G3 to G4
Gamma conversion

Total cross section and Mean free path

• Geant3
  S(E,Z): parametrisation from L. Urban (total: 18 parameters).
  Fit from 1.5 MeV to 100 GeV. Constant above 100 GeV.
  

• Geant4 standard
  Copied from Geant3
  

• Geant4 low energy
  Extracted from EPDL97 data set.
  from 1.2 MeV to 100 GeV.
  

Final state

• Geant3
  Sample the energy pair from Bethe-Heitler formula with Coulomb correction.
  (use an effective Z for the medium)
  Angle sampled from an energy-angle distribution derived from Tsai formula.
  e+ e- assumed to have a symetric angular distribution.
  (E+ and E- > Ethreshold)
  

• Geant4 standard
  Choose randomly an element within material.
  Sample the energy pair from Bethe-Heitler (adapted from Geant3).
  Final state as in Geant3.
  (E+ and E- > Ethreshold) or (range > safety)
  

• Geant4 low energy
  As in Geant4 standard.