positrons

Positrons are the antimatter counterparts of electrons. They have the same mass and spin as electrons but carry a positive electric charge. The symbol e+ is commonly used. Positrons were predicted by Paul Dirac in 1928 as solutions to the Dirac equation and were discovered in 1932 by Carl D. Anderson in cosmic ray experiments.

Positrons have the same mass as electrons (approximately 9.109 × 10^-31 kg) and a positive charge of +1e. They annihilate when they encounter electrons, producing gamma radiation, typically two 511 keV photons emitted in opposite directions. The annihilation rate and spectrum depend on the surrounding material and the formation of bound states such as positronium.

Positrons can form a hydrogen-like bound state with electrons called positronium. It exists in two spin configurations: parapositronium (singlet) with a lifetime of about 125 picoseconds and orthopositronium (triplet) with a lifetime of about 142 nanoseconds in vacuum; lifetimes are shortened in matter due to interactions with surrounding electrons.

Natural and artificial production: Positrons are produced in beta+ decay of proton-rich radionuclides and in high-energy processes such as cosmic ray interactions and pair production by high-energy photons. In medicine, radionuclides that emit positrons (for example fluorine-18) are used in PET imaging to visualize metabolic processes inside the body.

Applications and significance: Positrons are used in medical imaging (positron emission tomography, PET) and materials science (positron annihilation spectroscopy) to study defects in solids. They also provide a probe for fundamental tests of quantum electrodynamics and antimatter properties.

History: The concept arose from Dirac’s equation; discovery of the positron in 1932 confirmed the existence of antimatter and established a new class of particles.