Photomultiplier tubes

A photomultiplier tube (PMT) is a transducer which converts photons into electrons, which generate a current and voltage. Photomultiplier tubes are used to detect low levels of incident light, even as low as a single photon.

An incoming photon will strike the photocathode surface of the PMT which will emit electrons. These electrons are then accelerated toward additional electrodes by a potential difference of about 90 volts. Upon striking this electrode, more electrons are emitted. This process repeats nine times, each time generating more and more electrons. This process can generate 10⁶ to 10⁷ electrons for each individual photon. These electrons are then collected at the anode where the current and voltage is then measured.

Photomultiplier tubes are highly sensitive and have very quick response times. The sensitivity of the PMT can be improved by cooling it, to reduce temperature induced noise. Photomultiplier tubes must not be exposed to intense light, such as the sun or ambient light because the intensity can damage the components.

Photomultiplier tubes are commonly used for analysis techniques as well as medical and research purposes.

Photomultiplier Tubes Media

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  Pmside

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  Dynodes inside a photomultiplier tube

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  Fig.1: Schematic of a photomultiplier tube coupled to a scintillator. This arrangement is for detection of gamma rays.

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  Fig. 2: Typical photomultiplier voltage divider circuit using negative high voltage.

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  Internal metallisation as a protective screen against unwanted lights sources