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 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). The first scientist deemed to have captured positrons through electron-positron annihilation was Chung-Yao Chao, a graduate student at Caltech in 1930, though he did not realize what they were at that time.
Cloud chamber photograph by C.D. Anderson of the first positron ever identified. A 6 mm lead plate separates the upper half of the chamber from the lower half. The positron must have come from below since the upper track is bent more strongly in the magnetic field indicating a lower energy
Positrons may be generated by positron emission radioactive decay (a weak interaction), or by pair production from a sufficiently energetic 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.3 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.
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 (sometimes referred to as PAS) is used in materials research.
New research has dramatically increased the quantity of positrons capable of being created through experimental processes. Physicists at the Lawrence Livemore National Laboratory in California have used a short, ultra-intense laser to irradiate a millimetre-thick gold target and produce more than 100 billion particles of antimatter by causing the electrons to emit energy packets which decay into matter and anti-matter.4
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