can occur when the X-ray photon energy is greater than 1.02
MeV, but really only becomes significant at energies around 10 MeV. Pair
production occurs when an electron and positron are created with the annihilation
of the X-ray photon. Positrons are very short lived and disappear (positron
annihilation) with the formation of two
photons of 0.51 MeV energy. Pair production
is of particular importance when high-energy
photons pass through materials of a high
atomic number.