A detector in which a photon interacts with a solid surface, which is called the photocathode, or a gas, releasing a photoelectron. This process is called the external photoelectric effect. The photoelectrons are collected by an electrode at positive electric potential, i.e. the anode.
PAC, 1995, 67, 1745. 'Nomenclature, symbols, units and their usage in spectrochemical analysis-XI. Detection of radiation (IUPAC Recommendations 1995)' on page 1752 (https://doi.org/10.1351/pac199567101745)