A known amount of element that is simultaneously irradiated with the test portion in the context of activation analysis. If one comparator is used (single comparator method), it is essentially identical to a flux monitor (except that this term is not necessarily linked to activation analysis).
Source:
PAC, 1994, 66, 2513. 'Nomenclature for radioanalytical chemistry (IUPAC Recommendations 1994)' on page 2516 (https://doi.org/10.1351/pac199466122513)