Positron

This article is about the positron particle. For the computer game, see Positron (game).

File:Cloud chamber - visible trace of positron.JPG

The first detection of the positron in 1932 by Carl D. Anderson

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 positron annihilates with an electron, their mass is converted into energy in the form 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 2 m_ec² = 2×0.511 MeV = 1.022 MeV with matter (m_e represents 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 an inevitable consequence of the Dirac equation. In 1932, positrons were observed by Carl D. Anderson, who gave the positron its name. Anderson also unsuccessfully suggested renaming electrons "negatrons." 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 produced in enormous numbers in accelerator physics laboratories and used in electron-positron colliders.

The positron in fiction

The most famous use of the positron in fiction was Isaac Asimov's use in his robots' positronic brains. It is likely that he chose to use positrons because they were relatively newly-discovered when he was first writing about the robots. Perhaps in an homage to Asimov, in the Star Trek universe the android Data, his "brother" Lore, "daughter" Lal, and other sundry related androids also have positronic brains.

In Hideaki Anno's Neon Genesis Evangelion, the positron rifle, based upon the ATHENA tests in Europe, is used to defeat the invading Angels on numerous occasions.

In the Japanese anime sci-fi series Mobile Suit Gundam SEED and its sequel, Mobile Suit Gundam SEED Destiny, large-scale positron cannons are used on some advanced warships for space-faring combat. Some characters in the series have moral issues with the use of positron cannons within the Earth's atmosphere, as the gamma radiation produced by annihilation reactions can irradiate the surrounding environment in the same way that a nuclear explosion can, causing cancers and mutations. Either way, these cannons are typically portrayed as the single most powerful conventional weapon that can be deployed on a battlefield, more than enough to take down anything from a battlecruiser to an entire naval fleet in a single hit. A positron cannon was also deployed to break apart the remains of Junius Seven, a colony destroyed in the war that had fallen out of its orbit and was rapidly descending to Earth.

See also

• electron
• radioactive decay
• list of particles
• positron emission tomography

External links

• What is a Positron? (from the Frequently Asked Questions :: Center for Antimatter-Matter Studies)
• Positron information search at SLAC