Annihilation is the process in which the entire mass of two colliding particles,
one of matter and one of antimatter, is converted
into radiant energy in the form of gamma
rays or mesons. In the case of a collision
between an electron and a positron (anti-electron), which have a combined rest
mass of 1.02 MeV, the result is two gamma rays each with a characteristic
energy of 0.511 MeV. When nucleons annihilate
each other the result is mesons.