Photomultiplier Tube (PMT)

Photomultiplier tube (PMT) is among the most sensitive of all light sensors and are used to detect faint light.

In other words, a photomultiplier tube, useful for light detection of very weak signals, is a photo emissive device in which the absorption of a photon results in the emission of an electron. These detectors work by amplifying the electrons generated by a photocathode exposed to a photon flux.

Note at this point that tubes without a multiplication function are called phototubes (PT).

Photomultiplier tubes are mainly used as secondary electron detectors in scanning electron microscopes (SEM).

Photomultipliers acquire light through a glass or quartz window that covers a photosensitive surface (photocathode0 which then releases electrons that are multiplied by electrodes known as metal channel dynodes.

Most photomultiplier tubes have 9 to 12 dynodes and amplify to the 5th to 8th power of 10, and finally output as a current.

At the end of the dynode chain is an anode or collection electrode. Over a very large range, the current flowing from the anode to ground is directly proportional to the photo electron flux generated by the photocathode.

Further Reading: https://hamamatsu.magnet.fsu.edu/articles/photomultipliers.html ↗

There are two types of photomultiplier tubes - head-on type with a photosensitive area at the tip and side-on type with a photosensitive area at the side. And, there are numerous types of photocathodes with different photosensitive surface sizes and wavelength sensitivities.

Most photomultiplier tubes operate at voltages ranging from 500 V to 2000 V. Below you can see a high-voltage power supply module for PMTs (Hamamatsu HVPS Module).

In order to generate high internal gain, the electrons initially generated at the PMT's photocathode needs to be accelerated by a high-voltage (kV) potential, so that the potential causes the electrons to strike several dynodes that generate secondary electrons. The photocurrent that has accumulated after the final dynode stage represents the amplified signal and is collected by an anode.

So, Photomultiplier tubes are used to detect faint optical signals from weakly emitting sources. 

Compared to Avalanche Photodetectors (APDs), they offer significantly larger active areas, making them ideal for capturing signals that may be diverging due to scatter or nonlinear optical effects.

Since the first commercial photomultiplier tube was developed in the early 1940s, it has remained the detector of choice for experiments requiring fast response times and high sensitivity.

Today, the PMT is a staple for research in many fields including analytical chemistry, particle physics, medical imaging, industrial process control, astronomy, and atomic and molecular physics.

This PMT Tutorial (Thorlabs, Inc.) provides introductory material for the principle of operation and key specifications to consider when choosing a PMT for a given application. 

⫸ Finally here is the Wikipedia Article https://en.wikipedia.org/wiki/Photomultiplier_tube