When a charged particle moves through a transparent material with a velocity v, greater than the velocity of light in that material, it radiates light in the forward direction at an angle whose cosine is equal to c/vn, where n is the index of refraction of the material. This light is called Cerenkov radiation and can be detected with photomultiplier tubes as was the case with scintillation detectors (Figure 3). It is named after the Russian physicist Pavel Cerenkov who discovered it in 1934. The special theory of relativity limits particle velocities to values less than c, the velocity of light in a vacuum. Cerenkov detectors can be of two types. A threshold detector merely detects the fact that light is emitted and indicates that the velocity of the particle passing through it is greater than c/n. Other more complicated detectors can actually determine the velocity v by measuring the angle at which light is emitted.
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Science EncyclopediaScience & Philosophy: Overdamped to PeatParticle Detectors - Geiger Counter, Scintillation Detector, Solid State Detectors, Neutron Detectors, Cerenkov Detectors, Cloud Chambers And Bubble Chambers