Gamma-ray photons with energy greater than 1.02
MeV
may interact with a nucleus
to form an electron-positron pair. This amount of energy is just sufficient to
provide the rest masses of the electron and
positron (0.51 MeV each). Excess
energy will be carried away equally by these two particles which produce
ionisation as they travel in the material. The
positron is eventually captured by an electron and
annihilation of the two particles occurs. This
results in the release of two photons each of 0.51 MeV
known as annihilation
radiation . These two photons then lose
energy by Compton scattering or the photoelectric
effect . Pair production is
illustrated in Fig. 2.9
.
