THE EGS5 CODE SYSTEM

The various parameters needed to sample the secondaries’ energies for the bremsstrahlung and
pair production interactions are computed in the PEGS routine DIFFER. The parameters are stored
for each medium during execution of EGS in COMMON/BREMPR/.
One other use for the bremsstrahlung cross section is for computing the mean energy loss per
unit length of an electron due to emission of soft photons (i.e., those with energy below the photon
cutoff energy A p ). This is given by
which will be discussed in Section 2.13.
−
( )
∫ d Ĕ
AP
=
dx
Soft Brem
0
( )
d˘ΣBrem
˘k
d˘k
d˘k (2.148)
2.7.1 Bremsstrahlung Photon Angular Distribution
So far we have only discussed the selection of the energy of the secondaries. Since these are interactions
with three body final states, the polar angles of the secondary particles are not uniquely
determined by the secondary energies, and a complete simulation would sample from some appropriate
distributions. However the angles at which products from these reactions are usually emitted
are small compared to angular deviations resulting from multiple scattering. Previous versions of
EGS therefore assumed that the direction of an electron emitting bremsstrahlung is unchanged,
that a bremsstrahlung photon is emitted at an angle relative to the incident electron direction,
θ = m/Ĕ0, and that pair produced particles have production angles relative to the incident photon
direction given by θ = m/˘k. The azimuthal angle for the first product particle is chosen randomly
and the other product particle is given the opposite azimuth.
The above model of the angular distribution of newly created bremsstrahlung photons may be
overly simple for some applications. The angle given by m/˘k is a good estimate of the expected
average scattering angle, and at high energies, where the distribution is strongly peaked in the
forward direction, more accurate modeling of scattering angles does not significantly improve the
accuracy of shower simulations. Additionally, at low energies, particularly in thick targets, the effect
of multiple scattering of the initiating electrons greatly overwhelms the impact of photon angular
distributions in defining the development of the shower, and the extra effort and computing time
necessary to implement bremsstrahlung angular distribution sample is not worthwhile.
While it was recognized that the above argument may break down for applications involving
thin target bremsstrahlung spectra, it was discovered that the above assumption about multiple
scattering dominance does not apply even for thick targets at low energies (10 MeV or so) for
narrow beams such as those employed in some medical linacs for producing photon beams for
radiotherapy[54]. Thus, options for more accurate modeling of bremsstrahlung scattering angles in
EGS have been developed.
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