Activity is the number of nuclear transformations occurring per time unit in a radioactive substance. The measuring unit of activity is Becquerel (short symbol: Bq) with which the number of radioactive nuclear transformations per second is given.

As the radionuclides can be included in amounts of substance of different configuration, the activity is frequently also referred to them, e. g.

  • Becquerel per gram (Bq/g) in solid substances,
  • Becquerel per litre (Bq/l) in liquids or
  • Becquerel per cubic metre (Bq/m³) in air.

Solely stating the activity without knowing the radionuclides does not allow a statement on the radiation exposure.