A measurement in which the activity of a test portion is derived from the ratio between the count rates observed for the test portion and for a radioactive source of known activity.
Source:
PAC, 1994, 66, 2513. 'Nomenclature for radioanalytical chemistry (IUPAC Recommendations 1994)' on page 2525 (https://doi.org/10.1351/pac199466122513)