Improved logic for sampling landau straggling in MCNP5

Previous versions of the MCNP Monte Carlo transport code have relied on a Class I approach to electron energy-loss straggling. Parameters of the Landau theory were calculated for a predetermined set of energy groups and for the average ranges associated with those energy groups. During transport, energy-loss fluctuations were sampled based on these pre-computed parameters, and the energy loss for the actual transport step was interpolated from the sample appropriate to the selected energy-group range. The particular choice of energy group for a given transport step could be made in two ways, one originally developed for MCNP, and the other based on logic found in the Integrated TIGER Series codes. For electron transport in sufficiently large geometric regions, these interpolations cause few problems, but in small zones, artifacts begin to appear. In this paper, we describe an improved logic by which the sampling of straggling can be done for the specific energy and path length of the electron step without interpolation. We illustrate this improvement with a set of test calculations suitable for showing the artifacts found in the previous methods. (author)