Positron is a positively charged subatomic particle having the same mass and magnitude of charge as the electron and constituting the antiparticle of a negative electron. Positrons were the first of the antiparticles to be predicted and discovered. P.A.M. Dirac postulated their existence (1930-31) to account for the otherwise superfluous negative energy states of the electron that were predicted by his relativistic electron theory (1928), and Carl David Anderson established their existence (1932) while studying cloud-chamber photographs of cosmic rays.
Stable in a vacuum, positrons quickly react with the electrons of ordinary matter by annihilation to produce gamma radiation. Positrons are emitted in the positive beta decay of proton-rich (neutron-deficient) radioactive nuclei and are formed in pair production, in which the energy of a gamma ray in the field of a nucleus is converted into an electron-positron pair.
Excerpt from the Encyclopedia Britannica without permission.