Editorial Feature

What is a Photomultiplier?

Photomultipliers, or photomultiplier tubes (PMTs), belong to a class of vacuum tubes that convert photons into electric signal. PMTs are known for their ultra-fast response and high sensitivity. They are particularly useful in low-intensity applications.

Photomultipliers score over the other photosensitive devices on characteristics such as high internal gain, high response rate, very low noise and a choice of a larger photosensitive area. This datasheet will look into the working principle, construction and applications of PMTs.

Working Principle

Photomultiplier tubes operate using photoelectric effect and secondary emission. When light is incident on the photocathode, it emits electrons into the vacuum tube. These electrons are focused towards the electron multipliers (dynodes), which multiply the signal by secondary emission. These multiplied electrons are converted into an output signal by the anode.

Construction

The entire assembly of the photomultiplier is housed inside a high vacuum tube. The photocathode material can be chosen to optimize the photomultiplier for a particular region of the electromagnetic spectrum.

Any metal will exhibit some photoelectric properties - however, the materials most commonly used for photocathodes are alloys of alkali metals, or compound semiconductors, which tend to have a very low work function. Popular materials include S-20 Multialkali (alloy of sodium, potassium, antimony and caesium), and indium gallium phosphide (InGaAs).

Applications

Some of the typical applications of photomultiplier tubes are given below:

  • Spectrophotometers – fluorescence spectrophotometry, Raman spectroscopy
  • Liquid gas chromatography
  • X-Ray diffractometers
  • Electron microscopes
  • Biotechnology – flow cytometer, DNA micro array scanner
  • Pollution monitoring
  • Medical diagnostics
  • Photography
  • Radiation monitors.

Sources and Further Reading

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