Incorporation of single elastic scattering in the EGS4 Monte Carlo code system: tests of Molière theory

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To avoid prohibitively long computation times, conventional Monte Carlo e − transport algorithms (e.g. EGS4, ETRAN, ITS) employ multiple scattering theories and “condensed history” methods to model e − transport. Although highly successful for many calculations, these techniques do not model backscatter very well, particularly for high- Z materials. In an attempt to correct for this shortcoming, we have extended the EGS4 Monte Carlo code to allow for the simulation of single elastic scattering. The single scattering method also allows quantities to be scored in submicrometer dimension geometries where the Molière multiple scattering theory fails and the Ooudsmit-Saunderson multiple scattering equations converge very slowly. Two single scattering schemes have been implemented: (i) Screened Rutherford cross sections which form the basis of Molière's multiple scattering theory, (ii) Single scattering cross sections based upon phase-shift data. In this work we describe the implementation of single elastic scattering in the EGS4 Monte Carlo code system and employ it to verify the Molière multiple scattering theory in its range of validity. We demonstrate that the Molière multiple scattering formalism provides a good description of multiple scattering despite its use of a relatively crude cross section and that it may be employed with semiquantitative accuracy in the plural scattering regime, where electron step-lengths are so short that only as few as five atoms participate in the angular deflection. However, the remaining differences of the Molière distributions with the phase-shift data motivate the use of more accurate fundamental data, in particular, for applications involving high- Z elements.