Radiation, Cherenkov

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RADIATION, CHERENKOV

When a charged particle travels through a transparent material at a speed greater than that of light in that material, it emits a characteristic blue light, called Cherenkov radiation. Around 100 years ago, Marie and Pierre Curie enjoyed seeing a beautiful, if slightly eerie, blue glow coming from their concentrated radium solutions, but their observations occurred long before the complex light emitting effects in these solutions were understood or, indeed, before the health dangers of the ionizing radiation producing the effects were realized. Inventive experimental investigations to fully explore the phenomena now called Cherenkov radiation, carried out by Pavel Cherenkov between 1934 and 1944, with rudimentary apparatus and under trying physical conditions, were explained theoretically by Ilya Frank and Igor Tamm using classical electromagnetic theory, resulting in the award of the Nobel Prize to these three physicists in 1958.

Cherenkov light is an electromagnetic analog of the more familiar sonic boom, produced by an aircraft moving faster than the speed of sound in air, and is possible only because the phase velocity of light (υ_light) in transparent materials is slower than the speed of light (c ) in a vacuum. Even though Albert Einstein's Special Theory of Relativity prohibits a rapidly moving particle with velocity (υ_particle) from traveling faster than the speed of light in a vacuum, it may still exceed the speed of light in the material that it enters and, therefore, may exceed the Cherenkov threshold velocity (υ_t=υ_light) at which an electro-magnetic shock wave forms (Figure 1). This shock wave can be observed as a very fast pulse of light emitted uniformly in a cone around the particle direction with a characteristic Cherenkov cone-opening angle θc , the cosine (cos θ ) of which is given by Here, n = c /υ_light is an optical parameter of a material called the index of refraction. The Cherenkov angle θc approaches a maximum value as the particle speed approaches c .

Cherenkov radiation is emitted at all light frequencies for which the particle speed exceeds υ_light, with most of the light being observed at shorter wavelengths. This leads to the characteristic blue color. Above the threshold, the amount of light emitted increases as θ_c(υ_particle) increases and is proportional to the length of the particle's path in the material.

Particle detectors that use Cherenkov light are called Cherenkov counters. They are used in a number of scientific fields, such as elementary particle

FIGURE 1

physics, nuclear physics, studies of cosmic rays, and neutrino astronomy. Typically, they make particular use of one or more of the properties of Cherenkov light: (1) the fast emission, (2) the velocity threshold, and (3) the dependence of the light emission angle and the amount of light emitted on the particle velocity. The latter two features are especially useful in elementary particle physics, as they may be combined with a measurement of the momentum of the particle to determine the particle's mass and, thus, identify the kind of particle that has been observed. Detectors used in this manner are called particle identification detectors (PID).

Cherenkov counters have two main elements:(1) a radiator through which the charged particle passes and (2) a photodetector to interact with the emitted light and produce an electrical signal that can be further processed, counted, and displayed. At the very low light levels of the Cherenkov effect, light is usually detected as a small number of individual particles called photons. The most common kind of photon detector is the photomultiplier tube, which can detect a single, visible light photon with an efficiency of about 20 percent and its time of arrival to better than one nanosecond (10^-9 second). Other common photon detectors include wire chambers that can determine the positions of photons within the detector.

Radiators can be chosen from a wide variety of transparent materials. The choice between a gas (e.g., Nitrogen [υ_t= 0.9997c ]), a liquid (e.g., water [υ_t= 0.730c ]), or a solid (e.g., glass or plastic [υ_t= 0.667c ]) radiator is made to best match the velocity range of the particles under study. As the amount of light emitted per unit length is very small for the high velocity threshold materials, the radiator must be much longer for a gas counter than for a solid counter. An extremely light "designer" material called silica aerogel (made mainly from the same molecular material as ordinary glass but with much more space between the molecules) may be used to cover the velocity region between the gases and liquid or solid materials.

Cherenkov counters are classified as either imaging or threshold types, depending on whether they do, or do not, make use of the Cherenkov angle (θ_c) information. Imaging counters are sometimes used to track particles as well as to identify them.

Cherenkov counters may be of almost any size. Some threshold detectors fit in the palm of a hand. A typical high-energy physics detector is much larger. For example, the imaging Cherenkov counter called the DIRC (Detection of Internally Reflected Cherenkov Light) that is part of the B -factory detector BaBar at the Stanford Linear Accelerator Center has a radiator of pure fused silica glass that covers about 5m² and a photon detector with about 11,000 3-centimeter diameter photomultiplier tubes. The detectors used for neutrino astronomy, or cosmic ray studies, are even larger. For example, a large imaging neutrino detector in Japan called Super-Kamiokande is around 40 meters high, contains 50,000 tons of pure water as a radiator, and has over 11,000 very large (50 centimeter diameter) photomultipliers, while the AMANDA detector at the South Pole plans eventually to have 5,000 large photomultipliers imbedded in a cubic kilometer of ice.