Radioactive alpha decay, during which alpha particle is emitted.

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.