Positron

Positron

Infobox Particle
bgcolour =
name = Positron (antielectron)


caption =
num_types =
composition = Elementary particle
family = Fermion
group = Lepton
generation = First
interaction = Gravity, Electromagnetic, Weak
antiparticle = Electron
theorized = Paul Dirac, 1928
discovered = Carl D. Anderson, 1932
symbol = SubatomicParticle|Beta+, SubatomicParticle|Positron
mass = val|9.1093826|(16)|e=-31|u=kg [http://www.britannica.com/eb/article-9061025/positron positron - Britannica Online Encyclopedia ] ]
11836.15267261(85) u
val|0.510998918|(44)|u=MeV/c2 [ [http://scienceworld.wolfram.com/physics/ElectronMass.html Electron Mass - from Eric Weisstein's World of Physics ] ]
decay_time =
decay_particle =
electric_charge = val|1.602176462|(63)|e=-19|u=C
color_charge =
spin = ½
num_spin_states =
The positrons 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.

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. [cite journal |last=Anderson |first=Carl D. |authorlink= |coauthors= |year=1933 |month= |title=The Positive Electron |journal=Physical Review |volume=43 |issue=6 |pages=491–494 |doi=10.1103/PhysRev.43.491 |url= |accessdate= |quote= ] 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.

See also

* Beta particle
* Radioactive decay
* List of particles
* Positron emission tomography
* Electron

References

External links

* [http://www.positron.edu.au/faq.html What is a Positron?] (from the Frequently Asked Questions :: Center for Antimatter-Matter Studies)
* [http://www-search.slac.stanford.edu:8765/query.html?qt=positron Positron information search at SLAC]
* [http://positronannihilation.net Positron Annihilation as a method of experimental physics used in materials research.]


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