$ dependence predicted by the Coulomb interaction theory. Possible reasons for this deviation are discussed.