Activity, the number of disintegrations per second of radioactive-decay systems, or the number of reactive atomic nuclei that decay per second in a given sample. Activity is measured by measuring the number of particles and photons (pulses of electromagnetic energy) emitted from a radioactive source over a suitable time interval, with the aid of radiation detectors and electronic circuits. However, this experimental count must be viewed in the light of an in-depth understanding of the precise manner of radioactive decay in the sample material, as certain sources emit more than one particle or photon per decay.
SI Unit of activity is becquerel (Bq).