THE EGS5 CODE SYSTEM

Different scattering angles
L (= D)
D (= L)
Correct hinge step
Hinge steps too long, too small
Figure 2.16: Schematic illustrating the modified “broomstick” problem as used in EGS5.
energy from 2 keV 11 up to 1 TeV at values of 2, 3, 5, 7 and 10 in each of the 9 energy decades
spanning the energy range. The characteristic geometric dimensions in the data sets range from
10 −6 times the electron CSDA range at the low end, and up to half of the CSDA range at the high
end 12 . (Note that ignoring hard collisions and using unrestricted stopping powers at the upper
end of the energy range in question is physically unrealistic. Computations in this energy range
were made nonetheless to fill out the tables with overly-conservative estimates of the appropriate
step-size.)
Because K 1 is the integral over distance of scattering power, which is proportional to ρZ 2 /A
times the integral of the shape of the differential elastic scattering cross section, K 1 should be
roughly proportional to tρZ 2 /A, if t is the distance, and that was generally found to be the case.
Interpolation in the geometric dimension variable is therefore done in terms of tρ so that interpolation
between materials can be performed in terms of Z 2 /A. (To account for the effect of
soft collision electron scattering, interpolations are actually done in terms of Z(Z + 1) instead of
Z 2 .) Positron K 1 values are determined by scaling electron scattering strength by the ratio of the
positron and electron scattering power. The list of reference materials is given in Table 2.4.
11 For high Z materials for which the Bethe stopping power formula is inaccurate at 2 keV, the tables stop at 10
keV.
12 An additional constraint on the minimum characteristic dimension in EGS5 is the smallest pathlength for which
the Molière multiple scattering distribution produces viable results. Bethe [23] has suggested that paths which
encompass at least 20 elastic scattering collisions are necessary, though EGS5 will compute the distribution using as
few as e collisions, which is a numerical limit that simply assures positivity.
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