Evaluation of soft x-ray yield of Al from 27.557-MeV neutral particles

Extreme ultraviolet (XUV) emission from Al bombarded by 50-MeV H0 particles has been reported by James, Sauers, and Arakawa. However, the contribution to the observed signal from scattered electrons has not been evaluated, and the origin of XUV light was not determined since only broadband filters were used. We have therefore measured the XUV emission for 15 keV electrons incident on an Al target using a grazing incidence spectrometer. We find the emission is composed primarily of Al L23 soft x rays and determine a yield of 0.021 photons/electron. This value is large compared with previous work with high-Z materials, and our yield values are 100 times higher than standard theories which do not consider secondary electrons. We show that L-shell emission in low-Z materials is due primarily to ionizations from secondary electrons created near the surface. Since the number of secondary electrons created by an energetic particle is roughly proportional to the energy deposited by the particle, relative x-ray yields from energetic protons and electrons can be determined from a knowledge of stopping powers. Using stopping power calculations, we estimate that the Al L23 soft x-ray yield from 27.542-MeV proton bombardment is 0.027 photons/proton. A 27.557-MeV H0 particle is essentially equivalent to a 27.542-MeV proton and a 15-keV electron; thus, the yield of Al L23 soft x rays from 27.557-MeV H0 particle bombardment is 0.048 photons/particle.