Cherenkov radiation

A charged particle moving through a transparent material can polarize nearby atoms or molecules. If its speed exceeds the phase velocity of light in that material, the disturbance does not cancel in the usual way; the emitted electromagnetic waves combine coherently into a wavefront. This is analogous in geometry to a sonic boom, though the underlying interaction is electromagnetic. The condition is often written βn > 1, where β is the particle speed divided by the vacuum speed of light and n is the material's refractive index. Thus, the effect does not mean that a particle travels faster than light in vacuum; it exceeds light's slower phase velocity in the medium.

The radiation is emitted at an angle to the particle's direction that depends on its speed and the medium's refractive index. For a particle with speed v, the idealized relation is cos θ = c/(nv), when the threshold condition is met. The spectrum is broad, and the number of emitted photons generally increases toward shorter wavelengths in the transparent range. Water surrounding intense radioactive sources or reactor fuel can therefore appear blue, although the exact observed colour depends on the spectrum, medium, and viewing conditions.

Pavel Cherenkov first observed the bluish emission in 1934 while studying radioactive materials in water. Igor Tamm and Ilya Frank developed its theoretical explanation in 1937. Cherenkov, Frank, and Tamm shared the 1958 Nobel Prize in Physics for the discovery and interpretation of the effect. In his Nobel lecture, Cherenkov discussed radiation from particles moving faster than light in a medium and possible uses in experimental physics.

Cherenkov detectors use the emitted photons to infer properties of fast charged particles. Threshold counters can indicate whether a particle exceeds a selected speed; ring-imaging Cherenkov detectors measure the light-cone angle to help identify particle species over a range of momenta. Such systems are used in accelerator experiments, including the LHCb experiment at CERN. Cherenkov light is also detected in water or ice to study high-energy particles, including those associated with cosmic rays and neutrinos, and the effect is used in monitoring ionizing radiation in some materials.