photomultipliers

Photomultiplier tubes, commonly called PMTs, are vacuum electronic devices that convert light into an amplified electrical signal. When photons strike a photocathode, electrons are emitted via the photoelectric effect. These photoelectrons are accelerated toward a series of dynodes, each dynode stage releasing multiple secondary electrons. The cascade multiplies the initial signal, producing large output currents from a single photon event. The overall gain of a PMT typically ranges from about 10^6 to 10^7, allowing detection of very weak light.

A PMT consists of a glass or quartz envelope containing a photocathode, a high-voltage electrode system, and a chain of dynodes followed by an anode. The device requires a high-voltage power supply to create the electric fields that drive electron multiplication. PMTs are sensitive to magnetic fields, which can deflect electrons and distort the signal, so magnetic shielding is often used. They are available in various sizes, from compact 1–2 inch models to large 8–10 inch or larger tubes, with different photocathode materials to optimize sensitivity for specific wavelengths.

Key performance characteristics include quantum efficiency, the fraction of incident photons that produce photoelectrons, typically a few tens of percent in the blue–green region for common bialkali photocathodes. Time resolution, or transit-time spread, affects how precisely the arrival time of photons can be measured. PMTs may exhibit dark current, afterpulsing, and nonlinearity at high light levels. They remain widely used in research and industry for scintillation counters, neutrino and gamma-ray detectors, medical imaging, and astrophysical instruments, where fast, low-noise light detection is essential.