Positron stopping in elemental systems: Monte Carlo calculations and scaling properties.

Abstract

The scaling of positron-implantation (stopping) profiles has been reported by Ghosh [ital et] [ital al]., who used the BNL Monte Carlo scheme to generate stopping profiles in semi-infinite elemental metals. A simple scaling relationship reduced the stopping profiles of positrons implanted at different energies (ranging from 1--10 keV) onto a single universal curve for that particular metal. We have confirmed that the scaling relationship also applies to the quite different Jensen and Walker Monte Carlo scheme, for more materials, and over an expanded energy range of 1--25 keV. The mean depths of the stopping profiles calculated by the two Monte Carlo schemes are found to be different, mainly due to differences in the inelastic mean free paths and the energy-loss functions. However, after scaling, the profiles generated by the two schemes can be superimposed onto a single curve which can be appropriately parametrized. The scaled profiles are found to be only weakly material dependent. The mean depths, backscattered fractions, and scaled stopping profiles are fitted to simple parametric functions, and the values of these parameters are obtained for several elements.