Improved electron transport mechanics in the PENELOPE Monte-Carlo model

We describe a new model of electron transport mechanics, the method by which an electron is transported geometrically in an in®nite medium as a function of pathlength, s, the accumulated elastic multiple-scattering angular de ̄ection characterized by H…s†, the polar scattering angle, and U, a random azimuthal angle. This model requires only one sample of the multiple-scattering angle yet it reproduces exactly the following spatial moments and space±angular correlations: hzi, hx sin H cos Ui, hy sin H sin Ui, hz cos Hi, hx2i, hy2i and hz2i. Moreover, the distributions associated with these moments exhibit a good improvement over the PENELOPE transport mechanics model when compared selfconsistently with the results of analog simulations. When we split the transport step into two steps with equal pathlength, we observe excellent agreement with the distributions, indicating that the algorithm nearly matches higher order moments when employed in this way. The equations described herein are relatively inexpensive to employ in an iterative Monte-Carlo code. We have employed the new model to demonstrate the usefulness of the new mechanics for several examples that span the dynamic range of application. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 02.50Ng; 13.60Fz; 25.30Bf; 34.80Bm