orthopositronium

nuclear physics

Written by

The Information Architects of Encyclopaedia Britannica

Last updated

July 1, 2026

Quick Summary

Orthopositronium is a short-lived atom made of an electron and a positron, which is the antiparticle of an electron. It is similar to a hydrogen atom but with a positron instead of a proton.12

There are two forms of positronium: parapositronium and orthopositronium. Parapositronium annihilates into two photons with a mean life of about 0.1 nanoseconds. Orthopositronium annihilates into three photons with a mean life of about 100 nanoseconds.1

Positrons themselves are positively charged subatomic particles with the same mass and charge magnitude as electrons. They were discovered in cosmic rays and are emitted during the beta decay of radioactive nuclei. Positrons are used in medical imaging techniques like positron emission tomography (PET).3

Summary created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

3 Britannica Sources

Ask Britannica AI about this topic

Britannica doesn't have a full article on this topic, so we've curated a list of our relevant coverage.

positronium
…nanosecond is 10⁻⁹ second); and orthopositronium, in which the spins are in the same direction, annihilates into three photons with a mean life of about 100 nanoseconds (10^-7 second). The properties of positronium corroborate the quantum theory of electrodynamics for a two-particle system.
Read More
…nanosecond is 10⁻⁹ second); and orthopositronium, in which the spins are in the same direction, annihilates into three photons with a mean life of about 100 nanoseconds (10^-7 second). The properties of positronium corroborate the quantum theory of electrodynamics for a two-particle system.
Read More