K-shell ionization of Pb at zero impact parameter in 50- to 100- MeV Cl + Pb collisions

The absolute ionization probability at zero impact parameter has been measured in a coincidence experiment at incident Cl-ion energies of 50-100 MeV. Total $K$ -shell ionization cross sections are also reported over a larger energy range. The data are compared with theoretical predictions based on the assumption of pure Coulomb ionization as formulated in the binary-encounter approximation (BEA), the semiclassical approximation (SCA), and the plane-wave Born approximation. It is shown that the good agreement of the total cross sections with nonrelativistic calculations using a straight-line, constant-velocity projectile trajectory is to a large extent due to a fortuitous cancellation of the inadequacies of this oversimplified model. Recent impact-parameter formulations of the BEA model, including corrections for projectile retardation and deflection and an approximate treatment of relativistic effects, qualitatively reproduce the observed energy dependence of the cross sections and ionization probabilities but overestimate the absolute values by about a factor of 3. Approximate, closed-form, SCA results without these corrections also reproduce the energy dependence but underestimate the ionization probability by a factor of 2 to 3.