The curie (abbreviation Ci) is a former unit of radioactivity, equal to 3.700×1010decays per second. This is roughly the activity of 1 gram of the radiumisotope226Ra, a substance studied by the pioneers of radiology, Marie Curie and Pierre Curie (the unit is named for them). The Curie unit has been replaced by an SI derived unit, the becquerel (Bq), which equates to one decay per second (therefore 1 Bq = 2.7×10-11).
