How do scintillation counters use photomultiplier tubes to detect X-rays and gamma rays?

Scintillation counters detect X-rays and gamma rays by utilizing a two-stage process involving a scintillator material and a photomultiplier tube.12

Detection Process

1. Scintillation: When ionizing radiation, such as an X-ray or gamma ray, enters a scintillator (typically a crystal like thallium-activated sodium iodide), it interacts with the material to produce a brief flash of light. The intensity of this light is proportional to the energy of the incident radiation.134
2. Photoelectric Conversion: This light flash strikes the photocathode of the photomultiplier tube. Through the photoelectric effect, the photons eject a small number of electrons into the vacuum of the tube.42
3. Amplification: These initial electrons are accelerated toward a series of electrodes called dynodes. Each dynode is held at a higher positive potential, causing it to emit multiple secondary electrons when struck. This process repeats across several stages, amplifying the original signal by a factor of up to a million.5421
4. Measurement: The resulting pulse of electrons is collected at an anode, providing an electrical signal that can be counted and measured to determine the radiation's intensity and energy.14