THE EGS5 CODE SYSTEM

Equation 2.397. Additionally, the vacancies created in atomic sub-shells subsequent to the ejection
of photoelectrons can give rise to either characteristic x-rays or Auger electrons (and additional
vacancies in lower energy sub-shells) when the atom de-excites. Modeling the photoelectric effect
with this level of detail is crucial in many low-energy applications, such as the simulation of detector
response in at low energies.
A general treatment of photoelectric-related phenomena in elements, compounds and mixtures
was introduced into EGS4, and an improved method has been implemented in EGS5 by Hirayama
and Namito[72, 73]. K-, L1-, L2-, L3- and other sub-shell photoelectric cross sections taken from
the PHOTX data base are fitted to cubic functions in log-log form,
ln(σ s photo(Z, ˘k)) = M s 0(Z) + M s 1 (Z) ln(˘k) + M s 2 (Z) ln(˘k) 2 + M s 3 (Z) ln(˘k) 3 . (2.399)
It thus becomes possible to calculate the ratios of sub-shell photoelectric cross sections for each
sub-shell of each constituent element of any compound or mixture quickly and accurately inside
EGS, rather than approximately via the piece-wise linear fits supplied by PEGS. Once the correct
element and sub-shell have been determined (by sampling the discrete distributions of the branching
ratios), the photoelectron energy is given by
Ĕ = ˘k − Ĕs−edge(Z) + m , (2.400)
given that Ĕs−edge(Z) is the binding energy of the s-shell of element Z. Since the sub-shell vacancy
is thus known, atomic relaxation can be modeled and additional secondary particles generated
based on fluorescence and Auger transition probabilities and energies.
The fitted coefficients M0 s(Z), M 1 s(Z), M 2 s(Z) and M 3 s (Z) and the other associated data required
to model sub-shell level photoelectric effect and secondary particles from atomic relaxation are
initialized in a new BLOCK DATA subprogram of EGS5. The full data set required and the sources
for the data in EGS5 are given in Table 2.6.
Of the more than 50 possible transitions which may occur during the relaxation of L-shell
vacancies, 20 of the most important can be modeled in EGS5. All have relative intensities larger
than 1% of the L α1 transition for Fermium (Z = 100). Table 2.7 lists the atomic transitions which
produce these x-rays, along with their energies and their intensities relative to the L α1 line for lead.
Table 2.5: GMFP of Cu at K β1 (8.905 keV) and K β2 (8.977 keV) energies.
GMFP (µm)
K β1 (Error) K β2 (Error)
Exact ∗ 29.84 30.53
PWLF 11.80 (-60%) 6.295 (-79%)
PWLF-LEM 29.69 (-0.5%) 30.30 (-0.8%)
∗ Obtained using CALL option of PEGS.
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