positronium

[poz-i-troh-nee-uhm]
noun Physics.
a short-lived atomic system consisting of a positron and an electron bound together.

Origin:
1945; positron + -ium, coined by A. E. Ruark (born 1899), U.S. physicist

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positronium (ˌpɒzɪˈtrəʊnɪəm)
 
n
physics a short-lived entity consisting of a positron and an electron bound together. It decays by annihilation to produce two or three photons
 
[C20: from positron + -ium]

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Encyclopedia Britannica
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positronium

short-lived hydrogen-like atom composed of an electron and a positron (rather than an electron and a proton) arising as a positron is slowed down in matter and captured by an electron. Two forms are known. Parapositronium, in which the spins of the positron and electron are oppositely directed, decays by annihilation into two photons, with a mean life of about one-tenth of a nanosecond (or 10-10 second; a nanosecond is 109 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

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Example sentences
The atoms, called positronium, consist of an electron and positron orbiting each other.
Positron and positronium annihilation in low-dielectric-constant films studied by a pulsed positron beam.
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