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...adapted from euronuclear.org
