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| Geant 3.15 | GEANT User's Guide | PHYS220 ##
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Author(s) : G.N.Patrick, L.Urban Submitted: 26.10.84
Origin : GEANT2 Revised: 17.12.92
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|CALL GCOMPI |
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GCOMPI calculates the total cross-section for Compton scattering. It tabulates the mean free path, lambda= ((1)/(Sigma)) (in cm), as a function of the medium and of the energy (see JMATE data structure). The energy binning is set within the array ELOW (common /GCMULO/) in the routine GPHYSI. GCOMPI is called at initialization time by GPHYSI.
The mean free path, lambda, for a photon to interact via Compton scattering is given by
lambda =((1)/ (Sigma))= ((A)/(N rho))((1)/ (sigma(Z,E)))
Av
where:
N Avogadro's number
Av
Z,A atomic and mass number of the medium
rho density of the medium
sigma total cross-section per atom for Compton scattering
E energy of the photon.
For the total cross-section an empirical cross-section formula is used which reproduces the cross-section data rather well down to 10 keV:
2 2 3 -1
sigma(Z, E)= Z[P (Z)((log (1+ 2X))/(X))+ ((P (Z)+ P (Z)X+ P (Z)X )/(1 +aX +bX +cX ))]barn atom
1 2 3 4
where
m = electron mass
X = ((E)/ (m))
2
P (Z) = D +E Z +F Z
i i i i
The values of the parameters are put in the DATA statement within the routine GCOMPI. The fit was made over 511 data points chosen between:
1< =Z<=100 ; 10 keV<=E< =100GeV
The accuracy of the fit is estimated to be:
'10% for E ' 10keV- 20keV
((Deltasigma)/ (sigma))= { .
<=5- 6% for E >20 keV