A PMT (Photomultiplier Tube) assembly is a component used in scintillation detectors to detect and measure ionizing radiation. PMTs consist of a photocathode, a series of dynodes, and an anode. When a photon strikes the photocathode, it releases electrons through the photoelectric effect. These electrons are then accelerated and multiplied as they pass through the series of dynodes. Finally, the multiplied electrons reach the anode, generating an electrical pulse.