Cerenkov Radiation:

Cerenkov light is a consequence of the motion of a charged particle with a speed that is greater than the speed of light in the same medium. No particle can exceed the speed of light in a vacuum (c), but in materials with an index of refraction represented by n, the particle velocity v will be greater than the velocity of light if v > c/n. For materials with an index of refraction in the common range between 1.3 and 1.8, this velocity requirement corresponds to a minimum kinetic energy of many hundreds of MeV for heavy charged particles. Fast electrons with relatively small kinetic energy can reach this minimum velocity, however, and the application of the Cerenkov process to radiations with energy below 20 MeV is restricted to primary or secondary fast electrons.

Cerenkov light is emitted only during the time in which the particle is slowing down and therefore has very fast time characteristics. In contrast with the isotropically emitted scintillation light, Cerenkov light is emitted along the surface of a forward-directed cone centered on the particle velocity vector. The wavelength of the light is preferentially shifted toward the short-wavelength (blue) end of the spectrum. The total intensity of the Cerenkov light is much weaker than the light emitted from equivalent energy loss in a good scintillator and may be only a few hundred photons or less for a 1-MeV electron. Cerenkov detectors are normally used with the same type of light sensors employed in scintillation detectors.

Cherenkov radiation, when it is intense, appears as a weak bluish white glow in the pools of water shielding some nuclear reactors. The Cherenkov radiation in cases such as this is caused by electrons from the reactor traveling at speeds greater than the speed of light in water, which is 75 percent of the speed of light in a vacuum. The energetic charged particle traveling through the medium displaces electrons in some of the atoms along its path. The electromagnetic radiation that is emitted by the displaced atomic electrons combines to form a strong electromagnetic wave analogous to the bow wave caused by a power boat traveling faster than the speed of water waves or to the shock wave (sonic boom) produced by an airplane traveling faster than the speed of sound in air.

Excerpt from the Encyclopedia Britannica without permission.