THE EGS5 CODE SYSTEM

Final
Direction
x
Energy Hinges
φ
Θ
t(ζ∆ E)
t( (1 − ζ) K1)
t(K1)
∆x
y
t( ζ K1)
t( (1−ζ)∆E)
s
Θ
Mono−energetic transport
∆t = ∆K1/G1 in each segment
∆y
z
Material A
Material B
Figure 2.12: Electron transport across region boundaries.
hinge however, multiple scattering occurs only at hinge points. If a boundary is crossed during
either the pre-hinge (ζK 1 (t)) or post-hinge ((1 − ζ)K 1 (t)) portion of the step, the value of G 1 used
in updating the accumulated scattering strength is simply changed to reflect the new value of the
scattering power in the new media. Thus it is not necessary to apply multiple scattering at region
boundaries, and the expensive re-interrogation of the problem geometry required by PRESTA is
completely avoided. Inherent in this is the implication that multiple scattering distributions are
equivalent for different materials at a given energy and for pathlengths which correspond to the
same 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 1
K 1 ≃ 1 − 〈cos(Θ)〉. (2.375)
Thus, in preserving K 1 for cross boundary transport, the EGS5 method also roughly preserves the
average cosine of the scattering angle over the boundary.
Inspection of the implementation details reveals that the boundary crossing in EGS5 is analogous
to an energy hinge without energy loss. All step-size variables (rates and distances) need to be
updated, but otherwise transport to the next event is uninterrupted, as shown in Figure 2.12.
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