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)