Directional enhancement of characteristic relative to bremsstrahlung X-rays: Foil thickness optimization

X-ray emission spectra from thin targets show significant directional effects that have their origin in the polar dependence of the atomic bremsstrahlung cross section. This fact has led to the idea of enhancing the characteristic lines from an electron-impact X-ray source by observing X-rays from optically thin (foil) targets in a direction antiparallel to that of the incident electron beam. Efficiency considerations dictate the use of foils of maximum possible thickness consistent with the desired degree of characteristic line enhancement. Foil thickness and electron energy are crucial parameters determining the angular divergence of the electron beam within the foil and hence the extent of characteristic line enhancement. The aim of this article is to find a relationship between electron energy and foil thickness that preserves a high characteristic to bremsstrahlung X-ray ratio while maximizing efficiency. This is achieved by relating the root mean square electron deflection angle, derived from the mass scattering power, to the angular width of the "valley", centered on 180 degrees, within which bremsstrahlung emission can be neglected. Representative spectra are presented and conclusions for applications and for further work are briefly presented. (C) 2007 Elsevier Ltd. All rights reserved.