Cerenkov radiation

(electromagnetism) Light emitted by a high-speed charged particle when the particle passes through a transparent, nonconducting material at a speed greater than the speed of light in the material.

Light emitted by a high-speed charged particle when the particle passes through a transparent, nonconducting, solid material at a speed greater than the speed of light in the material. The blue glow observed in the water of a nuclear reactor, close to the active fuel elements, is radiation of this kind. The emission of Cerenkov radiation is analogous to the emission of a shock wave by a projectile moving faster than sound, since in both cases the velocity of the object passing through the medium exceeds the velocity of the resulting wave disturbance in the medium.

Particle detectors which utilize Cerenkov radiation are called Cerenkov counters. They are important in the detection of particles with speeds approaching that of light, such as those produced in large accelerators and in cosmic rays, and are used with photomultiplier tubes to amplify the Cerenkov radiation. These counters can emit pulses with widths of about 10⁻¹⁰ s, and are therefore useful in time-of-flight measurements when very short times must be measured. They can also give direct information on the velocity of the passing particle. See also Particle detector.

The properties of Cerenkov radiation have been exploited in the development of a branch of gamma-ray astronomy that covers the energy range of about 10⁵–10⁸ MeV. A high-energy gamma ray from a source external to the Earth creates in the atmosphere a cascade of secondary electrons and positrons. This cascade is generated by the interplay of two processes: electron-positron pair production from gamma rays, and gamma-ray emission as the electrons and positrons are accelerated by the electric fields of nuclei in the atmosphere (bremsstrahlung). For a primary gamma ray having an energy of 10¹² eV (1 teraelectronvolt), as many as 1000 or more electrons and positrons will contribute to the cascade. The combined Cerenkov light of the cascade is beamed to the ground over an area a few hundred meters in diameter and marks the arrival direction of the initiating gamma ray to about 1°. On a clear, dark night this radiation may be detected as a pulse of light lasting a few nanoseconds, by using an optical reflector. See also Bremsstrahlung; Electron-positron pair production.

This technique offers a means to study regions of the universe where charged particles are accelerated to extreme relativistic energies. Such regions involve highly magnetized, rapidly spinning neutron stars; supernova remnants; and active galactic nuclei. These same motivations drive the satellite observations of the EGRET instrument of the Compton Gamma-Ray Observatory at lower gamma-ray energies (up to about 10⁴ MeV). See also Gamma-ray astronomy.