sensitive volume

of a radiation detector
https://doi.org/10.1351/goldbook.S05604
That volume of the detector where an incident radiant power produces a measurable output.
Source:
PAC, 1995, 67, 1745. 'Nomenclature, symbols, units and their usage in spectrochemical analysis-XI. Detection of radiation (IUPAC Recommendations 1995)' on page 1751 (https://doi.org/10.1351/pac199567101745)