THE EGS5 CODE SYSTEM

FinalDirectionxEnergy HingesφΘt(ζ∆ E)t( (1 − ζ) K1)t(K1)∆xyt( ζ K1)t( (1−ζ)∆E)sΘMono−energetic transport∆t = ∆K1/G1 in each segment∆yzMaterial AMaterial BFigure 2.12: Electron transport across region boundaries.hinge however, multiple scattering occurs only at hinge points. If a boundary is crossed duringeither the pre-hinge (ζK 1 (t)) or post-hinge ((1 − ζ)K 1 (t)) portion of the step, the value of G 1 usedin updating the accumulated scattering strength is simply changed to reflect the new value of thescattering power in the new media. Thus it is not necessary to apply multiple scattering at regionboundaries, and the expensive re-interrogation of the problem geometry required by PRESTA iscompletely avoided. Inherent in this is the implication that multiple scattering distributions areequivalent for different materials at a given energy and for pathlengths which correspond to thesame scattering strength K 1 . This, of course, is not strictly true. It can be shown formally, however,that for a multiple scattering distribution expressed as a sum of Legendre polynomials in Θ,〈cos(Θ)〉 = exp(−K 1 ) (2.374)so that for small K 1K 1 ≃ 1 − 〈cos(Θ)〉. (2.375)Thus, in preserving K 1 for cross boundary transport, the EGS5 method also roughly preserves theaverage cosine of the scattering angle over the boundary.Inspection of the implementation details reveals that the boundary crossing in EGS5 is analogousto an energy hinge without energy loss. All step-size variables (rates and distances) need to beupdated, but otherwise transport to the next event is uninterrupted, as shown in Figure 2.12.103