The ratio between the quantity of the radiation emitted by the source and the quantity of the radiation as produced by the radioactive nuclei present in the source. Synonymous with source efficiency.
Source:
PAC, 1994, 66, 2513. 'Nomenclature for radioanalytical chemistry (IUPAC Recommendations 1994)' on page 2525 (https://doi.org/10.1351/pac199466122513)