Polynomial approximation to universal ionisation cross‐sections of K and L shells induced by H and He ion beams

In 1970s, Basbas et al. (Phys. Rev. A 1973, 7, 983; Phys. Rev. A 1978, 17, 1655) rose the possibility that PWBA ionisation cross‐sections could be expressed in terms of a universal ionisation cross‐section. Starting from ECPSSR ionisation cross‐sections and reproducing Basbas approach, a universal curve can be displayed against the reduced ion velocity, but data are too widespread to allow for a proper polynomial fit. In the present work, Basbas approach was revised and plotted against a rescaled, reduced ion velocity, eliminating most of the data spread and generating in practice a universal ionisation cross‐section curve. In this revised approach, the screening parameter, θ n , o , assumes a major role in reaching the universal behaviour. Polynomial approximations are then made to the universal curves for incident H + and He 2+ ions. Cross‐sections were then calculated using the polynomial approximations for target elements with atomic number from 11 to 92 and for ion beam energies from 50 keV to 10 MeV, for K‐ and L‐shell ionisation cross‐sections and are compared to experimental data from literature. Finally, X‐ray yields were simulated for various thick target samples using these polynomial approximations and the outcomes are compared to data obtained at the ITN PIXE set‐up. Copyright © 2011 John Wiley & Sons, Ltd.