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For other uses, see {{PAGENAME}} (disambiguation). The positron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1, a spin of 1/2, and the same mass as an electron. When a low-energy positron collides with a low-energy electron, annihilation occurs, resulting in the production of two gamma ray photons (see electron-positron annihilation). A positron may be generated by positron emission radioactive decay, or the interaction of photon with a charged particle (such as an atom's nucleus) with energy greater than 2mec2 = 2×0.511 MeV = 1.022 MeV with matter (me is the mass of one electron and c is the speed of light in vacuum). This process is called pair production, as it generates one electron and one positron from the energy of the photon. The existence of positrons was first postulated in 1928 by Paul Dirac as a consequence of the Dirac equation. In 1932, positrons were discovered by Carl D. Anderson, who gave the positron its name. The positron was the first evidence of antimatter and was discovered by passing cosmic rays through a gas chamber and a lead plate surrounded by a magnet to distinguish the particles by bending differently charged particles in different directions. Today, positrons are routinely produced in positron emission tomography (PET) scanners used in hospitals and in accelerator physics laboratories used in electron-positron collider experiments.
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