Photo-Multiplier Tubes (PMTs) convert light into electrical current, with an extremely high gain. Light strikes the photocathode, releasing electrons. These electrons are attracted to a series of elements called dynodes by an applied high voltage. Each time they strike a dynode, they cause the emission of secondary electrons, with a multiplying effect after each dynode. As a result, a single photon of light can result in a large current pulse at the output anode, which is easily measured. The gain is controlled by varying the applied voltage.
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