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The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1, a spin of , 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 or more gamma ray photons (see electron-positron annihilation). The existence of positrons was first postulated in 1928 by Paul Dirac as a consequence of the Dirac equation.

Positrons may be generated by positron emission radioactive decay (through weak interactions), or by pair production from a sufficiently energetic photon.

History

The first scientist deemed to have detected positrons through electron–positron annihilation was Chung-Yao Chao, a graduate student at Caltechmarker in 1930, though he did not realize what they were at that time. Positrons were discovered in 1932 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 cloud chamber and a lead plate surrounded by a magnet to distinguish the particles by bending differently charged particles in different directions. The positron was theoretically predicted by the Dirac equation in 1928, although Paul Dirac himself was slow to accept that the observed positron was actually the particle predicted by his equation.

Today, positrons, created through the decay of a radioactive tracer, are detected in positron emission tomography (PET) scanners used in hospitals and in accelerator physics laboratories used in electron–positron collider experiments. In the case of PET scanners, positrons provide a mechanism to show areas of activity within the human brain. In addition to the two above-mentioned applications of positrons in medicine and fundamental physics, an experimental tool called positron annihilation spectroscopy (PAS) is used in materials research.

New research has dramatically increased the quantity of positrons that experimentalists can produce. Physicists at the Lawrence Livermore National Laboratorymarker in California have used a short, ultra-intense laser to irradiate a millimetre-thick gold target and produce more than 100 billion positrons.

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