Cherenkov radiation

Cherenkov radiation (sometimes called Cerenkov or, Cherenkov written after the English spelling), named after its discoverer Pavel Alekseyevich Cherenkov, in the strict sense is a bluish luminous phenomenon that is caused faster electrons when passing through water. It is to be observed, for example in swimming pool reactors and spent fuel pools at nuclear power plants; the fast electrons are here partly beta radiation, released in part by collisions of neutrons and gamma rays from atomic shells.

In Russia, the radiation is after her co-discoverer of Sergei Ivanovich Vavilov also called Vavilov Cherenkov radiation.

Cherenkov radiation generally

In a broader sense Tscherenkowstrahlung the electromagnetic radiation which occurs when the charged particles move in the matter with a higher speed than the phase velocity of electromagnetic waves in said medium. More generally, it spoke of the Cherenkov effect. For example, the speed of light in water is only 225 000 000 m / s as compared to 299 792 458 m / s in a vacuum.

When a charged particle moves through a dielectric ( non-conductive ) medium, the atoms are polarized along the trajectory briefly by its charge, producing electromagnetic waves. Normally, the waves of adjacent atoms interfere destructively ( they cancel ), so that macroscopically no radiation occurs. The charged particles moving but faster than light in the medium, the waves of adjacent atoms can no longer cancel out, as always results in a common cone-shaped wavefront. These electromagnetic waves are the Tscherenkowstrahlung.

The direction of the emitted radiation along the flight path describes a so-called Mach cone. The angle between the particle and the direction of radiation depends on the ratio of the velocity of the particle and the velocity of light in the medium having the refractive index from:

The Cherenkov light is thus the optical analogue of the supersonic cone, which occurs when aircraft or other body move faster than sound.

The number of generated photons N per angular frequency and distance x for a particle of charge z ( fine structure constant and c the speed of light ) according to the Frank - Tamm formula

The minimum energy required for emission of Cherenkov radiation by electrons in water is 163 keV.

In 2001 it was discovered experimentally at the Stuttgart Max - Planck - Institute for Solid State Research and at the University of Michigan that conical Cherenkov radiation can also occur in sub- light speed.

Applications

The Cherenkov light is used for the detection of high-energy charged particles, in particular in particle physics, nuclear physics and astrophysics. In particle physics, the Cherenkov radiation of individual charged particles and to measure their speed is used. It eligible for various speed ranges different media such as glass, water or air in question.

In nuclear reactors, the intensity of the Cherenkov radiation is a measure of the number of fissions per second, since in this case pass energy electrons from the fuel rods in the water. After removal of the fuel rods from the reactor core and accommodated in a cooling pond, the intensity is a measure of remaining radioactivity.

Meeting very energetic particle on the Earth's atmosphere, formed new elementary through different processes depending on the type of particle that can produce Cherenkov light. It created this light flashes ( Cherenkov flashes ) of only about a billionth of a second duration, from which one can determine the direction of origin of the cosmic particles. This effect is so significant because, for example, gamma radiation from cosmic explosions, the earth's atmosphere can not penetrate and therefore can not be perceived directly by telescopes on Earth. Only consisting of the gamma quanta ( high energy photons ) resulting electromagnetic showers (consisting of electrons, positrons and photons lower-energy ) can of ground-based instruments ( Cherenkov telescopes ) are analyzed.

In the Super - Kamiokande and in the IceCube experiment, neutrinos are detected by the Cherenkov light is detected by secondary particles ( electrons, muons ) by highly sensitive photomultiplier which arise with water or ice at the extremely rare interactions of neutrinos.

In the case of light propagation in metamaterials, the refractive index can be negative. This then has the result ( in addition to other effects such as a reversed Doppler effect) that occurs Cherenkov radiation is emitted in the opposite direction of the particle, rather than into it.

Cherenkov telescopes and networks

• MAGIC
• High Energy Stereoscopic System
• Cherenkov Telescope Array