Multiple-Scattering Angular Deflections and Energy- Loss Straggling

Abstract

When energetic electrons pass through an extended medium, the number of Coulomb interactions with atomic nuclei and orbital electrons is so large that-even with the supercomputers now available—a direct Monte Carlo simulation of all these collisions is impractical. The transport codes of the type discussed in this book, with the exception of Chapter 16, therefore abandon direct simulation of every individual collision, and make use of a “condensed random walk” model in which multiple scattering theories are used to sample angular deflections and energy losses in successive short track segments. The lengths of these segments (also referred to as step-sizes) are chosen to satisfy two conditions: 1) a sufficient number of collisions must occur within each segment, so that the application of the multiple-scattering theories is justified; 2) the cumulative deflections and energy losses in each segment must be small enough so that the condensed random-walk model provides a sufficiently accurate simulation of electron-track generation, baundary crossings, the scoring of energy deposition, etc.