G3 to G4
Compton scattering

Total cross section and Mean free path

• Geant3
  S(E,Z): parameterisation from L. Urban (total: 15 parameters).
  Fit from 10 keV to 100 GeV/Z.
  but reasonnable behaviour below 10 keV. ---> 100 eV - 100 MeV plot
  

• Geant4 standard
  Copied from Geant3
  

• Geant4 low energy
  Extracted from EPDL97 data set.
  from 250 eV to 100 GeV.
  

Final state

• Geant3
  Sample the gamma energy (and angle) from Klein-Nishima formula.
  Final state from energy-momentum conservation.
  (Eelectron > Ethreshold)
  

• Geant4 standard
  Copied from Geant3
  (Eelectron > Ethreshold) or (range > safety)
  Optional: take account the photon polarisation.
  

• Geant4 low energy
  Sampled from [Klein-Nishima].[Scattering function]
  Scattering function read from EPDL97 data set.
  Final state from energy-momentum conservation.
  (Eelectron > Ethreshold) or (range > safety)