From Wikipedia, the free encyclopedia

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 3.7×10¹⁰ becquerels or 37 GBq. The becquerel is 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. 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 the second for the unit of activity measure to avoid dangerous mistakes: a measurement in becquerels is proportional to activity, and thus a more dangerous source of radiation gives a higher reading. A measurement in seconds is inversely proportional. As any SI unit, Bq can be prefixed; commonly used multiples are kBq (kilobecquerel, 10³ Bq), MBq (megabecquerel, 10⁶ Bq), and GBq (gigabecquerel, 10⁹ Bq).

When measuring radioactivity of a sample with a detector, a unit of "counts per second" (cps) is often used. Counts per second can be converted to the absolute activity of the sample in Bq if one applies a number of significant conversions, e.g., for the radiation background, for the detector efficiency, for the counting geometry, for self-absorption of the radiation in the sample.

The becquerel can be used for the frequency of aperiodic events; for periodic events, the hertz, which is also defined as s^–1, is used as unit.

1 Bq = 1 s^–1

• Derived units on the BIPM web site