Specific activity (symbol a) is the activity per unit mass of a radionuclide and is a physical property of that radionuclide.It is usually given in units of becquerel per kilogram (Bq/kg), but another commonly used unit of specific activity is the curie per gram (Ci/g). 1 Ci/g = 37 TBq/kg.

In the context of radioactivity, activity or total activity (symbol A) is a physical quantity