Characteristic X-Ray Production in theLIIIShell of Copper by Low-Energy (100- to 500-keV) Protons

Characteristic $L$-shell x rays produced when protons of 100- to 500-keV energy are stopped in thick targets of copper have been studied using proportional counter detection. As a consequence of self-absorption in the $L_{\mathrm{II}}$ and $L_{\mathrm{III}}$ shells, the relative intensities of the various $L$ lines change with depth of penetration (hence energy) of the protons. By use of additional copper absorbers, lines originating from the filling of vacancies in the $L_{\mathrm{III}}$ shell can be isolated. [A single-crystal (KAP) spectrometer was used to verify this for electron-excited spectra.] Presented here are the thick-target yields for the $L_{\mathrm{III}}$ lines and the highly self-absorbed lines as well as the x-ray production cross section for the $L_{\mathrm{III}}$ lines. With a fluorescent yield of 0.05, the ionization cross section (${\sigma }_I=3.6\mathrm{}\times {10}^{-22\mathrm{}}$ cm) at 100 keV is approximately two orders of magnitude lower than the value predicted on the basis of the Born approximation description of the proton trajectory.