sensitive area

of a radiation detector
https://doi.org/10.1351/goldbook.S05603
That area of the @R05054@ where an incident @R05046@ results in 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 [Terms] [Paper]