GPAIRG generates -pair production by photons. It uses a modified version of the random number techniques of Butcher and Messel [] to sample the secondary electron/positron energies from the Coulomb corrected Bethe-Heitler differential cross-section. For the angular distribution of the pair, it calls the function GBTETH.
Input: via COMMON /GCTRAK/
Output: via COMMON /GCKING/
GPAIRG is called from GTGAMA, when the photon reaches its pair production point.
GBTETHVALUE = GBTETH(ENER,PARTM,EFRAC) GBTETH calculates the angular distribution of the -pair for pair production and of the photon for bremsstrahlung. In case of -pair production by photons, it gives the scaled angle for an electron (mass PARTM) of energy ENER which is EFRAC times the initial energy of the photon. GBTETH is called by GPAIRG.