n.
Subatomic particle having the same mass as an electron but with an electric charge of +1 (an electron has a charge of -1).
It constitutes the antiparticle (see antimatter ) of an electron. The existence of the positron was a consequence of the electron theory of P. A. M. Dirac (1928), and the particle was discovered in cosmic rays by Carl D. Anderson (19051991) in 1932. Though they are stable in a vacuum, positrons react quickly with the electrons of ordinary matter, producing gamma rays by the process of annihilation . They are emitted in positive beta decay of proton-rich radioactive nuclei and are formed in pair production .