THE EGS5 CODE SYSTEM

particle on its very first track, its initial multiple scattering step is automatically set to be that
used for the smallest characteristic dimension treated in the data set. (This in effect is usually the
distance corresponding to the smallest value of K 1 for which the Molière distribution is defined.)
Subsequent multiple scattering steps for such particles are then taken to be the minimum of twice
the previous step and the default step given the characteristic dimension of the problem. This
approach is still approximate, however, and may be replaced by a single scattering model.
2.16 Photoelectric Effect
The total photoelectric cross sections used in the standard version PEGS4 were taken from Storm
and Israel[167]. In PEGS5 we use the more recent compilation in the PHOTX library[131], as
originally implemented by Sakamoto[143] as a modification to PEGS4. PHOTX provides data for
elements 1 through 100 in units of barns/atom, and PEGS subroutine PHOTTZ computes
˘Σ photo,partial (Z, ˘k) = N (
aρ 1 × 10 −24
M X cm 2 )
0
σ photo (Z,
barn
˘k) (barns) , (2.394)
where σ photo (Z, ˘k) is obtained by using PEGS function AINTP to do a log-log interpolation in energy
of the cross sections in the data base. The total cross section, as computed by PEGS routine PHOTTE,
is given by
˘Σ photo (˘k) =
N e ∑
i=1
p i ˘Σphoto,partial (Z i , ˘k) . (2.395)
This total photoelectric cross section is then used in the computation of the photon mean free path.
The run-time model of the photoelectric effect was rather simple in early versions of EGS,
which treated photoelectric event directly within subroutine PHOTON. Starting with EGS3, however,
subroutine PHOTO was created to provide flexibility in modeling the energies and angles of the ejected
secondary electrons. Since some of the photon energy imparted in a photoelectric absorption is
consumed in ejecting the electron from its orbit, the kinetic energy of a photoelectron is given by
the difference between the incident photon energy and the edge energy of the electron’s sub-shell.
For applications involving high energy gammas, the edge energy is negligible, but for applications
involving photons with energies on the order of several hundred keV or less, treating sub-shell edge
energies can be important. Thus the default version of EGS4 provided the weighted average K-edge
energy given by
∑ Ne
i=1 ˘Ē K−edge =
p i ˘Σ photo (A P )ĔK−edge(Z i )
∑ Ne
i=1 p i ˘Σ
. (2.396)
photo (A P )
In this implementation, photoelectrons are created with total energy
Ĕ = ˘k − ˘Ē K−edge + m, (2.397)
provided, of course, that the initial photon energy is greater than ˘Ē K−edge . To preserve the energy
balance, a photon of energy ˘Ē K−edge is created and then forcibly discarded. Photoelectric inter-
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