11/19/13 Olympus Microscopy Resource Center | Concepts in Digital Imaging Technology - Photomultiplier Tubes

www.olympusmicro.com/primer/digitalimaging/concepts/photomultipliers.html 1/2

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Photomultiplier Tubes

A photomultiplier tube, useful for light detection of very weak signals, is a photoemissive device in whichthe absorption of a photon results in the emission of an electron. These detectors work by amplifying theelectrons generated by a photocathode exposed to a photon flux.

Photomultipliers acquire light through a glass or quartz window that covers a photosensitive surface, calleda photocathode, which then releases electrons that are multiplied by electrodes known as metal channeldynodes. At the end of the dynode chain is an anode or collection electrode. Over a very large range, thecurrent flowing from the anode to ground is directly proportional to the photoelectron flux generated by thephotocathode.

The spectral response, quantum efficiency, sensitivity, and dark current of a photomultiplier tube aredetermined by the composition of the photocathode. The best photocathodes capable of responding tovisible light are less than 30 percent quantum efficient, meaning that 70 percent of the photons impactingon the photocathode do not produce a photoelectron and are therefore not detected. Photocathodethickness is an important variable that must be monitored to ensure the proper response from absorbedphotons. If the photocathode is too thick, more photons will be absorbed but fewer electrons will be emittedfrom the back surface, but if it is too thin, too many photons will pass through without being absorbed. Thephotomultiplier used in this tutorial is a side-on design, which uses an opaque and relatively thickphotocathode. Photoelectrons are ejected from the front face of the photocathode and angled toward thefirst dynode.

Side-On Photomultipliers

Discover how photomultipliers work by amplifying the electrons

generated by a photocathode exposed to a photon flux.

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Electrons emitted by the photocathode are accelerated toward the dynode chain, which may contain up to14 elements. Focusing electrodes are usually present to ensure that photoelectrons emitted near theedges of the photocathode will be likely to land on the first dynode. Upon impacting the first dynode, aphotoelectron will invoke the release of additional electron that are accelerated toward the next dynode,and so on. The surface composition and geometry of the dynodes determines their ability to serve aselectron multipliers. Because gain varies with the voltage across the dynodes and the total number ofdynodes, electron gains of 10 million (Figure 1) are possible if 12-14 dynode stages are employed.

11/19/13 Olympus Microscopy Resource Center | Concepts in Digital Imaging Technology - Photomultiplier Tubes

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Contributing Authors

Mortimer Abramowitz - Olympus America, Inc., Tw o Corporate Center Drive., Melville, New York, 11747.

Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State

University, Tallahassee, Florida, 32310.

Photomultipliers produce a signal even in the absence of light due to dark current arising from thermalemissions of electrons from the photocathode, leakage current between dynodes, as well as stray high-energy radiation. Electronic noise also contributes to the dark current and is often included in the dark-current value.

Channel Photomultipliers

Channel photomultipliers represent a new design that

incorporates a unique detector having a semitransparent

photocathode deposited onto the inner surface of the entrance

window.

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Channel photomultipliers represent a new design that incorporates a unique detector having asemitransparent photocathode deposited onto the inner surface of the entrance window. Photoelectronsreleased by the photocathode enter a narrow and curved semiconductive channel that performs the samefunctions as a classical dynode chain. Each time an electron impacts the inner wall of the channel, multiplesecondary electrons are emitted. These ejected photoelectrons have trajectories angled at the next bendin the channel wall (simulating a dynode chain), which in turn emits a larger quantity of electrons angled atthe next bend in the channel. The effect occurs repeatedly, leading to an avalanche effect, with a gainexceeding 100 million. Advantages of this design are lower dark current (picoamp range) and an increasein dynamic range.

Confocal microscopes, spectrophotometers, and many high-end automatic camera exposure bodies utilizephotomultipliers to gauge light intensity. Spectral sensitivity of the photomultiplier depends on the chemicalcomposition of the photocathode with the best devices having gallium-arsenide elements, which aresensitive from 300 to 800 nanometers. Photomultiplier photocathodes are not uniformly sensitive andtypically the photons are spread over the entire entrance window rather than on one region. Becausephotomultipliers do not store charge and respond to changes in input light fluxes within a fewnanoseconds, they can be used for the detection and recording of extremely fast events. Finally, thesignal to noise ratio is very high in scientific grade photomultipliers because the dark current is extremelylow (it can be further reduced by cooling) and the gain may be greater than one million.

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