The becquerel (Bq) is the derived unit of activity (of a radionuclide) in the SI system of units. The unit is named after the French physicist Henri Becquerel. One becquerel is defined as one disintegration per second. Often, in practice, kBq (= 1,000 Bq) and MBq (= 1,000,000 Bq) are used. The becquerel has largely replaced the formerly used unit, the curie, which equals 3.7 × 1010 Bq.