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An atomic-like system consisting of an electron and positron. Just as in the hydrogen atom, the energy levels of positronium are quantized, with the deepest levels bound by about 6.8 eV. The electron and positron spins can be aligned in the same direction (singlet states) or in opposite directions (triplet states). Annihilation of the positron and electron destroys the lowest-energy singlet state (parapositronium) in about 10-10 s, but the lowest triplet state (orthopositronium) survives longer, about 10-7 s. This allows sufficient time for precise measurement of the energy levels of triplet states. Because of the absence of nuclei in positronium, these measurements provide an accurate test of theories of the electromagnetic force (quantum electrodynamics) without interference from the strong force. See Atomic structure and spectra, Electron, Fundamental interactions, Positron

Since the formation of positronium requires the close approach of a positron and an electron, beams of slow positrons can be used as probes of the electron density in gases, in insulating solids, or near surfaces. Since the singlet and triplet forms of positronium have very different lifetimes, and transitions between the two states can be induced by neighboring electrons, study of the decay of positronium can also provide information about electron densities on a microscopic scale. This is especially useful in the study of density fluctuations in gases near the critical point for condensation into liquids or solids. See Critical phenomena

Annihilation radiation from positronium forms a component of the gamma-ray spectrum observed by astronomers, in particular from the galactic center.

McGraw-Hill Concise Encyclopedia of Physics. © 2002 by The McGraw-Hill Companies, Inc.


(poz-ă-troh -nee-ŭm) A positron-electron system that lasts for a measurable time before combining to produce annihilation radiation. Positronium can be thought of as an atom analogous to that of hydrogen in which the electron and positron move in Bohr orbits about the center of mass, which is halfway between them. During formation, approximately 25% is formed in a singlet spin state and the other 75% is formed in a triplet state. The annihilation radiation emitted by the combining of a positron-electron pair in a singlet state consists of two γ-ray photons emitted simultaneously with energies of 511 keV each; the annihilation radiation from the triplet state consists of three γ-ray photons emitted simultaneously each with energy less than 511 keV.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



a bound system of a positron e + and an electron e-. Sometimes symbolized ps, it is analogous to a hydrogen atom in which the proton is replaced by the positron. It was discovered in 1951 by M. Deutsch of the USA; the name was proposed in 1945 by the American physicist A. Ruark.

Positronium is formed during collisions of positrons with atoms. A positronium “atom” has a mass equal to two electron masses; in size, it is twice as large as the diameter of a hydrogen atom. Positronium can exist in ground and excited states. Because of the interaction of the spins of the electron and positron, the ground energy level of positronium is split into two sublevels with an energy difference of 8.41 × 10-4 electron volt (eV). The lower level corresponds to a state with particles having antiparallel spins (parapositronium), and the upper level to particles having parallel spins (orthopositronium). Annihilation of the positron and electron occurs in both states. Parapositronium is annihilated with the formation of two γ-quanta (e+e-2γ) in 1.25 × 10-10 sec, and orthopositronium with the formation of three γ-quanta (e+e- → 3γ) in 1.4 × 10-7 sec. The difference in the two decay modes is a result of the charge parities of parapositronium and orthopositronium being equal to +1 and — 1, respectively.

Investigation of the transformation of orthopositronium into parapositronium has confirmed the theoretical predictions of quantum electrodynamics, which gives the following value for the energy difference between parapositronium and orthopositronium:

Here, ℰ0 = (α2/2)mc2 = 27.21165 eV is the atomic unit of energy, and α = e2/ℏc = 1/137.03608 is the fine structure constant (ℏ is Planck’s constant and c is the speed of light). The energy difference ⋄ℰ is due to the difference in the interaction of the magnetic moments of the electron and positron in the para- and ortho-states and also to the annihilation interaction specific to positronium.

In its chemical properties, positronium is similar to the hydrogen atom and is therefore used as a tracer—that is, an atom that can be followed on the basis of its decay products. The properties and lifetime of positronium in a substance differ from the characteristics of free positronium and depend on the properties of the substance. As a result, positronium can be used to investigate, for example, rapid chemical reactions of atomic hydrogen that occur in a time comparable to the lifetime of positronium.


Landau, L. D., and E. M. Lifshits. Teoreticheskaia fizika, vol. 4, part 1. Moscow, 1968.
Gol’danskii, V. I. Fizicheskaia khimiia pozitrona ipozitroniia. Moscow, 1968.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


(particle physics)
The bound state of an electron and a positron.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
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