Cherenkov radiation (pronunciation: /tʃɛrɛnˈkɔv/) is an electromagnetic radiation emitted when a charged particle (such as an electron) passes through a dielectric medium at a speed greater than the phase velocity of light in that medium. The characteristic blue glow of an underwater nuclear reactor is due to Cherenkov radiation.
The radiation is named after the Soviet scientist Pavel Cherenkov, the 1958 Nobel Prize winner, who was the first to detect it experimentally under the supervision of Sergey Vavilov at the Lebedev Institute in 1934. Therefore it is also known as Vavilov–Cherenkov radiation. Cherenkov saw a faint bluish light around a radioactive preparation in water during experiments. His doctorate thesis was on luminescence of uranium salt solutions that were excited by gamma rays instead of less energetic visible light, as done commonly. He discovered the anisotropy of the radiation and came to the conclusion that the bluish glow was not a fluorescent phenomenon.
A theory of this effect was later developed in 1937 within the framework of Einstein's special relativity theory by Cherenkov's colleagues Igor Tamm and Ilya Frank, who also shared the 1958 Nobel Prize.
Cherenkov radiation as conical wave front had been theoretically predicted by the English polymath Oliver Heaviside in papers published between 1888 and 1889 and by Arnold Sommerfeld in 1904, but both had been quickly forgotten following the relativity theory's restriction of super-c particles until the 1970s. Marie Curie observed a pale blue light in a highly concentrated radium solution in 1910, but did not bother to look into details. In 1926, the French radiotherapists Lucien Mallet described the luminous radiation of radium irradiating water having a continuous spectrum.
While electrodynamics holds that the speed of light in a vacuum is a universal constant (c), the speed at which light propagates in a material may be significantly less than c. For example, the speed of the propagation of light in water is only 0.75c. Matter can be accelerated beyond this speed (although still to less than c) during nuclear reactions and in particle accelerators. Cherenkov radiation results when a charged particle, most commonly an electron, travels through a dielectric (electrically polarizable) medium with a speed greater than that at which light propagates in the same medium.
A common analogy is the sonic boom of a supersonic aircraft. The sound waves generated by the supersonic body propagate at the speed of sound itself; as such, the waves travel slower than the speeding object and cannot propagate forward from the body, instead forming a shock front. In a similar way, a charged particle can generate a light shock wave as it travels through an insulator.
Moreover, the velocity that must be exceeded is the phase velocity of light rather than the group velocity of light. 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 (see below) whereas ordinary Cherenkov radiation forms an acute angle with the particle velocity.
In their original work on the theoretical foundations of Cherenkov radiation, Tamm and Frank wrote,"This peculiar radiation can evidently not be explained by any common mechanism such as the interaction of the fast electron with individual atom or as radiative scattering of electrons on atomic nuclei. On the other hand, the phenomenon can be explained both qualitatively and quantitatively if one takes in account the fact that an electron moving in a medium does radiate light even if it is moving uniformly provided that its velocity is greater than the velocity of light in the medium.". However, some misconceptions regarding Cherenkov radiation exist: for example, it is believed that the medium becomes electrically polarized by the particle's electric field. If the particle travels slowly then the disturbance elastically relaxes back to mechanical equilibrium as the particle passes. When the particle is traveling fast enough, however, the limited response speed of the medium means that a disturbance is left in the wake of the particle, and the energy contained in this disturbance radiates as a coherent shockwave. Such conceptions do not have any analytical foundation, as electromagnetic radiation is emitted when charged particles move in a dielectric medium at subluminal velocities which are not considered as Cherenkov radiation.
In the figure on the geometry, the particle (red arrow) travels in a medium with speed such that
We define the ratio between the speed of the particle and the speed of light as
The emitted light waves (blue arrows) travel at speed
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 at some later time t. In the given time t, the particle travels the distance
whereas the emitted electromagnetic waves are constricted to travel the distance
So the emission angle results in
Cherenkov radiation can also radiate in an arbitrary direction using properly engineered one dimensional metamaterials. The latter is designed to introduce a gradient of phase retardation along the trajectory of the fast travelling particle ( ), reversing or steering Cherenkov emission at arbitrary angles given by the generalized relation:
Note that since this ratio is independent of time, one can take arbitrary times and achieve similar triangles. The angle stays the same, 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.
A reverse Cherenkov effect can be experienced using materials called negative-index metamaterials (materials with a subwavelength microstructure that gives them an effective "average" property very different from their constituent materials, in this case having negative permittivity and negative permeability). This means, when a charged particle (usually electrons) passes through a medium at a speed greater than the phase velocity of light in that medium, that particle will emit trailing radiation from its progress through the medium rather than in front of it (as is the case in normal materials with, both permittivity and permeability positive). One can also obtain such reverse-cone Cherenkov radiation in non-metamaterial periodic media where the periodic structure is on the same scale as the wavelength, so it cannot be treated as an effectively homogeneous metamaterial.
The Frank-Tamm formula describes the amount of energy emitted from Cherenkov radiation, per unit length traveled and per frequency . is the permeability and is the index of refraction of the material the charge particle moves through. is the electric charge of the particle, is the speed of the particle, and is the speed of light in vacuum.
Unlike fluorescence or emission spectra that have characteristic spectral peaks, Cherenkov radiation is continuous. Around the visible spectrum, the relative intensity per unit frequency is approximately 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 above which the equation can no longer be satisfied. The refractive index varies with frequency (and hence with wavelength) in such a way that the intensity cannot continue to increase at ever shorter wavelengths, even for very relativistic particles (where v/c is close to 1). At X-ray frequencies, the refractive index becomes less than unity (note that in media the phase velocity may exceed c without violating relativity) 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 frequencies just below the frequencies corresponding to core electronic transitions in a material, as the index of refraction is often greater than 1 just below a resonant frequency (see Kramers-Kronig relation and anomalous dispersion).
As in sonic booms and bow shocks, the angle of the shock cone is directly related to the velocity of the disruption. The Cherenkov angle is zero at the threshold velocity for the emission of Cherenkov radiation. The angle takes on a maximum as the particle speed approaches the speed of light. Hence, observed angles of incidence can be used to compute the direction and speed of a Cherenkov radiation-producing charge.
Cherenkov radiation can be generated in the eye by charged particles hitting the vitreous humour, giving the impression of flashes, as in cosmic ray visual phenomena and possibly some observations of criticality accidents.
Cherenkov radiation is widely used to facilitate the detection of small amounts and low concentrations of biomolecules. Radioactive atoms such as phosphorus-32 are readily introduced into biomolecules by enzymatic and synthetic means and subsequently may be easily detected in small quantities for the purpose of elucidating biological pathways and in characterizing the interaction of biological molecules such as affinity constants and dissociation rates.
More recently, Cherenkov light has been used to image substances in the body. These discoveries have led to intense interest around the idea of using this light signal to quantify and/or detect radiation in the body, either from internal sources such as injected radiopharmaceuticals or from external beam radiotherapy in oncology. Radioisotopes such as the positron emitters 18F and 13N or beta emitters 32P or 90Y have measurable Cherenkov emission and isotopes 18F and 131I have been imaged in humans for diagnostic value demonstration. External beam radiation therapy has been shown to induce a substantial amount of Cherenkov light in the tissue being treated, due to the photon beam energy levels used in the 6 MeV to 18 MeV ranges. The secondary electrons induced by these high energy x-rays result in the Cherenkov light emission, where the detected signal can be imaged at the entry and exit surfaces of the tissue.
Cherenkov radiation is used to detect high-energy charged particles. In pool-type nuclear reactors, beta particles (high-energy electrons) are released as the fission products decay. The glow continues after the chain reaction stops, dimming as the shorter-lived products decay. Similarly, Cherenkov radiation can characterize the remaining radioactivity of spent fuel rods. This phenomenon is used to verify the presence of spent nuclear fuel in spent fuel pools for nuclear safeguards purposes.
When a high-energy (TeV) gamma photon or cosmic ray interacts with the Earth's atmosphere, it may produce an electron-positron pair with enormous velocities. The Cherenkov radiation emitted in the atmosphere by these charged particles is used to determine the direction and energy of the cosmic ray or gamma ray, which is used for example in the Imaging Atmospheric Cherenkov Technique (IACT), by experiments such as VERITAS, H.E.S.S., MAGIC. Cherenkov radiation emitted in tanks filled with water by those charged particles reaching earth is used for the same goal by the Extensive Air Shower experiment HAWC, the Pierre Auger Observatory and other projects. Similar methods are used in very large neutrino detectors, such as the Super-Kamiokande, the Sudbury Neutrino Observatory (SNO) and IceCube. Other projects operated in the past applying related techniques, such as STACEE, a former solar tower refurbished to work as a non-imaging Cherenkov observatory, which was located in New Mexico.
Astrophysics observatories using the Cherenkov technique to measure air showers are key to determine the properties of astronomical objects that emit Very High Energy gamma rays, such as supernova remnants and blazars.
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 a Cherenkov radiation technique is the threshold counter, which gives an answer as to whether the velocity of a charged particle is lower or higher than a certain value (, where is the speed of light, and is the refractive index of the medium) by looking at 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 the 1980s. In a RICH detector, a cone of Cherenkov light is produced when a high speed charged particle traverses a suitable 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 Detector (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.
The Cherenkov effect can occur in vacuum. In a slow-wave structure, the phase velocity decreases and the velocity of charged particles can exceed the phase velocity while remaining lower than . In such a system, this effect can be derived from conservation of the energy and momentum where the momentum of a photon should be ( is phase constant) rather than the de Broglie relation . This type of radiation (VCR) is used to generate high power microwaves.
The Antarctic Muon And Neutrino Detector Array (AMANDA) is a neutrino telescope located beneath the Amundsen–Scott South Pole Station. In 2005, after nine years of operation, AMANDA officially became part of its successor project, the IceCube Neutrino Observatory.
AMANDA consists of optical modules, each containing one photomultiplier tube, sunk in Antarctic ice cap at a depth of about 1500 to 1900 metres. In its latest development stage, known as AMANDA-II, AMANDA is made up of an array of 677 optical modules mounted on 19 separate strings that are spread out in a rough circle with a diameter of 200 metres. Each string has several dozen modules, and was put in place by "drilling" a hole in the ice using a hot-water hose, sinking the cable with attached optical modules in, and then letting the ice freeze around it.
AMANDA detects very high energy neutrinos (50+ GeV) which pass through the Earth from the northern hemisphere and then react just as they are leaving upwards through the Antarctic ice. The neutrino interacts with nuclei of oxygen or hydrogen atoms contained in the surrounding water ice through the weak nuclear force, producing a muon and a hadronic shower. The optical modules detect the Cherenkov radiation from these latter particles, and by analysis of the timing of photon hits can approximately determine the direction of the original neutrino with a spatial resolution of approximately 2 degrees.
AMANDA's goal was an attempt at neutrino astronomy, identifying and characterizing extra-solar sources of neutrinos. Compared to underground detectors like Super-Kamiokande in Japan, AMANDA was capable of looking at higher energy neutrinos because it is not limited in volume to a manmade tank; however, it had much less accuracy because of the less controlled conditions and wider spacing of photomultipliers. Super-Kamiokande can look at much greater detail at neutrinos from the Sun and those generated in the Earth's atmosphere; however, at higher energies, the spectrum should include neutrinos dominated by those from sources outside the solar system. Such a new view into the cosmos could give important clues in the search for Dark Matter and other astrophysical phenomena.
After two years of integrated operation as part of IceCube, the AMANDA counting house (in the Martin A. Pomerantz Observatory) was decommissioned in July and August 2009.Askaryan radiation
The Askaryan radiation also known as Askaryan effect is the phenomenon whereby a particle traveling faster than the phase velocity of light in a dense dielectric (such as salt, ice or the lunar regolith) produces a shower of secondary charged particles which contain a charge anisotropy and thus emits a cone of coherent radiation in the radio or microwave part of the electromagnetic spectrum. It is similar to the Cherenkov radiation. It is named after Gurgen Askaryan, a Soviet-Armenian physicist who postulated it in 1962.
The radiation was first observed experimentally in 2000, 38 years after its theoretical prediction. So far the effect has been observed in silica sand, rock salt, ice, and Earth's atmosphere.The effect is of primary interest in using bulk matter to detect ultra-high energy neutrinos. The Antarctic Impulse Transient Antenna (ANITA) experiment uses antennas attached to a balloon flying over Antarctica to detect the Askaryan radiation produced as cosmic neutrinos travel through the ice. Several experiments have also used the Moon as a neutrino detector based on detection of the Askaryan radiation.CORSIKA
CORSIKA (COsmic Ray SImulations for KAscade) is a physics computer software for simulation of extensive air showers induced by high energy cosmic rays. It may be used up to and beyond the highest energies of 100 EeV.
The program utilizes the hadronic interaction models VENUS, QGSJET, and DPMJET, which are based on the Gribov-Regge theory and SIBYLL based on a minijet model for high
energies. Hadronic interactions at lower energies are described either by the GHEISHA module, by FLUKA, or by the UrQMD model. Electromagnetic interactions are treated by an adapted version of the EGS4 code,
customized by including the Landau–Pomeranchuk–Migdal effect relevant at higher energies.
It can be used to simulate the generation of Cherenkov radiation and atmospheric neutrinos.
A complete rewrite of CORSIKA in C++ named CORSIKA 8 is currently work in progress.Cherenkov detector
A Cherenkov detector (pronunciation: /tʃɛrɛnˈkɔv/; Russian: Черенко́в) is a particle detector using the speed threshold for light production, the speed-dependent light output or the speed-dependent light direction of Cherenkov radiation.Cherenkov luminescence imaging
Cherenkov luminescence imaging (CLI) is an emerging imaging modality, similar to bioluminescence imaging, that captures visible photons emitted by Cherenkov radiation. It basically is the optical imaging of radiotracers that emit charged particles traveling faster than the phase velocity of light in that particular medium. It can be used to quickly evaluate radio tracers in preclinical research but also to obtain clinical images in patients. While radioactivity itself can not be modified, the emitted light provides an opportunity to generate radioactivity-based activatable or "smart" imaging agents that sense for example enzymatic activity.DUMAND Project
The DUMAND Project (Deep Underwater Muon And Neutrino Detector Project) was a proposed underwater neutrino telescope to be built in the Pacific Ocean, off the shore of the island of Hawaii, five kilometers beneath the surface. It would have included thousands of strings of instruments occupying a cubic kilometer of the ocean.
The proposal called for two types of detectors: optical detectors to find the Cherenkov radiation emitted by secondary particles traveling faster than the speed of light in water, resulting from collisions by neutrinos, and hydrophones to listen for the acoustic signals generated by the interactions. Sophisticated signal processing would have combined the signals from many optical and acoustic sensors, allowing scientists to determine the direction from which the neutrino arrived, and to rule out false signals arising from other particles or acoustic sources. Because of the nature of the interaction between neutrinos and protons, DUMAND would have been most sensitive to ultra-high energy neutrinos, and completely insensitive to solar neutrinos.
Work began in about 1976, at Keahole Point, but the project cancelled in 1995 due to technical difficulties. Although it was never completed, DUMAND was in a sense a precursor of the Antarctic Muon And Neutrino Detector Array (AMANDA), and the water Cherenkov neutrino telescopes in the Mediterranean (ANTARES, NEMO and the NESTOR Project). The DUMAND hardware was also donated to NESTOR, to reduce costs and cut on development and construction time.Frank–Tamm formula
The Frank–Tamm formula yields the amount of Cherenkov radiation emitted on a given frequency as a charged particle moves through a medium at superluminal velocity. It is named for Russian physicists Ilya Frank and Igor Tamm who developed the theory of the Cherenkov effect in 1937, for which they were awarded a Nobel Prize in Physics in 1958.
When a charged particle moves faster than the phase speed of light in a medium, electrons interacting with the particle can emit coherent photons while conserving energy and momentum. This process can be viewed as a decay. See Cherenkov radiation and nonradiation condition for an explanation of this effect.Ilya Frank
Ilya Mikhailovich Frank (Russian: Илья́ Миха́йлович Франк) (23 October 1908 – 22 June 1990) was a Soviet winner of the Nobel Prize for Physics in 1958 jointly with Pavel Alekseyevich Cherenkov and Igor Y. Tamm, also of the Soviet Union. He received the award for his work in explaining the phenomenon of Cherenkov radiation. He received the Stalin prize in 1946 and 1953 and the USSR state prize in 1971.Ionized-air glow
Ionized-air glow is the fluorescent emission of characteristic blue–purple–violet light, of color called electric blue, by air subjected to an energy flux.Milagro (experiment)
Milagro (the Spanish word for miracle) was a ground-based water Cherenkov radiation telescope situated in the Jemez Mountains near Los Alamos, New Mexico at the Fenton Hill Observatory site. It was primarily designed to detect gamma rays but also detected large numbers of cosmic rays. It operated in the TeV region of the spectrum at an altitude of 2530 m. Like conventional telescopes, Milagro was sensitive to light but the similarities ended there. Whereas "normal" astronomical telescopes view the universe in visible light, Milagro saw the universe at very high energies. The light that Milagro saw was about 1 trillion times more energetic than visible light. While these particles of light, known as photons, are the same as the photons that make up visible light, they behave quite differently due to their high energies.
A cosmic ray or high-energy gamma ray striking an atom in the upper atmosphere generates a cascade of particles known as an air shower. This cascade of particles are traveling near the speed of light and generate Cherenkov radiation as they pass through the atmosphere and the water in the Milagro experiment. The photons of Cherenkov radiation are detected by an array of detectors or photomultiplier tubes which send a signal to a recorder. The data from the recorder can then be used to determine the energy and direction of the cosmic or gamma ray. The Milagro experiment used 700 sensitive light detectors submerged in the pond plus another 200 detectors arrayed around the pond.The Milagro Experiment stopped taking data in April 2008 after seven years of operation. There is a follow up experiment called the High Altitude Water Cherenkov Experiment (HAWC) located near the Large Millimeter Telescope at the Sierra Negra volcano, Mexico, which is expected to be 15 times more sensitive.
In November 2008 Milagro published the surprising result of observing cosmic ray anisotropy.NEVOD
NEVOD (Russian: НЕВОД, НЕйтринный ВОдный Детектор, Neutrino Water Detector; nevod means "dragnet" in Russian) is a neutrino detector and cosmic ray experiment that attempts to detect Cherenkov radiation arising from interactions between water and charged particles (mostly muons). It represents the first attempt to perform such measurements at the Earth's surface; it is because of this surface deployment that the experiment is also able to investigate cosmic rays. NEVOD is situated at the Moscow Engineering Physics Institute (MEPhI).
The term NEVOD experimental complex is used of the experimental complex built around the original water Cherenkov detector for the study of cosmic rays; as of 2018, the experimental complex consists of: the Cherenkov water detector (the eponymous NEVOD detector), a coordinate-tracking detector DECOR, an array of scintillation detectors forming the calibration telescopes system CTS, and PRISMA array of thermal neutron detectors. As of 2018, the experimental complex is being expanded by three new cosmic ray detectors: NEVOD-EAS (for determination of cosmic ray air shower parameters), URAN (neutron detector) and TREK (drift chamber detector). Part of the new detectors are under operation (in 2018).The experimental complex used to also have a muon hodoscope URAGAN which was operational in 2016 and years prior. Current (2019) status of URAGAN is unknown.Neutrino detector
A neutrino detector is a physics apparatus which is designed to study neutrinos. Because neutrinos only weakly interact with other particles of matter, neutrino detectors must be very large to detect a significant number of neutrinos. Neutrino detectors are often built underground, to isolate the detector from cosmic rays and other background radiation. The field of neutrino astronomy is still very much in its infancy – the only confirmed extraterrestrial sources so far as of 2018 are the Sun and the supernova 1987A in the nearby Large Magellenic Cloud. Another likely source (3 standard deviations ) is the blazar TXS 0506+056 about 3.7 billion light years away. Neutrino observatories will "give astronomers fresh eyes with which to study the universe."Various detection methods have been used. Super Kamiokande is a large volume of water surrounded by phototubes that watch for the Cherenkov radiation emitted when an incoming neutrino creates an electron or muon in the water. The Sudbury Neutrino Observatory is similar, but uses heavy water as the detecting medium. Other detectors have consisted of large volumes of chlorine or gallium which are periodically checked for excesses of argon or germanium, respectively, which are created by neutrinos interacting with the original substance. MINOS uses a solid plastic scintillator watched by phototubes; Borexino uses a liquid pseudocumene scintillator also watched by phototubes; and the NOνA detector uses a liquid scintillator watched by avalanche photodiodes.
The proposed acoustic detection of neutrinos via the thermoacoustic effect is the subject of dedicated studies done by the ANTARES, IceCube, and KM3NeT collaborations.Omega West Reactor
The Omega West Reactor (OWR) was an experimental nuclear reactor located at Los Alamos National Laboratory in Los Alamos NM. OMR was completed in 1956 and primarily used for scientific scale nuclear research until it was fully decommissioned in 1994. It operated 24 hours a day, five days a week until 1972, when it went to eight hours a day, five days a week operation. The original purpose of the reactor was to collect nuclear material properties in support of the United States nuclear weapons program. Other uses included production of useful medical isotopes. The reactor was capable of producing an external beam of neutrons via beam tubes which extended through the reactor shielding. These were provided for external neutron beam experiments including: neutron radiography, neutron capture studies, gamma ray studies, neutron cross section measurements and neutron activation studies.
The reactor's low-pressure design and tall vertical vessel made it possible for a lead glass window to be installed at the top, through which the active core could be viewed directly. Very few production nuclear reactors were ever designed with viewing windows. The blue glow seen in the picture is characteristic of Cherenkov radiation.Pavel Cherenkov
Pavel Alekseyevich Cherenkov (Russian: Па́вел Алексе́евич Черенко́в [ˈpavʲɪɫ ɐlʲɪˈksʲeɪvʲɪtɕ tɕɪrʲɪnˈkof], July 28, 1904 – January 6, 1990) was a Soviet physicist who shared the Nobel Prize in physics in 1958 with Ilya Frank and Igor Tamm for the discovery of Cherenkov radiation, made in 1934.Smith–Purcell effect
The Smith–Purcell effect was the precursor of the free electron laser (FEL). It was studied by Steve Smith, a graduate student under the guidance of Edward Purcell. In their experiment, they sent an energetic beam of electrons very closely parallel to the surface of a ruled optical diffraction grating, and thereby generated visible light. Smith showed there was negligible effect on the trajectory of the inducing electrons. Essentially, this is a form of Cherenkov radiation where the phase velocity of the light has been altered by the periodic grating.Solar tower (astronomy)
A solar tower, in the context of astronomy, is a structure used to support equipment for studying the sun, and is typically part of solar telescope designs. Generically, the term solar tower has many more uses especially for a type of power production using Earth's Sun. Solar tower observatories are also called vacuum tower telescopes.
Solar towers are used to raise the observation equipment above the atmospheric disturbances caused by solar heating of the ground and the radiation of the heat into the atmosphere. Traditional observatories do not have to be placed high above ground level, as they do most of their observation at night, when ground radiation is at a minimum.
The horizontal Snow solar observatory was built on Mount Wilson in 1904. It was soon found that heat radiation was disrupting observations. Almost as soon as the Snow Observatory opened, plans were started for a 60-foot-tall (18 m) tower that opened in 1908 followed by a 150-foot (46 m) tower in 1912. The 60-foot (18 m) tower is currently used to study helioseismology, while the 150-foot (46 m) tower is active in UCLA's Solar Cycle Program.
The term has also been used to refer to other structures used for experimental purposes, such as the Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE), which is being used to study Cherenkov radiation, and the Weizmann Institute solar power tower.Tachyon
A tachyon () or tachyonic particle is a hypothetical particle that always travels faster than light. Most physicists believe that faster-than-light particles cannot exist because they are not consistent with the known laws of physics. If such particles did exist, they could be used to build a tachyonic antitelephone and send signals faster than light, which (according to special relativity) would lead to violations of causality. No experimental evidence for the existence of such particles has been found.
The possibility of particles moving faster-than-light was first proposed by O. M. P. Bilaniuk, V. K. Deshpande, and E. C. G. Sudarshan in 1962, although the term they used for it was "meta-particle". In the 1967 paper that coined the term, Gerald Feinberg proposed that tachyonic particles could be quanta of a quantum field with imaginary mass. However, it was soon realized that excitations of such imaginary mass fields do not under any circumstances propagate faster than light, and instead the imaginary mass gives rise to an instability known as tachyon condensation. Nevertheless, in modern physics the term "tachyon" often refers to imaginary mass fields rather than to faster-than-light particles. Such fields have come to play a significant role in modern physics.
The term comes from the Greek: ταχύ, tachy, meaning "rapid". The complementary particle types are called luxons (which always move at the speed of light) and bradyons (which always move slower than light); both of these particle types are known to exist.Track Imaging Cherenkov Experiment
The Track Imaging Cherenkov Experiment (TrICE) is a ground-based cosmic ray telescope located at Argonne National Laboratory near Chicago, IL. The telescope, which contains a Fresnel lens, eight spherical mirrors, and a camera with 16 multianode photomultiplier tubes, uses the atmospheric Cherenkov imaging technique to detect Cherenkov radiation produced when cosmic rays interact with particles in the Earth's atmosphere.
The telescope is primarily a research and development tool for improving photomultiplier tube cameras and electronic systems for future gamma and cosmic ray telescopes. It is also used to study the energy and composition of cosmic rays in the TeV–PeV range, and the collaboration is currently conducting pioneering work in detecting direct Cherenkov signals from cosmic rays.Transition radiation
Transition radiation (TR) is a form of electromagnetic radiation emitted when a charged particle passes through inhomogeneous media, such as a boundary between two different media. This is in contrast to Cherenkov radiation, which occurs when a charged particle passes through a homogeneous dielectric medium at a speed greater than the phase velocity of electromagnetic waves in that medium.