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.
Source:
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)