The property of certain nuclides of emitting radiation by spontaneous transformation of their nuclei. Various units of (radio)activity have been used including curie (1 Ci = 3.7 x 1010 disintegrations per second) and becquerel ( 1 Bq = 1 disintegration per second). (Mettler FA Jr, Upton AC: Medical Effects of Ionizing Radiation, 3rd ed. Philadelphia, Pa: Saunders Elsevier, 2008, page 552)