The detection of alpha, beta and gamma radiation enables the radioactivity of a source, a material or an object to be measured. The unit used to measure radioactivity is the becquerel (Bq), which, since 1975, has replaced the curie (1 curie = 37 thousand million becquerels).

The activity of a source is measured in becquerels. One becquerel corresponds to the disintegration of one nucleus per second.

The becquerel is a very 'small' unit and activity levels cover a very wide range. Multiples of the becquerel such as the mega-, giga- or terabecquerel are therefore often used.