The becquerel (symbol Bq)'s the SI derived unit of radioactivity. One Bq's defined as the activity of a quantity of radioactive material in which one nucleus decays per second. It's therefore equivalent to s-1. The becquerel's named for Henri Becquerel, who shared a Nobel Prize with Pierre and Marie Curie for their work in discovering radioactivity. In a fixed mass of radioactive material, the number of becquerels changes with time. Therefore, a sample radioactive decay rate's always stated wi… (More on Becquerel)