Curie, KYUR ee, is a unit of radioactivity. Its symbol is Ci. One curie equals 37 billion spontaneous nuclear transformations per second. These transformations involve the emission of particles and rays by atomic nuclei. The radioactivity of 1 gram of radium equals about 1 curie. The curie was named for French physicist Pierre Curie. The curie has been largely replaced by the becquerel. One becquerel equals 1 spontaneous nuclear transformation per second.