Pair Production
Pair Production is the production of a pair of mutual antiparticles, an electron and a positron. This process only takes place for photon energies above1.02 MeV, the minimum energy required to create the two particles (according to the E=mc2 Einstein mass formula). The majority of gamma rcoming out of radioactive decay sdo not reach this threshold. Pair production occurs mainly with more energetic gamma produced by accelerators or found in cosmic rays.