View source for Annihilation photons

The electromagnetic radiation (photons) emitted as a positron–electron pair undergoes annihilation. Annihilation generally occurs after the positron has lost its initial kinetic energy to the absorbing medium, and two photons, each equivalent in energy to the rest mass of an electron (m0c2 = 0.511 MeV), are created in opposite directions. The number of annihilation photons is taken to be twice the number of positrons emitted in beta-plus decay. ([[ICRP Publication 107]], 2007)

''[[ICRP Glossary]] entry -  June 2019''