Projectile Charge Dependence of K -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 Z₁, at equal velocities, show pronounced deviations from the Z₁^_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.