High Energy Extension of the Semi-Empirical Model for Energy Deposition at Interfaces

A semi-empirical electron transport model is proposed for calculating dose profiles at material interfaces irradiated by gamma rays. The model is a one-energy-group, "two-flux" model of the Boltzmann equation-type; isotropic elastic scattering is assumed. It has the novel feature of employing electron scattering parameters that depend only upon the backscatter coefficient and electron range in each interface material. The model gives predictions similar to the semi-empirical model of Burke and Garth (1976) derived for x-ray energies, but extends the predictions to higher energies by taking into account the high degree of forward anisotropy of Compton and photoelectrons. Analytic expressions for the dose profiles on each side of the interface are derived and applied to the case of 1.25 MeV gamma rays irradiating a gold-aluminum interface. The relationship of this new model to the semi-empirical model is discussed.