A scintillator coupled to a photomultiplier tube. Incident X-ray photons are converted in the scintillator into bursts of visible light photons, some of which fall on the photocathode and can be measured. For incident photons having energies higher than the absorption edge of the elements contained in the scintillator, an escape peak can be observed.
PAC, 1995, 67, 1745. 'Nomenclature, symbols, units and their usage in spectrochemical analysis-XI. Detection of radiation (IUPAC Recommendations 1995)' on page 1753 (https://doi.org/10.1351/pac199567101745)