Positroni

Positroni are the antiparticles of electrons. Each positrone carries a positive electric charge equal in magnitude to that of the electron, has the same rest mass as an electron (about 9.11 x 10^-31 kg or 0.511 MeV/c^2), and a spin of 1/2. As antiparticles, positroni are produced in high-energy processes and annihilate with ordinary matter, producing gamma rays.

The positron was predicted by Paul Dirac and discovered experimentally by Carl D. Anderson in 1932 in cosmic rays. The identification relied on observing particles with electron-like mass but opposite charge. Today positroni are routinely produced in laboratories and in certain radioactive decay processes.

When a positrone encounters an electron, they annihilate, typically yielding two gamma photons of approximately 511 keV emitted in nearly opposite directions. A bound state of an electron and a positron, called positronium, can form briefly; para-positronium decays mainly into two gamma photons, while ortho-positronium decays into three gamma photons with different lifetimes.

Positroni are produced in beta-plus decay (a proton converts to a neutron, emitting a positrone and a neutrino) and by pair production when high-energy photons interact with nuclei. They are used in medicine and science, most notably in positron emission tomography (PET), where positron-emitting isotopes such as fluorine-18 enable functional imaging of metabolic processes. Other applications include positron annihilation spectroscopy in materials science and fundamental physics experiments that study antimatter behavior and symmetries.

In astrophysics, positrons are observed in cosmic sources and the interstellar medium, contributing to gamma-ray phenomena and helping probe high-energy processes in the universe.