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PhotomultiplierPhotomultipliers, or photomultiplier tubes (PMT) are extremely sensitive detectors of light in the ultraviolet, visible and near infrared. These detectors multiply the signal produced from the incident light to such an extent that detection of single photons is possible. Photomultipliers are constructed from a glass vacuum tube which houses a dynode and an anode. Incident photons strike the photocathode material which is present as a thin deposit on the entry window of the device, with electrons being produced as a consequence of the photoelectric effect. These photons are directed by the focusing electrode towards the electron multiplier, where electrons are multiplied by the process of secondary emission. The electron mutliplier consists of a number of electrodes, called dynodes. Each dynode is charged to a higher voltage than it the previous one. As the electrons head towards the first dynode they are accelerated by the difference in potential. On striking the first dynode, more electrons will be emitted. The geometery of the dynode chain is such that a cascade occurs with an ever-increasing number of electrons being produced at each stage. Finally the anode is reached where the accumulation of charge results in a sharp voltage pulse indicating the detection of a photon at the photocathode Amplification can be as much as 10^8 meaning that measurable pulses can be obtained from single photons. The combination of high gain, low noise, high frequency response and large area of collection have meant that these devices still find applications in particle physics, astronomy and medical imaging. Their replacement by semiconductor devices has long been predicted but has never been achieved. External links
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