Photomultiplier tube (PMT)

A photomultiplier tube detects light through the photoelectric effect in vacuum. A photon striking the photocathode, a thin layer of a low-work-function material such as a bialkali (antimony with potassium and caesium), can release an electron into the vacuum. An electric field accelerates the electron onto the first dynode, where it knocks out several secondary electrons; those are accelerated onto the next dynode, and so on down a chain of typically eight to twelve stages. With a secondary emission ratio $\delta$ per stage and $n$ stages, the gain is $\delta^n$: four secondaries per stage over ten stages gives $4^{10} \approx 1.05 \times 10^6$. The resulting pulse of about a million electrons at the anode is large enough to count single photons directly.

The gain is nearly noiseless compared with a semiconductor avalanche. The excess noise factor of a dynode chain is close to $\delta/(\delta - 1)$, about 1.33 for $\delta$ = 4 and lower when the first dynode has high gain, where an avalanche photodiode typically adds more. The photocathode can be centimeters across, collecting light from large or diffuse sources, and its thermionic emission is low, giving dark count rates from a few to thousands per second depending on the cathode and its temperature. These properties made the PMT the standard detector for scintillation counting, fluorescence, Raman spectroscopy, astronomy before CCDs, and the large neutrino and cosmic-ray detectors that use thousands of tubes.

The limitation is the photocathode's quantum efficiency. Bialkali cathodes peak around 400 nm with efficiencies of roughly 25 to 35% and fall off toward the red; multialkali and GaAs cathodes extend the response to 900 nm or so, and InP/InGaAs cathodes reach 1.7 µm with efficiencies of only a few percent, requiring cooling. In the near infrared, semiconductor detectors, SPADs at room temperature and SNSPDs with cryogenic cooling, outperform PMTs, and in the visible silicon photomultipliers (arrays of SPADs read out in parallel) are replacing them where compactness, magnetic-field immunity or low voltage matters. A PMT needs a high voltage, typically 1 to 2 kV, and its gain changes with that voltage and with magnetic fields.

Timing is set by the electrons' flight through the tube. The transit time spread, the variation in delay from photon to output pulse, ranges from a few hundred picoseconds in conventional tubes to tens of picoseconds in microchannel-plate PMTs, in which the dynode chain is replaced by a plate of micron-scale glass channels, each acting as a continuous electron multiplier.

References: Hamamatsu Photonics, Photomultiplier Tubes: Basics and Applications, 4th ed. (2017); R. H. Hadfield, Nat. Photonics 3, 696 (2009).