# Positron

The positron (symbol e⁺) is the antiparticle of the electron: an elementary particle with the same mass as the electron but a positive electric charge of +1, where the electron carries −1.<sup>[1](https://opendata.cern.ch/glossary/Electron)</sup> It was the first antiparticle ever discovered, found in cosmic-ray tracks by [Carl D. Anderson](https://www.edgepedia.org/carl-d-anderson) in 1932, and it remains the antiparticle most often met in everyday technology, since its annihilation with electrons is the signal on which positron emission tomography (PET) medical imaging is built.<sup>[2](https://www.aps.org/apsnews/2004/08/discovery-positron-1932)</sup><sup> • </sup><sup>[3](https://www-pub.iaea.org/MTCD/Publications/PDF/Pub1642web-16821314.pdf)</sup>

| Key fact | Detail |
|---|---|
| Identity | Antiparticle of the electron; charge +1, mass equal to the electron's, about 0.511 MeV/c², roughly 1/1836 of a proton's mass<sup>[1](https://opendata.cern.ch/glossary/Electron)</sup> |
| Prediction | Implied by Paul Dirac's 1928 relativistic equation for the electron, which had solutions for a particle with the electron's mass but opposite charge<sup>[4](https://home.cern/science/physics/antimatter/?from=NCCE)</sup> |
| Discovery | August 1932, by Carl D. Anderson in a cloud chamber studying cosmic rays at Caltech<sup>[2](https://www.aps.org/apsnews/2004/08/discovery-positron-1932)</sup> |
| Recognition | Nobel Prize in Physics 1936, divided equally between Anderson (positron) and Victor Franz Hess (cosmic radiation)<sup>[5](https://www.nobelprize.org/prizes/physics/1936/summary/-/)</sup> |
| Annihilation | A positron meeting an electron converts their rest mass into energy, typically two 511 keV photons emitted almost exactly 180° apart<sup>[6](https://www-pub.iaea.org/MTCD/Publications/PDF/P1861_web.pdf)</sup> |
| Natural production | Cosmic-ray interactions, pair production by photons above 1.022 MeV, and beta-plus decay of proton-rich nuclei<sup>[7](https://technav.ieee.org/topic/positrons/)</sup> |
| Main application | PET imaging, in which coincident detection of the two annihilation photons locates the decay of a positron-emitting tracer<sup>[3](https://www-pub.iaea.org/MTCD/Publications/PDF/Pub1642web-16821314.pdf)</sup> |

## Prediction and discovery

In 1928 the British physicist [Paul Dirac](https://www.edgepedia.org/paul-dirac) wrote down an equation combining quantum theory and special relativity to describe an electron moving at relativistic speed. The equation, which won Dirac the 1933 [Nobel Prize in Physics](https://www.edgepedia.org/nobel-prize-in-physics), had two kinds of solutions: one for an electron with positive energy, and one for a particle with negative energy. Physicists interpreted the second solution as an "anti-electron", a particle with the electron's mass but opposite charge.<sup>[4](https://home.cern/science/physics/antimatter/?from=NCCE)</sup> Dirac made the prediction of the antielectron explicit in 1931.<sup>[8](https://alpha.web.cern.ch/sites/default/files/2020-05/Tests_of_Fundamental_Symmetries_with_Trapped_Antihydrogen_v2.pdf)</sup>

The experimental discovery came from a different direction. From 1930, Carl D. Anderson, then a postdoctoral researcher at the [California Institute of Technology](https://www.edgepedia.org/california-institute-of-technology), studied cosmic rays with a cloud chamber, an instrument in which charged particles leave trails of droplets that curve in a magnetic field according to the particle's charge and energy. To tell negatively charged particles moving one way from positively charged ones moving the other, he placed a 6-millimeter-thick lead plate across the chamber's center: a particle passing through loses energy, so its track curves more sharply on the far side, revealing the direction of travel.<sup>[9](https://physics.aps.org/story/v17/st5)</sup>

In August 1932 Anderson recorded the photograph that settled the question: a positively charged particle of about the electron's mass, traveling upward through the plate. Of 1,300 cloud-chamber photographs he found 15 such tracks, and he could rule out protons, because a proton with the energy implied by the observed curvature would stop within a few millimeters, while these tracks ran for centimeters. Anderson's full paper in *Physical Review* argued the particles carried one positive unit of charge and had a mass no more than 20 times the electron's; the name "positron" followed a journal editor's suggestion. Anderson apparently did not know of Dirac's 1931 prediction when he made the find, and he later insisted the discovery was wholly accidental.<sup>[9](https://physics.aps.org/story/v17/st5)</sup>

Confirmation came quickly. On learning of Anderson's result, Patrick Blackett and [Giuseppe Occhialini](https://www.edgepedia.org/giuseppe-occhialini) at Cambridge ran cloud-chamber experiments that photographed positive and negative electrons being produced simultaneously in cosmic-ray collisions, and announced the pairs as direct evidence for Dirac's antiparticle, a conclusion Anderson accepted.<sup>[9](https://physics.aps.org/story/v17/st5)</sup> The 1936 Nobel Prize in Physics was divided equally between Anderson for the positron and Victor Franz Hess for the discovery of cosmic radiation itself.<sup>[5](https://www.nobelprize.org/prizes/physics/1936/summary/-/)</sup>

## Production

Positrons arise through three main routes.<sup>[7](https://technav.ieee.org/topic/positrons/)</sup>

**Beta-plus decay.** A nucleus with too many protons can convert a proton into a neutron, emitting a positron and a neutrino. The reaction must supply the rest energy of the created positron, so the decay energy must exceed twice the electron's rest energy, 1.022 MeV; where it does not, proton-rich nuclei decay instead by electron capture, a closely related process.<sup>[10](https://www.physicsbook.gatech.edu/Radioactive_Decay_Processes)</sup> The first artificial positron emitter was found by Jean Frédéric Joliot-Curie in 1934, who observed phosphorus-30 emitting positrons with a half-life of about three minutes.<sup>[11](https://escholarship.org/uc/item/3w1281sp.pdf)</sup> Short-lived emitters such as fluorine-18, carbon-11, nitrogen-13, and oxygen-15 underpin nuclear medicine, while longer-lived isotopes such as sodium-22 serve laboratory work.<sup>[11](https://escholarship.org/uc/item/3w1281sp.pdf)</sup>

**Pair production.** A photon with energy of at least 1.022 MeV, the combined rest energy of an electron and a positron, can convert into an electron-positron pair in the strong electric field near an atomic nucleus, which absorbs a small recoil to conserve momentum. Energy above the threshold becomes kinetic energy shared between the two particles.<sup>[12](https://www.sciencedirect.com/topics/physics-and-astronomy/pair-production)</sup> Accelerator facilities use this process to make intense positron beams for collider experiments and materials research.<sup>[7](https://technav.ieee.org/topic/positrons/)</sup>

**Cosmic rays.** Positrons are produced continuously in the upper atmosphere through secondary particle cascades, and it was cosmic-ray showers of both charge signs that first revealed the particle to Anderson.<sup>[7](https://technav.ieee.org/topic/positrons/)</sup>

## Annihilation

When a positron has slowed in matter, it annihilates with an electron, converting the rest mass of both into radiation. For a pair essentially at rest, conservation of energy and momentum requires two photons of 511 keV each, emitted simultaneously in almost exactly opposite directions.<sup>[6](https://www-pub.iaea.org/MTCD/Publications/PDF/P1861_web.pdf)</sup> The positron's life in ordinary matter is short: Dirac's theory predicts an average lifetime of the order of one billionth of a second or less for a positron passing through common substances.<sup>[13](https://www.nobelprize.org/uploads/2018/06/anderson-lecture.pdf)</sup>

Before annihilating, a positron and an electron can briefly form a bound state called positronium, the system of just two point-like particles. Its lifetime is 142 billionths of a second before it annihilates into gamma rays, and because of its simplicity it is a favored testbed for precision measurements.<sup>[14](https://home.cern/aegis-experiment-paves-way-new-set-antimatter-studies-laser-cooling-positronium/)</sup>

## Applications

**PET imaging.** [Positron emission tomography](https://www.edgepedia.org/positron-emission-tomography) relies on radiotracers labeled with a positron-emitting radionuclide, most commonly fluorine-18 attached to the glucose analog fluorodeoxyglucose for cancer diagnosis.<sup>[15](https://www.ib.edu.ar/fisica-exp/images/9/98/IPFN-E_B_Podgorsak.pdf)</sup> The emitted positron travels typically within a sub-millimeter distance in tissue before annihilating, and the two 511 keV photons fly apart at the speed of light, 30 cm/ns. Detectors arranged in a ring register pairs in coincidence within a timing window of 4 to 12 ns, which identifies a line of response along which the decay occurred; because the annihilation is nearly back-to-back, PET needs no mechanical collimator, unlike single-photon imaging.<sup>[3](https://www-pub.iaea.org/MTCD/Publications/PDF/Pub1642web-16821314.pdf)</sup> Time-of-flight PET measures the arrival-time difference of the two photons to localize the decay along that line; with a timing resolution of about 0.5 ns the position uncertainty is about 7 cm, which does not sharpen the image directly but improves the reconstruction.<sup>[3](https://www-pub.iaea.org/MTCD/Publications/PDF/Pub1642web-16821314.pdf)</sup>

**Materials science.** Positron annihilation spectroscopy exploits two positron properties: annihilation with an electron emits two 511 keV gamma rays, and positrons become trapped in vacancy defects in solids, making the technique a probe of vacancy defects and free volumes in materials.<sup>[16](https://conferences.iaea.org/event/403/contributions/35150/attachments/19624/33711/09%20Mod-PMI%202025%2026-28%2005-2025%20V5.pdf)</sup>

**Antimatter research.** Positrons are one of the two ingredients of antihydrogen, the bound state of an antiproton and a positron that experiments at CERN's Antimatter Factory produce to test whether antimatter falls and behaves like matter.<sup>[17](https://home.cern/science/experiments/atrap/)</sup> The ALPHA experiment derives its positrons from a sodium-22 source, whose roughly 2.6-year half-life makes it a convenient long-term supply; the source emits positrons with energies up to about 545 keV, which are slowed and accumulated in traps before use.<sup>[18](https://alpha.web.cern.ch/positron-accumulator)</sup> In 2024 ALPHA reported that cooling the positron cloud by sympathetic cooling with laser-cooled beryllium ions, down to around −266 °C, raised antihydrogen production eightfold, allowing over 15,000 atoms to be accumulated in under seven hours.<sup>[19](https://home.cern/breakthrough-antimatter-production/)</sup> Other experiments use positronium as a transfer medium: AEgIS directs positronium beams into trapped antiprotons, which take up the positron to form antihydrogen, and in 2024 it laser-cooled positronium for the first time, halving a sample's temperature from 380 to 170 kelvin.<sup>[14](https://home.cern/aegis-experiment-paves-way-new-set-antimatter-studies-laser-cooling-positronium/)</sup> The GBAR collaboration reported the first production of an antihydrogen beam by charge exchange of 6.1 keV antiprotons with a positronium cloud, with a production significance of 3 to 4 standard deviations.<sup>[20](https://cds.cern.ch/record/2862642)</sup>

## Positrons in the universe

Antimatter does not occur naturally in bulk, except for single particles produced in nuclear or cosmic-ray interactions.<sup>[4](https://home.cern/science/physics/antimatter/?from=NCCE)</sup> Positrons nonetheless reach Earth steadily as part of the cosmic-ray flux, and their numbers carry astrophysical information: calculations show that positrons from the beta-plus decay chains of the radioactive nuclei nickel-56, titanium-44, and aluminum-26, produced in Galactic supernovae, can account for the observed features of the diffuse 511 keV annihilation radiation measured across the Galaxy by the INTEGRAL/SPI instrument.<sup>[21](https://arxiv.org/abs/0711.3008)</sup> Positrons have even been detected from terrestrial weather: gamma rays from lightning are energetic enough to trigger photonuclear reactions that produce unstable isotopes, whose beta-plus decay yields positrons that annihilate, leaving a characteristic annihilation line in the gamma-ray spectrum.<sup>[22](https://arxiv.org/abs/1711.08044)</sup>

## References

1. [Electron, CERN Open Data Portal glossary](https://opendata.cern.ch/glossary/Electron)
2. [August 1932: Discovery of the Positron, APS News (2004)](https://www.aps.org/apsnews/2004/08/discovery-positron-1932)
3. [IAEA Human Health Series, PET physics (Pub1642)](https://www-pub.iaea.org/MTCD/Publications/PDF/Pub1642web-16821314.pdf)
4. [Antimatter, CERN](https://home.cern/science/physics/antimatter/?from=NCCE)
5. [The Nobel Prize in Physics 1936, NobelPrize.org](https://www.nobelprize.org/prizes/physics/1936/summary/-/)
6. [IAEA Human Health Series, annihilation radiation (P1861)](https://www-pub.iaea.org/MTCD/Publications/PDF/P1861_web.pdf)
7. [Positrons, IEEE Technology Navigator](https://technav.ieee.org/topic/positrons/)
8. [Tests of Fundamental Symmetries with Trapped Antihydrogen, ALPHA/CERN](https://alpha.web.cern.ch/sites/default/files/2020-05/Tests_of_Fundamental_Symmetries_with_Trapped_Antihydrogen_v2.pdf)
9. [Landmarks: The First Antiparticle, Physics (APS)](https://physics.aps.org/story/v17/st5)
10. [Radioactive Decay Processes, Physics Book (Georgia Tech)](https://www.physicsbook.gatech.edu/Radioactive_Decay_Processes)
11. [Positron sources, UC San Diego eScholarship](https://escholarship.org/uc/item/3w1281sp.pdf)
12. [Pair Production, ScienceDirect Topics](https://www.sciencedirect.com/topics/physics-and-astronomy/pair-production)
13. [Carl D. Anderson, Nobel Lecture (1936)](https://www.nobelprize.org/uploads/2018/06/anderson-lecture.pdf)
14. [AEgIS experiment paves the way for new set of antimatter studies by laser-cooling positronium, CERN (2024)](https://home.cern/aegis-experiment-paves-way-new-set-antimatter-studies-laser-cooling-positronium/)
15. [Radiation Physics for Medical Physicists, Podgorsak](https://www.ib.edu.ar/fisica-exp/images/9/98/IPFN-E_B_Podgorsak.pdf)
16. [Positron Annihilation Spectroscopy for vacancy defects, IAEA conference presentation (2025)](https://conferences.iaea.org/event/403/contributions/35150/attachments/19624/33711/09%20Mod-PMI%202025%2026-28%2005-2025%20V5.pdf)
17. [ATRAP, CERN](https://home.cern/science/experiments/atrap/)
18. [Positron Accumulator, ALPHA Experiment, CERN](https://alpha.web.cern.ch/positron-accumulator)
19. [Breakthrough in antimatter production, CERN (2024)](https://home.cern/breakthrough-antimatter-production/)
20. [Production of antihydrogen atoms by 6 keV antiprotons through a positronium cloud, CERN Document Server (2023)](https://cds.cern.ch/record/2862642)
21. [The Galactic Positron Annihilation Radiation and the Propagation of Positrons in the Interstellar Medium, arXiv](https://arxiv.org/abs/0711.3008)
22. [Photonuclear Reactions in Lightning Discovered from Detection of Positrons and Neutrons, arXiv](https://arxiv.org/abs/1711.08044)

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*Topic: Encyclopedia › Physical world and mathematics › Physics › Particles and nuclei › Particle physics › Standard Model particle content › Quarks and leptons › Charged leptons (electron, muon, tau)*

*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*

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