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Cherenkov radiation (also spelled Cerenkov, scientific transliteration: Čerenkov) is electromagnetic radiation emitted when a charged particle passes through an insulator at a speed greater than the speed of light in that medium. The characteristic "blue glow" of nuclear reactors is due to Cherenkov radiation. It is named after Soviet scientist Pavel Alekseyevich Cherenkov, the 1958 Nobel Prize winner who was the first to rigorously characterize it.

Contents 1 Physical origin 2 Characteristics 3 Uses 3.1 Nuclear reactors 3.2 Astrophysics experiments 3.3 Particle physics experiments 4 See also 5 References 6 External links

While relativity holds that the speed of light in a vacuum is a universal constant (c), the speed of light in a material may be significantly less than c. For example, the speed of light in water is only 0.75c. Matter can be accelerated beyond this speed during nuclear reactions and in particle accelerators. Cherenkov radiation results when a charged particle, most commonly an electron, exceeds the speed of light in a dielectric (electrically insulating) medium through which it passes.

Moreover, the velocity of light that must be exceeded is the phase velocity rather than the group velocity. The phase velocity can be altered dramatically by employing a periodic medium, and in that case one can even achieve Cherenkov radiation with no minimum particle velocity — a phenomenon known as the Smith-Purcell effect. In a more complex periodic medium, such as a photonic crystal, one can also obtain a variety of other anomalous Cherenkov effects, such as radiation in a backwards direction (whereas ordinary Cherenkov radiation forms an acute angle with the particle velocity).

As a charged particle travels, it disrupts the local electromagnetic field (EM) in its medium. Electrons in the atoms of the medium will be displaced and polarized by the passing EM field of a charged particle. Photons are emitted as an insulator's electrons restore themselves to equilibrium after the disruption has passed. (In a conductor, the EM disruption can be restored without emitting a photon.) In normal circumstances, these photons destructively interfere with each other and no radiation is detected. However, when the disruption travels faster than the photons themselves travel, the photons constructively interfere and intensify the observed radiation.

A common analogy is the sonic boom of a supersonic aircraft or bullet. The sound waves generated by the supersonic body do not move fast enough to get out of the way of the body itself. Hence, the waves "stack up" and form a shock front. Similarly, a speed boat generates a large bow shock because it travels faster than waves can move on the surface of the water.

In the same way, a superluminal charged particle generates a photonic shockwave as it travels through an insulator.

In the figure, v is the velocity of the particle (red arrow), β; is v/c, n is the refractive index of the medium. The blue arrows are photons.

The left corner of the triangle represents the location of the superluminal particle at some initial moment (t=0). The right corner of the triangle is the location of the particle as some later time t. In the given time t, the particle can travel

whereas the electromagnetic waves are constricted to travel at

so, in this time the waves have traveled

So:

Note that since this ratio is independent of time, one can take arbitrary time periods and achieve similar triangles. The angle remains unchanged, meaning that subsequent waves generated between the initial time t=0 and final time t will form similar triangles with coinciding right endpoints to the one shown.

Intuitively, the overall intensity of Cherenkov radiation is proportional to the velocity of the inciting charged particle and to the number of such particles. Unlike fluorescence or emission spectra that have characteristic spectral peaks, Cherenkov radiation is continuous. The relative intensity of one frequency is proportional to the frequency. That is, higher frequencies (shorter wavelengths) are more intense in Cherenkov radiation. This is why visible Cherenkov radiation is observed to be brilliant blue. In fact, most Cherenkov radiation is in the ultraviolet spectrum - it is only with sufficiently accelerated charges that it even becomes visible; the sensitivity of the human eye peaks at green, and is very low in the violet portion of the spectrum.

There is a cut-off frequency for which the equation above cannot be satisfied. Since the refractive index is a function of frequency (and hence wavelength), the intensity doesn't continue increasing at ever shorter wavelengths even for ultra-relativistic particles (where v/c approaches 1). At X-Ray frequencies, the refractive index becomes less than unity and hence no X-Ray emission (or shorter wavelength emissions such as gamma rays) would be observed. However, X-rays can be generated at special energies corresponding to core electronic transitions in a material, as the index of refraction is often greater than 1 at these energies.

As in sonic booms and bow shocks, the angle of the shock cone is inversely related to the velocity of the disruption. Hence, observed angles of incidence can be used to compute the direction and speed of a Cherenkov radiation-producing charge.

Cherenkov radiation is used to detect high-energy charged particles. In pool-type nuclear reactors, the intensity of Cherenkov radiation is related to the frequency of the fission events that produce high-energy electrons, and hence is a measure of the intensity of the reaction. Cherenkov radiation is also used to characterize the remaining radioactivity of spent fuel rods.

When a high-energy cosmic ray interacts with the Earth's atmosphere, it may produce an electron-positron pair with enormous velocities. The Cherenkov radiation from these charged particles is used to determine the source and intensity of the cosmic ray, which is used for example in the Imaging Atmospheric Cherenkov Technique (IACT), by experiments such as VERITAS, H.E.S.S., and MAGIC. Similar methods are used in very large neutrino detectors, such as the Super-Kamiokande, the Sudbury Neutrino Observatory (SNO) and IceCube.

Cherenkov radiation can also be used to determine properties of high-energy astronomical objects that emit gamma rays, such as supernova remnants and blazars. This is done by projects such as STACEE, a gamma ray detector in New Mexico.

Cherenkov radiation is commonly used in experimental particle physics for particle identification. One could measure (or put limits on) the velocity of an electrically charged elementary particle by the properties of the Cherenkov light it emits in a certain medium. If the momentum of the particle is measured independently, one could compute the mass of the particle by its momentum and velocity (see Four-momentum), and hence identify the particle.

The simplest type of particle identification device based on Cherenkov radiation technique is the threshold counter, which gives an answer on whether the velocity of a charged particle of lower or higher than a certain value by looking on whether this particle does or does not emit Cherenkov light in a certain medium. Knowing particle momentum, one can separate particles lighter than a certain threshold from those heavier than the threshold.

The most advanced type of a detector is the RICH, or Ring imaging Cherenkov detector, developed in 1980s. In a RICH detector a cone of Cherenkov light is produced when a high speed charged particle traverses a suitable gaseous or liquid medium, often called radiator. This light cone is detected on a position sensitive planar photon detector, which allows reconstructing a ring or disc, the radius of which is a measure for the Cherenkov emission angle. Both focusing and proximity-focusing detectors are in use. In a focusing RICH detector the photons are collected by a spherical mirror and focused onto the photon detector placed at the focal plane. The result is a circle with a radius independent of the emission point along the particle track. This scheme is suitable for low refractive index radiators, i.e. gases, due to the larger radiator length needed to create enough photons. In the more compact proximity-focusing design a thin radiator volume emits a cone of Cherenkov light which traverses a small distance – the proximity gap – and is detected on the photon detector plane. The image is a ring of light the radius of which is defined by the Cherenkov emission angle and the proximity gap. The ring thickness is determined by the thickness of the radiator. An example of a proximity gap RICH detector is the High Momentum Particle IDentification (HMPID), a detector currently under construction for ALICE (A Large Ion Collider Experiment), one of the six experiments at the LHC (Large Hadron Collider) at CERN.

• Bremsstrahlung
• List of light sources
• Frank-Tamm formula

• L. D. Landau, E. M. Liftshitz, and L. P. Pitaevskii, Electrodynamics of Continuous Media (Pergamon: New York, 1984).
• J. V. Jelley, Cerenkov Radiation and Its Applications (Pergamon: London, 1958).
• S. J. Smith and E. M. Purcell, Phys. Rev. 92, 1069 (1953).
• Chiyan Luo, Mihai Ibanescu, Steven G. Johnson, and J. D. Joannopoulos, "Cerenkov Radiation in Photonic Crystals," Science 299, 368–371 (2003).

• Hyperphysics on Cherenkov radiation