Statistical approach of inner-shell ionization in Cu + Au collisions

Measurements of inner-shell vacancy production cross sections σn and associated multiple ionization mean probabilities of the outer shells pn (defined as the ratio between the average number of vacancies in the n shell created in the collision and the occupancy number of that shell), in heavy ion - atom collisions in the K-L level matching region, have been in progress in our laboratory for several years. In the present work, we report experimental data obtained in the collision system Cu + Au, in the energy range of 0.1-1.625 MeV/u, and compare them with the predictions of the statistical model. The measurements have been performed with beams of Cu obtained from the Bucharest HVEC-FN Van de Graaff tandem accelerator. By normalizing the measured X-ray yields to the elastically scattered projectiles the absolute ionization cross sections of the Cu K-shell and Au Li-subshells (i=1,2,3) were determined. The multiple ionization probabilities have been obtained from the measured energy and yield shifts of the K and L X-ray lines and by comparing with calculations in the independent electron approximation. The inner-shell ionization treatment as electron diffusion to the continuum, although not clearly justified, is still an effective manner of representing different kinds of data. The model of Mittleman and Wilets has only one adjustable parameter, the diffusion constant Dn , and R0 - the interaction radius - is usually taken as the Thomas-Fermi radius. The energy dependencies of the Cu K-shell and Au L1,2- and L3-subshell ionization cross sections, as well as of the Au M-shell multiple ionization probability, pM , were found in good agreement with the statistical model predictions. The dependence of the diffusion constant, determined for different final vacancy states, on the excitation (ionization) energy σn is in fairly good agreement with the model prediction, Dn∝σn2. (authors)