THE EGS5 CODE SYSTEM

Table 2.8: Total cross section (10 −24 cm 2 /molecule) for coherent scattering from water.
Photon Energy Free O + Free H 2 O Liquid
(keV) 2 Free H (b) Molecule Water
20 2.65 2.92 2.46
60 0.417 0.444 0.392
100 0.161 0.170 0.151
(a ) From Johns and Yaffe [84]. Note that effects on scattering angle are more dramatic than for total cross
sections.
(b) Default values used in EGS/PEGS.
2.17 Coherent (Rayleigh) Scattering
The total coherent (Rayleigh) scattering cross sections used in EGS are from Storm and Israel[167]
and are available for elements 1 through 100. As with the photoelectric effect cross sections, the
data file is in units of barns/atom. The PEGS routine COHETZ computes
˘Σ coher,partial (Z, ˘k) = N ( )
aρ 1 × 10 −24
M X cm 2
0
σ coher (Z,
barn
˘k) (barns) , (2.405)
where σ coher (Z, ˘k) is obtained by using the PEGS function AINTP to do a log-log interpolation in
energy of the Storm and Israel cross sections. To obtain the total cross section, we treat all the
atoms as if they act independently. That is, in PEGS routine COHETM we compute
˘Σ coher (˘k) =
N e ∑
i=1
p i ˘Σcoher,partial (Z i , ˘k) . (2.406)
To permit PEGS to retain complete generality when treating compounds and mixtures, the
independent atom approximation is also used when calculating form factors for coherent scattering.
However, as shown in Table 2.8, this assumption is known to be poor, as both the molecular
structure (row 3 of Table 2.8) and the structure of the medium (row 4 of Table 2.8) can affect
coherent scattering [110, 84]. For simulations involving problems in which the correlation effect
between molecules on coherent scattering is important, PEGS5 provides for the direct input of
interference coherent cross sections and form factors (if such data is available). Interference coherent
cross sections and form factors for selected materials, including water, are provided in the EGS5
distribution, and the procedure for invoking this option is described in Appendix C of this report,
The PEGS5 User Manual.
Rather than passing ˘Σ coher (˘k) to the EGS code, PEGS passes the ratio ( ˘Σ tot − ˘Σ coher )/˘Σ tot ,
which EGS uses as a correction factor to include coherent scattering only if the user requests it.
Let us now develop a method for sampling the coherent scattering angle.
coherent scattering cross section is given by
The differential
dσ R (θ)
dΩ = r2 0
2 (1 + cos2 θ)[F T (q)] 2 , (2.407)
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