views updated

# Cerenkov Effect

KEY TERMS

The Cerenkov effect is the emission of light from a substance like water or glass when a charged particle, such as an electron, travels through the material with a speed faster than the speed of light in that material. The speed of light in a vacuum is a universal speed limit, but light travels more slowly in gasses, solids, and liquids, so it is possible for particles to travel faster than light in those media. In a vacuum, nothing can travel as fast as or faster than light.

The Cerenkov effect was discovered by Russian experimentalist P. A. Cerenkov (19041990) in 1934 and explained by Russian theorists I. Y. Tamm (18651971) and I. M. Frank (19081990). For this work, all three scientists received the Nobel prize in physics in 1958.

The electric field of a fast-moving charged particle shifts the electrons of the atoms of a nonconducting material as the particle passes through. When the particle travels at speeds faster than the speed of light in the material, the atoms respond by

## KEY TERMS

Cerenkov detector a device built to measure the intensity and angle of emission of Cerenkov light. A typical detector consists of a radiating medium through which the charged particle passes (often-times a cylinder of gas), a focusing mirror that collects and concentrates the Cerenkov light, and a photo multiplier tube that converts the light to an electrical signal.

Index of refraction Ratio of the speed of light in a vacuum to the speed of light in a specific material. Numerical values of the index of refaction of materials used in Cerenkov detectors range from 1.000035 for helium gas to 1.5 for Lucite plastic.

emitting light in a cone at an angle determined by the index of refraction of the material. The process can be compared to that of a shock wave of sound generated when an airplane exceeds the speed of sound in air.

The index of refraction is computed by dividing the speed of light in a vacuum (3 × 108 m/sec or 186, 000 mi/sec) by the speed of light in the medium through which the particle passes. The speed of light inlucite or heavy lead glass, for example, is 2 × 108m/sec; therefore the index of refraction for those substances is 1.5.

Cerenkov detectors use the properties of Cerenkov radiation in high energy physics and cosmic ray physics observations. Since the radiation is only emitted when the velocity of the particle is above a predetermined speed, a threshold value for the particle velocity can be set on the detector to discriminate against slow particles. For a particle velocity above the threshold value, an angular measurement of the Cerenkov light relative to the particle direction determines the velocity of the particle. Installation of a large Cerenkov detector at CERN (Organisation Europe´enne pour la Recherche Nucle´-aire, European Organization for Nuclear Research), began in July 2006.

James OConnell