X-ray cross sections in helium for electron capture to excited states by bare nuclei with5≦Z1≦9

Abstract

Experimental data are presented for $K$-shell x-ray cross sections for electron capture to excited states by bare nuclei of F, O, N, C, and B in the energy range from ∼0.25 to 2.3 MeV/amu. For these projectiles the capture to excited states dominates the capture of the loosely bound electrons available in helium, and the projectile x-ray cross section is closely related to the total capture cross section. The experimental x-ray production cross sections are compared to predictions based on a closed-form electron-capture calculation in the Brinkman-Kramers (BK) approximation, even though the magnitude of the theoretical results is a factor of 7 in excess of experiment. Using a single normalization constant, the calculated cross sections for electron capture to excited states preserved the trends of the experimental data with these bare nuclei over the entire energy range of the experiment. At the lowest energy, the experimental and normalized BK cross sections are still below, but rapidly approaching, the maximum in the capture cross section estimated from calculations for bare nuclei in hydrogen by Salop and Olson using a Landau-Zener model.