Britannica
Sources used in this answer
How do photomultiplier tubes detect individual photons?
Created from Britannica articles
using AI.
AI can make mistakes, so verify using Britannica articles.
electromagnetic radiation
Diagram of photosynthesis showing how water, light, and carbon dioxide are absorbed by a plant to produce oxygen, sugars, and more carbon dioxide.
Photomultiplier tubes detect individual photons by leveraging the photoelectric effect to convert light into an electrical signal that is then massively amplified.12
The process occurs in two primary stages:
- Photoemission: The tube contains a photosensitive photocathode. When an incident photon strikes this surface, it may eject a low-energy electron into the vacuum of the tube. The probability of this conversion is known as the quantum efficiency.32
- Electron Multiplication: Because the initial electron pulse is too small to measure, the tube utilizes a series of electrodes called dynodes. Each dynode is held at a successively higher positive potential. When an electron strikes a dynode, it triggers the emission of several additional electrons through secondary emission. This process repeats across multiple stages, typically resulting in an amplification factor of to .32
The final, significantly amplified pulse of electrons is collected at an anode, providing an output signal that allows for the detection and measurement of extremely weak light sources, including single photons.312