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. This radiation, first predicted by P. A. Cerenkov in 1934 and later substantiated theoretically by I. Frank and I. Tamm, is used as a signal for the indication of high-speed particles and as a means for measuring their energy in devices known as Cerenkov counters. See also: Shock wave
Direction of emission
Cerenkov radiation is emitted at a fixed angle θ to the direction of motion of the particle, such that cos θ = c/nυ, where υ is the speed of the particle, c is the speed of light in vacuum, and n is the index of refraction of the medium. The light forms a cone of angle θ around the direction of motion. If this angle can be measured, and n is known for the medium, the speed of the particle can be determined. The light consists of all frequencies for which n is large enough to give a real value of cos θ in the preceding equation.
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; Photomultiplier
Dielectrics such as glass, water, or clear plastic may be used in Cerenkov counters. Choice of the material depends on the velocity of the particles to be measured, since the values of n are different for the materials cited. By using two Cerenkov counters in coincidence, one after the other, with proper choice of dielectric, the combination will be sensitive to a given velocity range of particles. See also: Dielectric materials
The counters may be classified as nonfocusing or focusing. In the former type, the dielectric is surrounded by a light-reflecting substance except at the point where the photomultiplier is attached, and no use is made of the directional properties of the light emitted. In a focusing counter, lenses and mirrors may be used to select light emitted at a given angle and thus to give information on the velocity of the particle.
Cerenkov counters may be used as proportional counters, since the number of photons emitted in the light beam can be calculated as a function of the properties of the material, the frequency interval of the light measured, and the angle θ. Thus the number of photons which make up a certain size of pulse gives information on the velocity of the particle.
Gas, notably carbon dioxide (CO2; Fig. 1), may also be used as the dielectric in Cerenkov counters. In such counters the intensity of light emitted is much smaller than in solid or liquid dielectric counters, but the velocity required to produce a count is much higher because of the low index of refraction of gas.
Cerenkov gamma-ray astronomy
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 (Fig. 2). 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 (Fig. 3). 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
The idea of using the atmosphere as an essential component of a very high energy gamma-ray detector dates from the 1960s. However, the development of the field was hindered for several decades by the enormous background to any possible gamma-ray signal that arises from cosmic-ray-induced air showers. Such showers are hundreds of times more numerous (within a 1° field of view) than the showers from even the strongest gamma-ray source. However, the technique remains attractive since it appears to be the only viable technique for gamma-ray astronomy in this energy range. See also: Cosmic rays
Rapid development of the field dates from the late 1980s and follows the recognition that cosmic-ray air showers differ substantially from gamma-ray air showers. In order to distinguish between them, it is necessary to image each individual air shower. The imaging is accomplished through the use of an array of fast photomultipliers, operating in the focal plane of the gamma-ray telescope. Comparison of observed image parameters for each shower with the image expected for a gamma-ray shower results in rejection of more than 99% of background cosmic-ray showers, while more than 50% of gamma-ray showers are retained.
A number of imaging Cerenkov telescopes are in operation with, in most cases, focal-plane imaging detectors consisting of more than 100 pixels. By 1998, seven sources had been reliably observed with good signal-to-noise ratios.
Spin-powered neutron stars
Three of these sources are associated with spin-powered neutron stars, the Crab pulsar, Vela pulsar, and PSR B1706-44, all located within the Milky Way Galaxy less than 7000 light-years from the Earth. Unlike earlier detections, however, there is no evidence for pulsations at the spin periods of the neutron stars. (This behavior is in contrast to observations at lower gamma-ray energies from the EGRET instrument, in which the gamma rays are dominantly pulsed.) For these sources the unpulsed gamma rays are presumably associated with a surrounding nebula. For the Crab, the nebula is clearly observed, but for the other two objects there is some ambiguity concerning the presence of any nebulae.
The observations of the Crab Nebula at teraelectronvolt energies combined with observations at lower gamma-ray energies generally support a theoretical model of the high-energy emission process in which the highest-energy radiation is due to Compton scattering between extremely relativistic electrons, with energies up to about 1016 eV, and lower-energy photons in the nebula. In this picture, the lower-energy photons have been created by the relativistic electrons themselves as they spiral in the magnetic field of the nebula. The underlying power for all of this high-energy activity is the rotation of a rapidly spinning neutron star (spin period 33 milliseconds) which itself is the remnant of the relatively recent supernova explosion in the year 1054. See also: Compton effect; Crab Nebula; Neutron star; Pulsar; Supernova
Shell-type supernova remnants
Another of the very high energy gamma-ray sources is a shell-type supernova remnant associated with a historical supernova of the year 1006. Shell-type supernova remnants are suspected to be the dominant source of cosmic rays with energies less than about 1014 eV. The shock waves that occur at shell boundaries accelerate cosmic rays confined by magnetic fields near the shocks to these energies. Although SN 1006 is a shell-type supernova, it does not meet expectations. The teraelectronvolt gamma rays appear to arise principally from electrons rather than the protons and heavier nuclei that make up the bulk of the cosmic rays, since the location of the teraelectronvolt gamma-rays generally coincides with the location of nonthermal x-ray emission attributed to relativistic electrons.
Observations of other shell-type supernova remnants have failed to detect any very high energy gamma-ray emission with, in some cases, upper limits that are slightly below theoretical expectations. If the common view of the origins of cosmic rays is correct, the next generation of imaging Cerenkov detectors should easily detect the nearer shell-type remnants and map their teraelectronvolt photon emission.
Active galactic nuclei
The remaining sources seen by atmospheric Cerenkov telescopes are all extragalactic objects associated with active galactic nuclei: Markarian 421, Markarian 501, and 1ES 2344 + 514. Active galactic nuclei in general are thought to be powered by supermassive black holes (106–109 solar masses) accreting material from a surrounding disk of gas. In many cases, a jet of relativistic material and radiation is formed perpendicular to the accretion disk. In the case of the objects seen at teraelectronvolt energies, and at 100 MeV–1 GeV energies as well, the jets are apparently directed toward the Earth, so that individual gamma rays are boosted in energy and the apparent luminosity of the sources is enhanced by several orders of magnitude. Active galactic nuclei with jets aligned near to the line of sight to the Earth are called blazars. See also: Black hole
Variations at very short time scales have been observed in the emission of the blazars. The gamma-ray flux from Markarian 421, for example, has been observed to rise and fall by a factor of 2 within 15 min. Because of the large collection area associated with Cerenkov telescopes (Fig. 2), they have the ability to probe for flux variations on a time scale unobtainable by any other detector operating above a few megaelectronvolts. The observations of Markarian 421 show the the teraelectronvolt gamma-ray emission is best characterized by succession of rapid flares with a baseline level below the sensitivity limit of the detector, consistent with little steady emission.
When the spectrum obtained at teraelectronvolt energies is combined with lower-energy spectra, a consistent picture emerges in which the teraelectronvolt photons are due to Compton scattering of radiation from a population which is dominantly relativistic electrons. However, many questions remain, and alternative scenarios in which the dominant particle species are protons are possible. The fact that gamma rays up to energies of 1012 eV are observed suggests that this type of source is a copious producer of high-energy cosmic rays. Their contribution to the observed cosmic ray flux at Earth has yet to be assessed. See also: Galaxy, external; High-energy astrophysics