Positronium (Ps) is a system consisting of an electron and its anti-particle, a positron, bound together into an exotic atom, specifically an onium. The system is unstable: the two particles annihilate each other to produce two gamma-ray photons after an average lifetime of 125 picoseconds or three gamma-ray photons after 142 nanoseconds in vacuum...
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http://en.wikipedia.org/wiki/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 ... [3 related articles]
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http://www.britannica.com/eb/a-z/p/99
A neutral, atom-like system formed between an electron and a positron (the electron's antiparticle), held together by electromagnetic attraction. The name 'positronium' was introduced by A. E. Ruark in 1945 and the system was first observed in 1951. One of the two particles in positroniu...
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http://www.daviddarling.info/encyclopedia/P/positronium.html
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