THE EGS5 CODE SYSTEM

2.15.5 EGS5 Transport Mechanics AlgorithmThe transport between hard collisions (bremsstrahlung or delta-ray collisions) is superimposed onthe decoupled hinge mechanics as an independent, third possible transport process. To retain thedecoupling of geometry from all physics processes, for hard collisions EGS5 holds fixed over allboundary crossing an initially sampled number of mean free paths before the next hard collision,updating the corresponding distance (computed from the new total cross section), when enteringa new region. Again, while the random energy hinge preserves the average distance between hardcollisions, it does not preserve the exact distribution of collision distances if the hard collision crosssection exhibits an energy dependence between the energy hinges. In practice, however, this leadsto only small errors in cases where the energy hinge steps are very large and the hard collisionmean free path is sharply varying with energy.Thus the dual random hinge transport mechanics can be described as follows: at the beginningof the particle simulation, four possible events are considered: an energy loss hinge (determinedby a hinge on specified energy loss ∆E); a multiple scattering hinge (determined by a hinge ona specified scattering strength K 1 ); a hard collision (specified by a randomly sampled number ofmean free paths); and boundary crossing (specified by the problem geometry). The distances toeach of these possible events is computed using the appropriate stopping power, scattering power,total cross section or region geometry, and the particle is transported linearly through the shortestof those 4 distances. The appropriate process is applied, values of the stopping power, scatteringpower, cross section and boundary condition are updated if need be, and new values of the distancesto the varies events are computed to reflect any changes. Transport along all hinges then continuesthrough to the next event. Effectively, there are four transport processes occurring simultaneouslyat each single translation of the electron.The details of the implementation of the dual random hinge, because it is such a radical departurefrom other transport mechanics models, can sometimes lead to some confusion (and, in anycase, the energy hinge definitely leads to important implications for many common tallies), and sowe present an expanded explication here. Ignoring hard collisions and boundary crossings for themoment and generalizing here for the sake of brevity, we note that a step t involves transport overthe initial step prior to the hinge a distance t init given by (ζt) followed by transport through theresidual step distance t resid by ((1 − ζ)t). In practice, once a particle reaches the hinge point att init , we do not simply transport the particle through t resid to the end of the current step, becausenothing actually occurs there, as the physics was applied at the hinge point. So instead, immediatelyafter each hinge the distance to the next hinge point is determined and total step that theparticle must be transported before it reaches its next hinge is given by t step = t 1 resid + t2 init , wherethe superscripts refer to the 1 st and 2 nd hinges steps. We thus have the somewhat counter-intuitivesituation in that when a particle is translated between two hinge points, it is actually being moveda distance which corresponds to the residual part of one transport step plus the initial part of thesecond transport step. Thus we distinguish between translation hinge steps, over which particlesare moved from one hinge point to the next, linearly and with constant energy, and transport steps,which refer to the conventional condensed history Monte Carlo distances over which energy lossesand multiple scattering angles are computed and applied. Translation hinge steps and transport104