Radiation and Radiological Disasters. Activity is the term used to characterise the number of nuclei which disintegrate in a radioactive substance per unit time usually measured in Becquerel (Bq); a Bq is 1 disintegration per second. Replaces the former unit curie, symbol: Ci. 1 Ci is equal to 37 000 000 000 Bq. Note: "Activity" is a quantitative term whereas 'radioactivity' is a qualitative term used to describe atoms that decay