The becquerel (symbol Bq) is the SI derived unit of radioactivity, defined as the activity of a quantity of radioactive material in which one nucleus decays per second. It is therefore equivalent to s-1. The older unit of radioactivity was the Curie (Ci), defined as 3.7E10 Bq or 37 GBq. In a fixed mass of radioactive material, the number of becquerels changes with time. Sometimes, amounts of radioactive material are given after adjustment for some period of time. For example, one might quote a ten-day adjusted figure, that is, the amount of radioactivity that will still be present after ten days. This de-emphasizes short-lived isotopes. SI uses the becquerel rather than its equivalent, the reciprocal second, for the unit of activity measure to eliminate any possible source of confusion regarding the meaning of the units, because errors in specifying the amount of radioactivity, no matter how far-fetched, could have such serious consequences.
Often used quantities are kBq for ground contamination and PBq for nuclear reactor inventories:
kBq = Kilobecquerel = E3 Bq = 1,000 Bq
PBq = Petabecquerel = E15 Bq = 1,000,000,000,000,000 Bq