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.

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SI Unit of activity is becquerel (Bq).