Cherenkov detectors

Type of physical science: Elementary particle (high-energy) physics

Field of study: Techniques

If certain radioactive sources are placed near a transparent medium such as water, a bluish-white light may be observed. This radiation, now known as "Cherenkov radiation," takes its name from Pavel Alekseyevich Cherenkov, who was the first to study its origins extensively in a series of experiments from 1934 to 1938. The effect occurs when the velocity of a charged particle passing through a medium exceeds the velocity of light in that medium.

Overview

According to classical electromagnetic theory, a charged particle moving through a vacuum will radiate energy only if it is accelerating. Ilya Mikhailovich Frank and Igor Yevgenyevich Tamm discovered that a charged particle traveling in some medium faster than the speed of light in that medium will also radiate energy. Excited atoms in the vicinity of the particle become polarized and coherently emit radiation at the characteristic angle (θ sub C) at each small increment along the path. The frequencies will be continuous, but concentrated in the blue end of the spectrum. The light looks blue because the number of photons per unit path length per unit wavelength is proportional to 1/λ² and thus is concentrated in the short (blue) wavelength portion of the spectrum.

For particles whose velocities exceed the speed of light threshold, the Cherenkov radiation will be emitted along the axis of a cone. This configuration of radiation emission is analogous with the "shock wave" accompanying supersonic flight or a boat traveling through the water. The half angle of the cone is given by the Cherenkov relation, θc = 1/βn where n is the index of refraction of the medium and β is the ratio of the velocity of the particle to the velocity of light in free space. This preferential emission is in contrast to ordinary scintillation detectors, which emit light isotropically.

In order to emit Cherenkov light, the velocity of particles must be greater than the speed of light in the medium of the detector. In other words, the product of n and beta must be greater than one. This corresponds to a minimum threshold energy. For example, most plastics and glasses have a refractive index of about 1.5, requiring a minimum kinetic energy on the order of 175,000 electronvolts for electrons. For other charged particles, the threshold is higher in proportion to their rest mass energy as compared with that of the electron.

Electromagnetic radiation or light comes in massless packets of energy called photons.

A small number of these photons are emitted at the Cherenkov angle, given by the velocity of the particle and the index of refraction of the medium. The light may then be collected onto a photomultiplier tube to form a counter. A threshold Cherenkov counter detects the presence of a particle whose velocity exceeds some minimum amount, while a differential Cherenkov counter can measure the velocity of a particle within a certain range. A total absorption counter measures the energy of the particle.

Like scintillation detectors, Cherenkov detectors usually use a photomultiplier tube to convert the emitted photons into an electronic signal that may be interpreted by modern electronics modules and computers, which analyze the event. In a photomultiplier tube, the light strikes a photosensitive surface called the photocathode. The photoelectric effect (explained by Albert Einstein in 1905) results in the release of at most one electron (here, called a photoelectron) from the photocathode. The photoelectron is accelerated by a potential difference in the tube, where it strikes another metal surface (called a dynode) from which more secondary electrons are emitted. These secondary electrons are in turn accelerated and multiplied in, usually, ten stages of dynodes, each at a higher electric potential. This results in a total multiplication of about a million. It is these million or so electrons that form the output pulse of the photomultiplier tube. The typical potential difference between adjacent dynodes is about 100 volts; thus, electrons strike the dynodes with about 100 electronvolts of energy. The dynodes are made from materials with a high probability of secondary electron emission. It may take 2 to 3 electronvolts of energy to release an electron; thus, a gain in the number of electrons by a factor of 30 is possible. Because the electrons are released in random directions in the material, however, relatively few will actually be released at the surface, and a gain by a factor of 4 or 5 at each dynode is more typical. Even so, with a ten-dynode tube, the overall gain would be 4 to the power of 10, which is about 1 million, or 5¹⁰, which is about 10 million.

A drawback of the Cherenkov counter is its relatively low level of light output. Only one part in a thousand of the energy of the particle is converted into detectable light. This is only 1 percent of the efficiency of a scintillation counter, which emphasizes the need to collect as many photons as possible. The Cherenkov radiator should be transparent to the emitted radiation over the range of wavelengths. Also, the radiator should have no impurities that produce scintillation light. Scintillation light is emitted in all directions, and its presence would destroy the directionality of the emitted Cherenkov light. It is desirable to use a radiator that has a small density and a low atomic number (the number of protons in an atomic nucleus) because this minimizes undesirable effects such as ionization loss and multiple scattering.

Cherenkov counters using a gas radiator are particularly useful for detecting high-energy particles that are moving with velocities near the speed of light in a vacuum (ratio of velocity is greater than 0.99). The refractive indices of gases in the visible and ultraviolet region of the electromagnetic spectrum depend on the presence of absorption bands. The index of refraction of the gas is related to its density through the Lorentz law (named for Hendrik Antoon Lorentz). The index of refraction increases with the pressure of the gas. Some values at normal atmospheric pressure range from 1.00004 for helium to 1.00171 for pentane.

Gas counters are particularly advantageous as threshold Chrenekov counters for high-energy particles, since indices near 1.0 are required, and those for gases can be controlled by varying the pressure of the gas. The radiator gas is contained in a high-pressure, light-tight container. Cherenkov light is reflected by a mirror and transmitted through a pressure window onto a photomultiplier tube. The length of the counter is determined by the required photon yield.

The diameter of the counter must be matched to the maximum angle of radiation emission.

A differential Cherenkov counter can measure the velocity of a particle by accepting only the light in a small annulus, or ring, around some angle. This type of counter can provide a signal for the presence of a particle of a given mass. Differential gas counters are suitable only for use in a well collimated beam of charged particles--that is, only with particles along a particular narrow path. It is desirable to measure the velocities of the secondary particles that leave an interaction region along different paths and directions. One scheme for doing this is a ring-imaging Cherenkov detector. Photons emitted at various points along a particle's path, when reflected from a spherical mirror, are focused onto a circle whose radius is a simple function of the Cherenkov angle and the radius of curvature of the mirror. If the particle's path is inclined at an angle to the optical axis of the mirror, then the center of the circle is displaced a distance from the axis that is given by a simple function of the radius of curvature of the mirror and the angle.

Thus, measurement of the center of the Cherenkov circle determines the particle direction.

Inversely, external measurement of the particle direction determines the center of the circle, so that each detected photon gives a measure of the velocity of the particle. If the momentum is measured using a magnetic field, then the mass of the particle can be determined.

Since the number of photons is expected to be small, most ring-imaging Cherenkov detector designs concentrate on collecting photons from the ultraviolet portion of the spectrum.

The yield is larger there than in the visible portion, and the efficiency of converting these photons using the photoelectric effect is nearly 100 percent. Since the gas must be transparent to ultraviolet radiation, the radiator is restricted to a few choices, including the noble gases and nitrogen. An oxygen contamination of two parts per million would cause a significant loss of photon transmission.

In a total absorption Cherenkov counter, all the kinetic energy of the incident particle is absorbed by the radiator medium. Two classes of reactions are commonly used. The first involves detecting incident electrons or photons. These particles initiate an electromagnetic shower in the detector through the combined processes of bremsstrahlung and pair production.

The Cherenkov radiation emitted by the electrons and positrons in the shower is detected by the counter. In the second type of reaction, an incident particle interacts in the medium, and one of the particles gains enough kinetic energy in the reaction to emit Cherenkov radiation. These secondary particles include the recoil proton in neutron-proton elastic scattering and electrons knocked out of an atom by neutrino interactions.

Arrays of lead glass blocks are widely used as detectors for photons and electrons.

Large total-absorption water Cherenkov detectors are used to study neutrinos and to search for proton decay.

Applications

There is a large range of threshold Cherenkov counters that have been used in particle physics experiments. For example, the TASSO experiment (1976-1986) using the PETRA positron-electron collider at the Deutsches Elektronen-Synchrotron (DESY) in Hamburg, Germany, incorporated two gas and one aerogel threshold counters in a particle spectrometer.

Physicists studied the particles produced in electron-positron collisions. One of the counters used Freon and the other used carbon dioxide, both at standard atmospheric pressure. At the other extreme is the MD1 detector at Novosibirsk in the Soviet Union, which used ethylene at 25 atmospheres. Scientists also studied the particles produced in electron-positron collisions (1980-1985). The TASSO detectors employed two concave ellipsoid mirrors, with one focus at the positron-electron interaction point and the other at the photomultiplier tube. The mirrors were formed from heat-treated Lucite sheets with a vacuum-deposited aluminum surface.

An exotic type of counter is the silica aerogel threshold counter. Silica aerogel consists of strings of small (about 4 x 10^-7 centimeter-diameter) spheres of amorphous silica surrounding spheres (about fifteen times bigger) of trapped air. The index of refraction of the solid material can be lowered from the value for pure silica (1.46) by increasing the amount of trapped air. The TASSO aerogel blocks were constructed with an index of 1.024. One difficulty with collecting the light from an aerogel counter is that visible light has a high probability for scattering. The average distance between scatterings is about 2.5 centimeters for blue light. In this case, a light collection system using diffuse scatterings was more efficient than one using focusing mirrors. The mean number of photoelectrons from this counter is between two and four, depending on the velocity of particles in the narrow range of acceptance.

An example of a differential gas counter is that used in a hyperon (particle of the baryon group) beam at the Centre Europeen de Recherche Nucleaire (CERN) in Geneva, Switzerland. This counter used an adjustable optical system to correct for dispersion and geometric aberrations. The accepted velocity could be changed by varying the pressure in the chamber. The velocity resolution achieved was excellent, with an uncertainty in the ratio of velocity of 50 parts per million.

In 1980, a group at the Clinton P. Anderson Meson Physics Facility in Los Alamos, New Mexico, constructed a large total-absorption water Cherenkov counter for use as a neutrino detector. It detected low-energy electrons or positrons created in neutrino reactions on free protons or deuterons. A Cherenkov counter was chosen over a scintillation counter for this application in order to avoid background signals from recoil protons in fast neutron interactions.

The counter consisted of 6,000 liters of water enclosed in a cube with nearly 2-meter sides and about one hundred photomultiplier tubes. The energy resolution was about 12 percent of the energy of the electron.

Large-volume water Cherenkov counters have been placed deep underground in mines, where they are shielded from the cosmic-ray background. They have been used to look for the decay of the proton and for solar neutrinos, and have seen neutrinos from Supernova 1987A. One detector is from the University of California at Irvine, University of Michigan, and Brookhaven National Laboratory collaboration. The detector is located in the Morton Thiokol salt mine near Cleveland, Ohio. It consists of 10,000 metric tons of water and twenty-four hundred photomultiplier tubes. The Kamiokande-II detector is located in a deep mine near Kamioka, Japan, and consists of 948 photomultiplier tubes that view more than 2,000 metric tons of water.

Neither of these detectors has seen evidence of proton decay. On February 27, 1987, they both saw the neutrino burst from the Supernova 1987A that resulted from the collapse of the core of a blue supergiant star in the Large Magellanic Cloud. This observation was a true bonus to both experiments and demonstrates that scientific investigations give unforeseen dividends.

Context

Prior to Pavel Alekseyevich Cherenkov's work, the Cherenkov radiation was observed by many others, although they did not pursue an investigation. Among them was Marie Curie, who told how she was mystified to find bottles of her radium solution "aglow with this uncanny blue light." In 1926, Lucien Mallet was able to give only a qualitative description of the effect.

These early papers on the subject were, unfortunately, largely overlooked. In 1934, Cherenkov, unaware of Mallet's work, resumed work on the effect, conducting extensive and varied experiments. In 1937, Ilya Mikhailovich Frank and Igor Yevgenyevich Tamm were the first to offer a theory based on classical electrodynamics to explain correctly the cause of the radiation.

For their combined efforts, the 1958 Nobel Prize in Physics was awarded to Frank, Tamm, and Cherenkov.

Modern nuclear and particle physics experiments employ Cherenkov counters for particle identification or rejection following scattering reactions. The number of particles detected varies from one to several hundred. The detector may consist of magnetic devices, scintillators, wire chambers, and Cherenkov detectors. Signals are entered into fast electronics that enable experimental scientists to determine that the various signals in the detectors came from one common event. Information may include position and angles, energy deposited in the counter, and time difference from one point to another.

Cherenkov detectors are useful to measure neutral mesons such as pions and etas, which decay very soon after their creation in a reaction. Both pions and etas have a large chance of decaying into two high-energy γ rays. Lead glass Cherenkov detectors are useful for detecting the two photons and their positions by the Cherenkov light emitted by the electron-positron pairs that are created when the γ rays interact with the lead atoms in the glass. For example, the SPESO-2π detector constructed by the Institut de Physique Nucleaire in Orsay, France, consists of about twenty lead glass bars that are about 70 centimeters long and 8 centimeters thick. The bars are arranged vertically around a target and have a photomultiplier tube attached to each end. They have an index of refraction of 1.63, which requires a minimum electron energy of 140,000 electronvolts and a velocity of about six-tenths the speed of light. The low efficiency of the lead glass bars to slow protons was one crucial factor in the operation of the spectrometer.

Ring-imaging Cherenkov detectors were proposed in 1977 by J. Seguinot and Thomas Ypsilantis in an imaginative technical realization of an idea that A. Roberts presented in 1960.

By the late 1980's, the technology advanced sufficiently to be included on the list of useful detection devices that can be employed in the design of new experiments. A high-energy physics experiment that has used a ring-imaging Cherenkov detector is a fixed-target experiment at Fermi National Accelerator Laboratory near Chicago that looks for particles produced following proton-proton collisions. The purpose of the ring-imaging Cherenkov detctor is the identification of very high momentum electrons, pions, kaons, and protons.

Principal terms

BREMSSTRAHLUNG: the energy loss of charged particles (usually electrons) resulting from the electromagnetic radiation emitted in the violent accelerations that occur during collisions with atomic electrons

INDEX OF REFRACTION: the ratio of the speed of light in free space to the speed of light in the medium

IONIZATION: the passage of radiation through materials disturbs the electrons in the atoms and sometimes ejects electrons from the atoms, leaving behind charged atoms or ions; as a result, α, β, and γ rays are sometimes called ionizing radiation

PAIR PRODUCTION: the process by which a photon encounters a virtual photon in the electromagnetic field of the atom and creates an electron-positron pair that has the total energy of the original photon (except for the small recoil momentum of the nucleus)

PHOTOMULTIPLIER TUBE: a light-sensitive vacuum tube used to convert the extremely weak light output of a scintillation pulse or of Cherenkov radiation into a corresponding electrical signal that is proportional to light intensity

Bibliography

Carrigan, Richard A., Jr., and W. Peter Trower, eds. PARTICLES AND FORCES AT THE HEART OF MATTER: READINGS FROM "SCIENTIFIC AMERICAN" MAGAZINE. New York: W. H. Freeman, 1990. Includes twelve articles, with added postscripts and notes.

Carrigan, Richard A., Jr., PARTICLE PHYSICS IN THE COSMOS: READINGS FROM "SCIENTIFIC AMERICAN" MAGAZINE. New York: W. H. Freeman, 1989. Twelve articles discuss elementary particle physics. Includes notes.

Jelley, John V. CHERENKOV RADIATION AND ITS APPLICATIONS. Elmsford, N.Y.: Pergamon Press, 1958. Although dated, this is a good reference source on Cherenkov. Gives suggestions for further readings. For the specialist and nonspecialist alike.

Schwarzchild, Bertram. "Solar Neutrino Update: Three Detectors Tell Three Different Stories." PHYSICS TODAY 43 (October, 1990): 17. A brief report on results of solar neutrino measurements using different detectors.

Schwarzchild, Bertram. "Underground Experiments Will Look for Proton Decay." PHYSICS TODAY 33 (January, 1980): 17. A report on different detectors that were constructed to look for proton decay.

Willis, William J. "The Large Spectrometers." PHYSICS TODAY 31 (October, 1978): 32. A report on the large detectors used in high-energy physics.

Detectors on High-Energy Accelerators

Electron Emission from Surfaces

Electrons and Atoms

Radiation: Interactions with Matter