positron

A positron is the elementary particle with the symbol e+; it is the antiparticle of the electron. It has the same mass as the electron but opposite electric charge, +1e, and a half-integer spin of 1/2, making it a lepton in the same family as the electron. Its existence was predicted by the Dirac equation and discovered experimentally by Carl D. Anderson in 1932 in cosmic-ray studies.

When a positron encounters an electron, the two particles annihilate, converting their mass into energy in the form of gamma rays. In the rest frame of the annihilating pair, the dominant outcome is two 511 keV photons emitted in nearly opposite directions. If the positron forms a short-lived bound state with an electron, called positronium, the annihilation can proceed through two photons (para-positronium) with a lifetime of about 125 picoseconds, or three photons (ortho-positronium) with a lifetime of about 142 nanoseconds in vacuum.

Positrons are produced in nuclear decay processes (beta-plus decay) and in high-energy interactions. In medicine, radiotracers that emit positrons, such as fluorine-18, carbon-11, nitrogen-13, and oxygen-15, are used in positron emission tomography (PET) to image metabolic processes. Positrons can also be generated in laboratories by particle accelerators or photon-induced reactions for research purposes.

Applications extend beyond medicine to materials science, where positron annihilation spectroscopy probes microscopic voids and defects, and to fundamental physics and astrophysics, where measurements of positron flux and annihilation radiation inform models of antimatter and cosmic processes.