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| Geant 3.16 | GEANT User's Guide | PHYS350 ##
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Author(s) : L.Urban Submitted: 26.10.84
Origin : Same Revised: 19.12.92
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|CALL GANNII |
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GANNII tabulates the mean free path at initialization time as a function of the medium and of the energy (see JMATE data structure). The energy binning is set within the array ELOW (common block /GCMULO/) in the routine GPHYSI. GANNII is called from GPHYSI.
The mean free path, lambda, for a positron to interact via annihilation is
lambda =((1)/ (sum ))= ((A)/(Nrhosigma(Z,E))) (1)
where
For the annihilation cross-section the formula of Heitler is used. The total cross-section is given by [bib-EGS3], [bib-HEIT]:
2 2 2 2 2
sigma(Z, E)= ((Zpir )/(gamma+1))[((gamma +4gamma+1)/ (gamma -1))ln(gamma+sqrt(gamma - 1))-((gamma+3)/(sqrt(gamma -1)))](2)
0
where r is the classical radius of the electron and gamma= E/m. In the 0
case of a compound/mixture the effective atomic number and weight (evaluated in GSMIXT are used in computing the cross-section. This treatment of the compounds/mixtures is correct in those processes where the cross-section depends on the atomic number Z linearly (annihilation, Moeller and Bhabha scattering, Compton scattering).