Cherenkov radiation

Cherenkov Radiation in the Advanced Test Reactor, Idaho National Laboratory

Cherenkov radiation (also written as Čerenkov or - in English transliteration - Cherenkov radiation ) is in the narrower sense the bluish luminous phenomenon that occurs when relativistic, charged particles (e.g. electrons ) pass through a translucent dielectric .

It occurs in swimming pool reactors and in the cooling pools of nuclear power plants . The causing, fast electrons are partly part of the beta radiation , partly they arise from Compton scattering of gamma quanta on atomic shells .

Cherenkov radiation is named after its discoverer Pavel Alexejewitsch Cherenkov . In Russia, the radiation is also called Vawilow Cherenkov radiation after its co-discoverer Sergei Iwanowitsch Wawilow .

Cherenkov radiation in general

Cherenkov effect
(ideal case without dispersion )

In a broader sense, Cherenkov radiation is the electromagnetic radiation that is created when charged particles in matter move at a higher speed than the phase speed of electromagnetic waves in this medium . The Cherenkov effect is generally spoken of. In contrast to the speed of light in a vacuum of 299,792.458 km / s , z. B. the speed of light in water is only about 225,000 km / s .

When a charged particle moves through a non-conductive dielectric medium, atoms along the flight path are briefly polarized by its charge and generate electromagnetic radiation in the process. Normally, the waves from neighboring atoms interfere destructively and cancel each other out, so that macroscopically no radiation occurs. If, however, charged particles move faster in a medium than light in this, the waves of neighboring atoms no longer cancel each other out, as there is always a common conical wave front. These electromagnetic waves are Cherenkov radiation.

The direction of the emitted radiation along the flight path describes a so-called Mach cone . The angle between the particle path and the direction of radiation depends on the ratio of the speed of the particle and the speed of light in the medium with the refractive index : ${\ displaystyle \ theta}$ ${\ displaystyle v = \ beta c}$ ${\ displaystyle c '= c / n}$ ${\ displaystyle n}$

{\ displaystyle \ cos (\ theta) = {\ frac {c '} {v}} = {\ frac {1} {n \ beta}}}

Cherenkov light is thus the optical analogue of the supersonic cone that is created when aircraft or other bodies move faster than sound .

The frequency spectrum of the Cherenkov radiation produced can be calculated using the Frank-Tamm formula :

{\ displaystyle {\ frac {d ^ {2} E} {dx \, d \ omega}} = {\ frac {q ^ {2}} {4 \ pi}} \ mu (\ omega) \ omega {\ left (1 - {\ frac {c ^ {2}} {v ^ {2} n ^ {2} (\ omega)}} \ right)}}

This formula describes the amount of energy that is emitted per angular frequency and distance for a particle of the charge . is the frequency-dependent magnetic permeability and the frequency-dependent refractive index of the medium. ${\ displaystyle \ omega}$ ${\ displaystyle x}$ ${\ displaystyle q}$ ${\ displaystyle \ mu (\ omega)}$ ${\ displaystyle n (\ omega)}$

In the visible range of the electromagnetic spectrum, the magnetic permeability and the refractive index can be assumed to be approximately constant in the case of water as the medium. The spectrum in this area can thus be estimated for each location along the path of the charged particle as follows:

{\ displaystyle E \ propto \ omega ^ {2}}

At higher frequencies, more photons are emitted than at lower frequencies, which explains the blue color of the Cherenkov radiation in the picture shown above. N is the number of photons. ${\ displaystyle E = N \ hbar \ omega}$

The minimum kinetic energy of a particle of mass necessary for the emission of Cherenkov radiation in a medium with the refractive index is: ${\ displaystyle E}$ ${\ displaystyle m}$ ${\ displaystyle n}$

{\ displaystyle E = mc ^ {2} \ left ({\ frac {n} {\ sqrt {n ^ {2} -1}}} - 1 \ right)}

.

Divided by the elementary charge you get the energy in eV, divided by your own charge you get the necessary acceleration voltage. ${\ displaystyle e}$ ${\ displaystyle q}$

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

Cherenkov telescope MAGIC

Cherenkov telescope FACT

Applications

Cherenkov light is used to detect high-energy charged particles, especially in particle physics , nuclear physics and astrophysics . In particle physics, Cherenkov radiation is also used to measure the speed of individual charged particles. Different media such as glass, water or air can be used for different speed ranges.

In water-moderated and -cooled nuclear reactors , the intensity of Cherenkov radiation is a measure of the instantaneous radioactivity of the fission products in the nuclear fuel (and thus for the reactor output), since high-energy beta electrons from the fuel get into the water. After the fuel elements have been removed from the reactor core and placed in a cooling pool , the intensity is a measure of the remaining radioactivity.

When very energetic cosmic particles hit the earth's atmosphere , depending on the type of particle, new elementary particles are formed through different processes, which Cherenkov light can generate. This creates flashes of light ( Cherenkov flashes ) with a duration of billionths of a second, from which one can determine the direction of origin of the cosmic particles. This effect is important for observation because z. B. Gamma radiation from cosmic explosions does not penetrate the earth's atmosphere and therefore cannot be directly perceived by telescopes on earth. Only the electromagnetic shower (consisting of electrons, positrons and lower -energy photons) arising from the gamma quanta (high-energy photons) can be analyzed by earth-based measuring devices (Cherenkov telescopes).

In the Super-Kamiokande , IceCube and ANTARES experiments , cosmic neutrinos are detected by photomultipliers detecting the Cherenkov light from secondary particles ( electrons and muons ), which are created during the extremely rare interaction of neutrinos with water or ice.

In the case of light propagation in metamaterials , the refractive index can become negative. This has the consequence (in addition to other effects such as a reverse Doppler effect ) that Cherenkov radiation that occurs is not emitted in the direction of the particle movement, but in the opposite direction.

Nobel Prize

Igor Evgenjewitsch Tamm , Pawel Alexejewitsch Tscherenkow and Ilja Michailowitsch Frank received the 1958 Nobel Prize in Physics "for the discovery and interpretation of the Cherenkov effect".

Cherenkov telescopes and networks

Individual evidence

↑ DESY kworkquark.net ( Memento of December 13, 2007 in the Internet Archive ) and TE Stevens, JK Wahlstrand, J. Kuhl, R. Merlin: Cherenkov Radiation at Speeds Below the Light Threshold. Phonon-Assisted Phase Matching . In: Science . January 26, 2001
^ DR Smith , JB Pendry and MCK Wiltshire: Metamaterials and Negative Refractive Index . In: Science . Volume 305, August 6, 2004

literature

Dieter Meschede : Gerthsen Physics . 23rd edition. Springer, Berlin, 2006, ISBN 3-540-25421-8 .
Gerhard Musiol, Johannes Ranft, Roland Reif: Nuclear and elementary particle physics . Wiley-VCH, 1987, ISBN 3-527-26886-3 , pp. 1127 .

Web links

Commons : Cherenkov Radiation - Collection of Images, Videos and Audio Files

Cern Courier on Wawilow and the discovery of Cherenkov radiation

[1] DESY kworkquark.net ( Memento of December 13, 2007 in the Internet Archive ) and TE Stevens, JK Wahlstrand, J. Kuhl, R. Merlin: Cherenkov Radiation at Speeds Below the Light Threshold. Phonon-Assisted Phase Matching . In: Science . January 26, 2001

[2] DR Smith , JB Pendry and MCK Wiltshire: Metamaterials and Negative Refractive Index . In: Science . Volume 305, August 6, 2004