# cherenkov radiation ![[advancedtestreactor.jpg|300]] cherenkov radiation glowing in the core of the advanced test reactor at idaho national laboratory cherenkov radiation () (also known as čerenkov or cerenkov radiation) is electromagnetic radiation emitted when a charged particle (such as an electron) passes through a dielectric medium (such as distilled water) at a speed greater than the phase velocity (speed of propagation of a wavefront in a medium) of light in that medium. a classic example of cherenkov radiation is the characteristic blue glow of an underwater nuclear reactor. its cause is similar to the cause of a sonic boom the sharp sound heard when faster-than-sound movement occurs. the phenomenon is named after soviet physicist pavel cherenkov 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. ir doctorate thesis was on luminescence of uranium salt solutions that were excited by gamma rays instead of less energetic visible light as done commonly. ey 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 wavefronts 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 dismissed following the relativity theory's restriction of superluminal particles until the 1970s. marie curie observed a pale blue light in a highly concentrated radium solution in 1910 but did not investigate its source. in 1926 the french radiotherapist lucien mallet described the luminous radiation of radium irradiating water having a continuous spectrum in 2019 a team of researchers from dartmouth's and dartmouth-hitchcock's norris cotton cancer center discovered cherenkov light being generated in the vitreous humor of patients undergoing radiotherapy. the light was observed using a camera imaging system called a cdose which is specially designed to view light emissions from biological systems. for decades patients had reported phenomena such as "flashes of bright or blue light" when receiving radiation treatments for brain cancer but the effects had never been experimentally observed while the speed of light in vacuum is a universal constant (c = 299-792-458 m/s) the speed in a material may be significantly less as it is perceived to be slowed by the medium. for example in water it is only 0.75c. matter can accelerate to a velocity higher than this (although still less than c the speed of light in vacuum) during nuclear reactions and in particle accelerators. cherenkov radiation results when a charged particle most commonly an electron travels through a dielectric (can be polarised electrically) medium with a speed greater than light's speed in that medium ![[cherenkovradiationduringmaintainence.jpg|300]] cherenkov radiation during scheduled refueling and maintenance outage of arkansas nuclear one unit 2 (ano-2) the effect can be intuitively described in the following way. from classical physics it is known that accelerating charged particles emit em waves and via huygens' principle these waves will form spherical wavefronts which propagate with the phase velocity of that medium (ie the speed of light in that medium) when any charged particle passes through a medium the particles of the medium will polarize around it in response. the charged particle excites the molecules in the polarizable medium and on returning to ir ground state the molecules re-emit the energy given to them to achieve excitation as photons. these photons form the spherical wavefronts which can be seen originating from the moving particle. if that is the velocity of the charged particle is less than that of the speed of light in the medium then the polarisation field which forms around the moving particle is usually symmetric. the corresponding emitted wavefronts may be bunched up but they do not coincide or cross and there are therefore no interference effects to consider. in the reverse situation ie the polarisation field is asymmetric along the direction of motion of the particle as the particles of the medium do not have enough time to recover to ir "normal" randomised states. this results in overlapping waveforms (as in the animation) and constructive interference leads to an observed cone-like light signal at a characteristic angle: cherenkov light ![[220px-cherenkovradiation-animation.gif]]] animation of cherenkov radiation a common analogy is the sonic boom of a supersonic aircraft. the sound waves generated by the aircraft travel at the speed of sound which is slower than the aircraft and cannot propagate forward from the aircraft instead forming a conical shock front. in a similar way a charged particle can generate a "shock wave" of visible light as it travels through an insulator 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 using 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 ![[reactor-core-from-above-1400-opttcm18-278435.jpg|300]] cherenkov radiation in the university of massachusetts lowell radiation laboratory in ir 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 into 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" ![[cherenkov.svg.png]] the geometry of the cherenkov radiation shown for the ideal case of no dispersion in the figure on the geometry the particle (red arrow) travels in a medium with speed # # arbitrary emission angle 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 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 # # reverse cherenkov effect 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 ir constituent materials in this case having negative permittivity and negative permeability.) this means that 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 emits 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 cherenkov effect can occur in vacuum # # collective cherenkov radiation with the same properties of typical cherenkov radiation can be created by structures of electric current that travel faster than light. by manipulating density profiles in plasma acceleration setups structures up to nanocoulombs of charge are created and may travel faster than the speed of light and emit optical shocks at the cherenkov angle. electrons are still subluminal hence the electrons that compose the structure at a time t = t0 are different from the electrons in the structure at a time t > t0 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 # # detection of labelled biomolecules 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 # # medical imaging of radioisotopes and external beam radiotherapy ![[cherenkov-breast.png]] cherenkov light emission imaged from the chest wall of a patient undergoing whole breast irradiation using 6 mev beam from a linear accelerator in radiotherapy 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 electron beams or photon beams with energy in the 6 mv to 18 mv 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. the cherenkov light emitted from patient's tissue during radiation therapy is a very low light level signal but can be detected by specially designed cameras that synchronize ir acquisition to the linear accelerator pulses. the ability to see this signal shows the shape of the radiation beam as it is incident upon the tissue in real time ![[220px-trigareactorcore.jpg|300]] cherenkov radiation in a triga reactor pool cherenkov radiation is used to detect high-energy charged particles. in open pool 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 **+** askaryan radiation similar radiation produced by fast uncharged particles **+** blue noise **+** bremsstrahlung radiation produced when charged particles are decelerated by other charged particles **+** faster-than-light about conjectural propagation of information or matter faster than the speed of light **+** frank-tamm formula giving the spectrum of cherenkov radiation **+** light echo **+** list of light sources **+** non-radiation condition **+** radioluminescence **+** tachyon **+** transition radiation **+** landau l. d.; liftshitz e. m.; pitaevskii l. p. (1984.) electrodynamics of continuous media. new york: pergamon press. 75-1 **+** jelley j. v. (1958.) cerenkov radiation and its applications. london: pergamon press **+** smith s. j.; purcell e. m. (1953.) "visible light from localised surface charges moving across a grating." physical review. 92 (4): 1069. bibcode:1953phrv...92.1069s. doi: 10.1103/physrev.92.1069 // republic of bob