The positron, usually denoted , is the electron’s antiparticle: it has the same mass as an electron but the opposite electric charge. It is a constituent of antimatter, rather than a positively charged form of ordinary matter. Discovered by Carl Anderson in 1932, it provided the first experimental identification of an antiparticle. Positrons occur in radioactive processes and high-energy interactions, and their annihilation with electrons supplies the physical basis of positron emission tomography. (home.cern)
Physical properties
A positron carries charge , where is the elementary charge. Its rest mass is approximately kilograms, equivalent to . Its spin quantum number is , the same as the electron’s. Although “positive electron” is a historical description, the positron is a distinct antiparticle, not an electron whose charge has simply been altered. (pdg.lbl.gov)
Within the Standard Model, electrons and positrons belong to the charged lepton family and are fermions. They are treated as elementary particles, with no known internal constituents. Their positive and negative charges allow magnetic fields to distinguish their trajectories, while their equal masses distinguish positrons from much heavier positively charged particles such as protons. (atlas-public.web.cern.ch)
Prediction and discovery
The theoretical background arose from Paul Dirac’s attempt to combine quantum mechanics with special relativity. His 1928 Dirac equation contained solutions that ultimately led to the prediction of an electron counterpart with equal mass and opposite charge. In 1931, Dirac explicitly proposed the antielectron, before experimental evidence had established its existence. (home.cern)
On August 2, 1932, Anderson photographed a particle track produced by cosmic radiation in a cloud chamber at the California Institute of Technology. A magnetic field curved the track, while a lead plate slowed the particle. The increase in curvature after passage through the plate established its direction of travel; combined with the direction of bending, this showed that the particle was positively charged. Its properties identified it as a positive electron rather than a proton. (digital.archives.caltech.edu)
Patrick Blackett and Giuseppe Occhialini reported corroborating evidence in 1933. Anderson received half of the 1936 Nobel Prize in Physics for discovering the positron; the other half went to Victor Hess for discovering cosmic radiation. (cds.cern.ch)
Production mechanisms
One source of positrons is the positive branch of beta decay, written . In an energetically permitted nuclear transition, a proton within an unstable nucleus is converted into a neutron, with emission of a positron and an electron neutrino:
This notation describes a transformation within the nucleus; the available energy depends on the masses and binding energies of the initial and final nuclear systems. The positron is created in the transition, rather than released from a pre-existing store inside the nucleus. (energy.gov)
Positrons also arise through pair production. A sufficiently energetic photon interacting with matter can produce an electron–positron pair. The pair’s combined rest energy is approximately , setting the characteristic threshold scale for conversion in a nuclear field. Energy must also account for recoil and the particles’ kinetic motion. Pair production connects the creation of matter and antimatter directly with mass–energy equivalence. (geant4.web.cern.ch)
Annihilation and positronium
In matter, an emitted positron generally loses kinetic energy through successive interactions before undergoing electron–positron annihilation. In the common two-photon process, the electron and positron disappear and their energy is carried away by photons. For a pair approximately at rest, each photon has an energy near 511 keV, and the photons travel in nearly opposite directions. The process conserves energy and momentum; it does not destroy energy. (www-pub.iaea.org)
Before annihilation, a positron may bind with an electron to form positronium, an electrically neutral, short-lived system. Its annihilation behavior depends on the combined spin state. Three-photon decay is characteristic of ortho-positronium, whereas two-photon annihilation occurs in the singlet state. Positronium also forms in voids within some materials, making its lifetime useful for probing microscopic structure. (geant4.web.cern.ch)
Scientific and technological uses
Positron emission tomography (PET) uses radioactive tracers that emit positrons. Detectors record pairs of annihilation photons arriving within a short timing interval. Each accepted coincidence identifies a line along which annihilation occurred; many such measurements permit reconstruction of the tracer distribution. PET therefore detects annihilation radiation rather than directly photographing positrons. The distance between positron emission and annihilation, together with the photons’ slight departure from exact opposition, contributes to limits on spatial resolution. (www-pub.iaea.org)
In materials research, positron annihilation lifetime spectroscopy probes defects, voids, and free volume on subnanometre scales. The measured interval between positron injection and annihilation reflects the local electronic environment; in suitable polymer systems, longer positronium lifetimes correspond to larger voids. (nist.gov)
Accelerated positron beams also serve as collision partners for electrons. CERN’s Large Electron–Positron Collider operated from 1989 to 2000, producing large samples of Z bosons and, after an energy upgrade, W-boson pairs. These collisions enabled detailed tests of the electroweak interaction and the Standard Model through measurements of particle production and decay. (home.cern)