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Radioactive decay

The process by which the unstable nucleus of an atom releases some of its energy in the form of ionizing radiation, thereby changing to a more stable nucleus. Typical radioactive decays include alpha (in which an alpha particle is released), beta (in which an electron or positron is released) and gamma (in which a high-energy photon is emitted). A material containing the unstable nuclei of an atom is referred to as radioactive.