Projectile Charge Dependence ofK-Shell Ionization by Swift Light Nuclei

$K$-shell x-ray yields were measured for Al and Ni bombarded by H, D, He, and Li of kinetic energies between 1 and 7.5 MeV/amu. Ratios of $K$-shell ionization cross sections for projectiles of different atomic numbers $Z_1$, at equal velocities, show pronounced deviations from the $Z_1^{}{}_{}{}^2$ dependence predicted in the plane-wave Born approximation. The effect can be understood in terms of distortions of the $K$ shell in the field of the ionizing particles.