G3 to G4
e+ e- annihilation

Total cross section and Mean free path

• Geant3
  S(E,Z): Heitler formula.
  

• Geant4 standard
  Copied from Geant3
  

Final state

• Geant3
  if e+ not at rest: sample the gamma energies from Heitler formula.
  Final state from energy-momentum conservation.
  

• Geant4 standard
  Adapted from Geant3
  if e+ at rest : trivial AtRestDoIt() method.