positronelectron

Positronelectron, often described as the electron–positron pair, refers to the two-particle system formed by an electron and its antiparticle, the positron. In practice the term covers both free electron–positron pairs produced in high-energy processes and the bound state known as positronium. The interaction between the two particles is governed by quantum electrodynamics and, when they meet, they can annihilate or form a bound system.

When not bound, a positron and an electron annihilate into photons. The dominant channel in the center-of-mass frame is two-photon annihilation, producing gamma rays of 511 keV each in opposite directions. Depending on the quantum state and kinematics, three-photon annihilation can also occur, especially for certain spin configurations. The resulting photons carry away the rest mass energy of the pair, converted into radiation.

Positronium is the hydrogen-like bound state of an electron and a positron. There are two ground-state configurations: parapositronium (singlet spin) and orthopositronium (triplet spin). Parapositronium decays primarily into two gamma photons with a lifetime around 125 picoseconds, while orthopositronium decays into three gamma photons with a longer lifetime near 142 nanoseconds. The binding energy of positronium is about 6.8 eV, and its Bohr radius is roughly 1.058 angstroms, reflecting the reduced mass of the system being half that of hydrogen.

Positronelectron pairs are produced in high-energy collisions and gamma-gamma interactions, and they play roles in areas ranging from medical imaging, via positron emission tomography, to materials science and astrophysical plasmas. See also: electron, positron, positronium, pair production, annihilation radiation.