Class II algorithm for charged-particle transport simulation - Paper 142
Simulation of the transport of high-energy charged particles is difficult because of the large number of interactions experienced by these particles before being brought to rest. Possible strategies to deal with this difficulty were analyzed by Berger. Most modern Monte Carlo codes use a mixture of complete-grouping (class I) and mixed (class II) schemes. In class I simulations, the global effect of all the interactions that occur along a trajectory segment of a given length s is described by means of precalculated distributions derived from multiple-scattering theories. In class II simulations, hard (or catastrophic) interactions (with energy loss W or polar scattering angle {theta} larger than given cutoff values) are sampled individually, while the effect of soft interactions (with energy losses and scattering angles smaller than the respective cutoffs) is described using a multiple-scattering approach. Although Berger effectively employed class II simulation only for energy-loss events, he did recognize the advantages of class II over class I also for multiple elastic scattering. On the one hand, because hard interactions are described exactly according to the adopted differential cross sections (DCSs), class II schemes are more accurate. On the other hand, multiple-scattering approximations, which are still required to describe the cumulative effect of soft interactions, are simpler and more accurate when individual interactions are limited to small energy transfers and angular deflections. For instance, for path lengths that are not too small, but still much less than the CSDA range, the energy-loss distribution due to multiple soft events is approximately Gaussian, by virtue of the central limit theorem. Similarly, the angular distribution caused by multiple small-angle deflections is also approximately Gaussian. However, class II schemes are more difficult to implement because of their dependence on specificities of the DCSs. In this article we briefly present the essentials of the class II algorithm for charged particles implemented in the last version of the penelope code, and provide evidence of its accuracy and stability with the simulation parameters. (authors)