In nuclear physics, activity is defined as the number of atoms of a radioactive substance that disintegrate per unit time. The SI unit of activity is the becquerel (Bq), named after Henri Becquerel. 1 Bq = 1 event of radiation emission per second. This is such a small unit that multiples of the Bq unit – kBq (kilobecquerel = 1000 Bq), MBq (megabecquerel = 1000 kBq), and GBq (gigabecquerel = 1000 MBq) – are commonly used.
An old and still popular unit of measuring radioactivity is the curie (Ci). 1 Ci = 37 GBq = 37000 MBq. One curie is a large amount of radioactivity. Commonly used subunits are mCi (millicurie = 10-3 Ci), μCi (microcurie = 10-6 Ci), nCi (nanocurie = 10-9 Ci), and pCi (picocurie = 10-12 Ci). Another useful conversion formula is: 1 Bq = 27 pCi.
The specific activity (Asp) is the quotient of the activity (A) of a material and the mass (m) of this material, i.e. Asp = A/m. The unit of specific activity is Bq/kg.
The activity concentration (Aconc) is the quotient of the activity of a material and the volume (V) of this material; Aconc = A/V. The unit of activity concentration is Bq/m3.
Note: "Activity" is a quantitative term whereas "radioactivity" is a qualitative term used to describe atoms that decay.