The response history Monte Carlo method for electron transport.

Abstract

The Response History Monte Carlo (RHMC) method, a new Monte Carlo method developed for electron transport problems, has significant advantages over existing electron transport methodologies. Among these current methods, the condensed history methodology, used in most existing Monte Carlo electron transport codes, is inadequate for low energy electron transport problems. Condensed history requires a number of restrictive assumptions about the scattering characteristics in order to analytically determine the electron state (energy, direction, and position) after several collisions. These assumptions, which are valid at high energies, fail with decreasing energy and increasing atomic number. Analog Monte Carlo is sometimes used as an alternative to the condensed history method but modeling individual interactions for electron transport problems is very time consuming. However, the results from analog Monte Carlo calculations are typically very accurate. The RHMC method combines the computational speed of the condensed history method with accuracy comparable to analog Monte Carlo. This is done by sampling from pre-constructed probability distribution functions (PDFs) that represent the electron state after the electrons have traveled a certain distance. These PDFs are similar to the multi-scatter distributions used in condensed history, but are constructed through an analog Monte Carlo simulation. Hence, the PDFs are free from the assumptions required to construct the condensed history PDFs. The RHMC method is divided into two separate calculations: a local calculation which is used to generate the PDFs over a small region; and a global calculation which samples from these predetermined PDFs to simulate electron transport through a thicker material. The method has been shown to be effective for electron transport problems at all energies for which analog Monte Carlo is valid and is more accurate than condensed history calculations for many low energy (below 200 keV) problems. The RHMC method is particularly attractive for use on sub-MeV electrons, because analog Monte Carlo calculations are too time-consuming and condensed history calculations are inaccurate.