Activity is the number of nuclear fissions per time unit occurring in a radioactive substance. The measure of activity is becquerel (abbr.: Bq) with which the number of radioactive nuclear transformations per second is given.

As radionuclides may be contained in different media (solids, liquids, gases) or adhere to surfaces, the activity is indicated in different ways such as

  • mass-related as "specific activity" (e.g. in becquerel per gram (Bq/g)),
  • volume-related as "activity concentration" (e.g. in becquerel per litre (Bq/l) or becquerel per cubic metre (Bq/m3)), or
  • area-related as "area activity" (e.g. in becquerel per square metre (Bq/m²)).

If the activity is given without relation to mass, volume or area and without stating the radionuclide, it is not possible to make a statement about the radiation exposure.