positroner

Positroner, commonly called positrons in English, are the antimatter counterparts of electrons. They have the same mass as electrons but carry a positive electric charge and half-integer spin. As antiparticles, positrons annihilate with electrons to produce gamma rays, typically two photons of 511 keV emitted in opposite directions in the center-of-mass frame.

Positrons were predicted by Paul Dirac in 1928 from the Dirac equation and were discovered experimentally in 1932 by Carl Anderson using cosmic-ray tracks.

They are produced in beta-plus decay of proton-rich radioactive isotopes, in high-energy photon interactions (pair production) near nuclei, and in particle accelerators. In medicine, cyclotrons produce positron-emitting isotopes used for PET imaging, such as fluorine-18.

Positronium is a short-lived bound state of an electron and a positron. It exists in two spin configurations: para-positronium and ortho-positronium, with lifetimes of about 125 picoseconds and 142 nanoseconds in vacuum, respectively. Annihilation yields gamma rays, with energy roughly 511 keV per photon in the center-of-mass frame.

Applications include medical imaging with positron emission tomography (PET), materials science via positron annihilation spectroscopy to study defects and voids, and fundamental physics experiments examining antimatter interactions and symmetries.

Safety and handling follow radiation-protection procedures. The energy released during annihilation is contained within a small region, and clinical use relies on short-lived isotopes, controlled dosing, and shielding to minimize exposure.

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