Based on the Random House Dictionary, © Random House, Inc. 2012.
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| the splitting of the nucleus of an atom into nuclei of lighter atoms, accompanied by the release of energy |
| the smallest component of an element having the chemical properties of the element; the number of protons determines the identity of the element |
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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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