Inner shell ionization of copper, silver, and gold by electron impact in the energy range 20-140 keV

Abstract

Absolute total cross sections for K-shell ionization have been measured with a Ge(Li) detector at incident electron energies of 20-135 keV for copper; 30-140 keV for silver; and 90-140 keV for gold. Comparison is made with the nonrelativistic calculations of Burhop and the partially relativistic calculations of Arthurs and Moiseiwitsch. In addition, the absolute total ionization cross section for the LIII subshell of gold has been measured at incident energies of 30-140 keV. These results are compared to the calculations of Burhop for the LIII subshell of mercury. The results of this experiment represent the first measurements near the ionization thresholds of these elements. The K-ionization results for copper, silver, and gold are consistently higher in the energy region near the ionization threshold than the corresponding theoretical calculations. This steep rise in the cross section as a function of energy near threshold is not predicted by current theoretical models. A similar pattern is also observed for the LIII-ionization cross section of gold. The general breakdown of the first Born approximation for the impact energy range near the ionization threshold is suggested to be the most important reason for the observed differences between this experiment and present theoretical calculations. Failure to treat the bound and ejected electrons relativistically and the neglect of exchange effects between the scattered and ejected electrons may also contribute to the discrepancy between experiment and theory. A relativistic model for L ionization would be useful to compare with the LIII-ionization results of this experiment.