Upon the collision of a particle and an anti-particle, e.g. electron and positron, these are “annihilated” as particles and the mass of these particles converted into energy. electron and positron have a rest mass which is together equal to an energy of 1.02 MeV. Upon the “annihilation” of both particles, two gamma quanta of 0.511 MeV each are generated.

Occurrence of annihilation radiation upon collision of electron and positron. Two gamma quanta of 0.511 MeV each result.