Effect of Target Thickness on Characteristic Projectile X Rays and REC in Ion-Atom Collisions

A systematic investigation of projectile K x-ray production as a function of target thickness has been conducted for 20-80 MeV Cl ions incident on thin Cu targets. Measured intensities for characteristic x-rays and radiative electron capture (REC) are parametrized according to the model of Betz et al. From the fitting parameters the mean fluorescence yield and the radiative lifetime for K-shell decay for the highly ionized Cl projectiles are determined. The fluorescence yield is found to increase by a factor of about six over the energy range studied having a value of ~ 0.1 at 20 MeV. The radiative lifetime for K-shell decay is found to be ~ 3 × 10-14 sec. Parametrization of the REC intensity allows a determination of the REC cross section per K vacancy. The resulting cross sections are in good agreement with the free electron theory of Bethe and Salpeter if it is assumed that each of the "loosely" bound (i. e., M- and N-shell) electrons in Cu contribute equally to the REC process. By combining the results obtained for the characteristic x-rays and REC, the fluorescence yield for K-shell capture events is determined and found to have values in the range (2-4) × 10-3.