THE EGS5 CODE SYSTEM

actions involving photons with energies below ˘Ē K−edge are treated as being completely absorbed,
discarded by a call to user routine AUSGAB with IARG=4.
For applications involving energies on the order of the K-edge energy of the materials being
modeled, this treatment is not suitable. Thus, provided with the EGS4 distribution as part of a
sample user code, was a substitute version of PHOTO[45] which allowed for more explicit modeling
of K-shell interactions, including the generation of K α1 and K β1 fluorescent photons. This version
of PHOTO is the basis for the much more generalized version of PHOTO which has become the default
in EGS5.
The microscopic photoelectric absorption cross section σ photo (Z, ˘k) of Equation 2.394 is actually
the sum over all the constituent atomic sub-shells of the cross section for each sub-shell s which
has edge energy less than ˘k, as in
σ photo (Z, ˘k) = ∑ s
σ s photo(Z, ˘k) (2.398)
where σ s photo (Z, ˘k) is the photoelectric cross section for sub-shell s of element Z at energy ˘k.
Evaluation of cross sections near absorption edges The energy dependence of gamma cross
section is modeled in PEGS and EGS using a piece-wise linear fit, which can result in large errors in
the vicinity of photon absorption edges. For example, material data created by PEGS for element
copper with UP=1.0 MeV,AP=0.001 MeV and 200 energy bins produces errors of 60% and 79% in
the gamma mean-free path (GMFP) at the energies of the Cu K β1 and K β2 x-rays as shown in
Table 2.5. From Figure 2.21, we can see that the linearly fitted gamma mean free path (GMFP)
differs significantly from its exact value in the 64th energy interval in the fitted data, which contains
the absorption edge. This GMFP error leads to an underestimation of K β x-ray production by a
factor of 2, as shown in Figure 2.22.
To circumvent this problem, a method called the “local extrapolation method” (LEM), has
been devised to specially treat energy intervals containing absorption edges [121]. For such energy
intervals, an extrapolation is performed using either the next higher or next lower energy bin,
depending on whether the gamma energy is higher or lower than the edge energy. also employed.
The gamma mean free path of Cu at K β1 and K β2 energies evaluated using LEM agree with
exact values to within 1% as shown in Table 2.5. Figure 2.21 shows that accurate prediction of
the intensity of the characteristic x-rays can be achieved by using LEM. EGS5 employs the LEM
method for K-, L1-, L2- and L3- edges by default.
2.16.1 General Treatment of Photoelectric-Related Phenomena
Accurate modeling of the energy of ejected photoelectrons in the general case thus requires resolving
the total photoelectric cross section σ photo (Z, ˘k) for a given species into the appropriate sub-shell
cross sections, so that the correct sub-shell atomic binding energy can be substituted for ˘Ē K−edge in
121