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 (1905–1991) 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.
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