THE EGS5 CODE SYSTEM

As a final comment on continuous energy loss, we note that when an electron is transported a
given distance, it is assumed that its energy loss due to sub-cutoff collisions is equal to the distance
traveled times the mean loss per unit length as evaluated using Equation 2.253. In actuality, the
energy loss over a transported distance is subject to fluctuations and gives rise to a restricted
Landau distribution. Fluctuations due to discrete interactions can be properly accounted for in
EGS in most applications by setting the cutoff energy for charged particle production sufficiently
low. For example, for 20 MeV electrons passing through a thin slab of water, Rogers[136] has shown
that the energy straggling predicted by considering all secondaries down to 1 keV (i.e., AE=512
keV) is in good agreement with the Blunck-Leisegang modification of the energy loss straggling
formalism of Landau (except for large energy loss events where the modified Landau distribution
appears to be wrong). The creation of large numbers of low-energy secondary electrons, even when
they are discarded immediately, adds significant inefficiency to EGS shower simulations, however,
and so a correct energy loss fluctuation model is currently under development for EGS5.
2.14 Multiple Scattering
When an electron passes through matter, it undergoes a large number of elastic collisions with the
atomic nuclei. These have the effect of changing the electron’s direction, but do not significantly
change its energy. As noted previously, the number of such collisions is so great that direct simulation
of individual scattering events is almost never practical 8 . Instead, elastic scattering is typically
treated by bundling large numbers of collisions together into large “steps” and then assuming that
the electron transport over these larger steps can be characterized by the particle’s longitudinal
translation and lateral displacement during the step plus its aggregate scattering angle Θ over the
step (the azimuthal angle is assumed to be uniform), which is taken from an appropriate “multiple
scattering” distribution function. The details of the current implementation of this approach are
provided in the next section. The remainder of this section is devoted to discussion of multiple
scattering probability density function (p.d.f.) for Θ. EGS5 currently offers two choices for the
multiple scattering p.d.f., one based on the theory of Molière, and a new one based on the approach
of Goudsmit and Saunderson (GS) [63, 64].
2.14.1 The Molière Multiple Scattering Distribution
All previous versions of EGS have treated electron elastic scattering using Molière’s [107]theory of
multiple scattering as formulated by Bethe [23]. The details of computing multiple scattering in
mixtures, and a good introduction to the subject is given in the the review article by Scott [147],
to which we make frequent reference in the discussion that follows.
In Versions 1 and 2 of EGS the method of sampling scattering angles was based on a scheme
of Nagel’s whereby one of 29 discrete representative reduced angles was selected and then used to
8 Even in PENELOPE [14], which was originally developed for modeling the transport of electrons at low energies,
only large angle collisions are treated explicitly.
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