Cherenkov radiation

Physical mechanism

Cherenkov radiation arises when a charged particle passes through a dielectric medium faster than the phase velocity of light in that medium. The relevant speed is the phase velocity, not the group velocity, and the threshold is v > c/n, or beta > 1/n. The light is emitted in a cone about the particle's direction of motion, with the cone angle set by the medium's refractive index n and the particle speed through cos(theta) = 1/(n*beta).

The Frank-Tamm formula gives the energy emitted per unit path length and per unit frequency. Cherenkov light emerges in pulses of roughly femtosecond duration and is polarized. Its continuous spectrum, whose intensity grows toward shorter wavelengths, distinguishes it from fluorescence and from discrete line spectra.

History of the discovery

The effect was predicted theoretically before it was studied experimentally. Oliver Heaviside (1888-1889) and Arnold Sommerfeld (1904) pointed to the possibility; Marie Curie saw a pale blue light in concentrated radium solution in 1910 without investigating it, and Lucien Mallet described the luminous radiation that radium induces in water in 1926-1929.

Systematic experimental study began in 1934 at the Institute of Physics of the Soviet Academy of Sciences in Moscow, the Lebedev Institute. Working under Sergei Vavilov, Pavel Cherenkov tested sixteen pure liquids, among them distilled water, paraffin and alcohols, and found that the emission was mainly blue-violet, did not vary with the liquid, and was not suppressed by fluorescence inhibitors such as potassium iodide or silver nitrate. In 1934 Vavilov proposed to explain the light as bremsstrahlung from electrons slowed by gamma rays; in 1936 Cherenkov's experiments on the influence of a magnetic field showed that the light came from electrons produced in the liquid while remaining inconsistent with bremsstrahlung.

In 1937 Ilya Frank and Igor Tamm gave the theoretical explanation within special relativity, predicting a cone of light along the particle's direction of motion; Cherenkov verified the cone angle and intensity in 1938, and Collins and Reiling independently confirmed it. The three shared the 1958 Nobel Prize in Physics.

From effect to detector

The development of the photomultiplier tube made single-particle detection possible. In 1951 John V. Jelley detected Cherenkov emission from a single fast charged particle in distilled water, and by the mid-1950s Cherenkov detectors had spread through particle physics and cosmic-ray research.

Distinctions and applications

Cherenkov radiation is distinguished from bremsstrahlung, which arises when charged particles are decelerated by other charged particles, and from fluorescence, which occurs in narrow discrete spectral bands.

Cherenkov detectors are classified as threshold or imaging (differential, ring imaging) types and by radiator medium (gas, aerogel, liquid, solid, metamaterial); Jelley classifies them as focusing or non-focusing. They are contrasted with scintillation counters and semiconductor detectors, and cannot be used for low-energy particles below about 175 keV.

In 1955 Emilio Segre and colleagues used a liquid-filled Cherenkov detector at the Bevatron to identify the antiproton. Only pions exceeded the detector's light threshold while antiprotons did not, and the first evidence was obtained on 21 September 1955.

Large water Cherenkov detectors such as Kamiokande II detected the neutrino burst from the 1987 supernova and confirmed the solar neutrino deficit. Super-Kamiokande uses 50,220 tonnes of ultrapure water and distinguishes muons from electrons by ring sharpness. VERITAS, H.E.S.S. and MAGIC apply the Imaging Atmospheric Cherenkov Technique, while HAWC, the Pierre Auger Observatory, Super-Kamiokande, the Sudbury Neutrino Observatory and IceCube use water or ice Cherenkov detection. The same technique explains the blue glow of open-pool nuclear reactors and serves in safeguards work such as the verification of spent fuel.