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positronium
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 positronium acts as a nucleus with the other particle orbiting around it. The positronium is unstable and annihilates forming two photons each with an energy of 511 keV, the rest mass of the electron. In ortho positronium, which has a mean life of 1.39 × 10-7 s, the spins of the positron and electron are parallel, in a triplet state. In para positronium, which has a mean life of 1.25 × 10-10 s, the spins are antiparellel, in a singlet state.
Related category
• PARTICLE PHYSICS
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