Cherenkov radiation
Cherenkov radiation is the blue glow that fills the water around an underwater nuclear reactor. Physicists named it after the Soviet scientist Pavel Cherenkov. It is a form of electromagnetic radiation, produced when a charged particle such as an electron moves through an insulating material, a dielectric, such as distilled water.
The setting sounds ordinary, yet the effect has become a working tool in medicine, nuclear safety, astrophysics and particle physics. What exactly is happening inside the water to make light? Why does that light come out blue? Who first noticed it, and why was an early explanation brushed aside? And how can a glow once associated with reactors now help doctors watch a radiation treatment as it happens?
In water, light travels at only about 0.75 times its speed in a vacuum, and that gap is where the whole effect begins. The vacuum speed is a universal constant, 299,792,458 metres per second. Inside a material it can be much lower. Matter can move faster than that reduced speed, while still staying below the vacuum value, during nuclear reactions and inside particle accelerators.
When a charged particle crosses a medium, the particles of the medium polarize around it in response. The disturbed molecules are excited, and as they return to their ground state they re-emit the energy as photons. Those photons form spherical wavefronts that spread out from the moving particle.
If the particle is slower than light in that medium, the polarization around it stays roughly symmetric. The wavefronts may bunch up, but they never cross, so nothing interferes. If the particle is faster, the medium cannot return to its relaxed state in time. The polarization becomes lopsided along the direction of travel, the wavefronts overlap, and constructive interference produces a cone of light at a characteristic angle.
A supersonic aircraft offers the everyday comparison. It outruns its own sound waves, which cannot spread forward and instead pile into a conical shock front. A charged particle in an insulator does the same with light, creating a shock wave of visible light. The speed that must be beaten is the phase velocity of light, not the group velocity.
For water, the refractive index is about 1.33 at 20 degrees Celsius, so the particle must travel at between 0.75 and 1 times the vacuum speed of light. Call the particle's speed as a fraction of the vacuum speed beta. The cosine of the emission angle then equals one divided by the product of the refractive index and beta. At the threshold speed the angle is zero, and it widens to a maximum as the particle nears the speed of light. Measuring the angle therefore reveals both the direction and the speed of the particle that made it.
The angle is only half the story, because the light also has a color, and that color is not what the physics first suggests.
Most of the light in Cherenkov radiation is ultraviolet, which no human eye can see. The glow looks blue because of a trade-off between the radiation and the eye. Around the visible range, intensity per unit frequency is roughly proportional to frequency, so shorter wavelengths shine brighter. Human vision peaks at green and is very weak toward violet, so blue wins the compromise.
The spectrum is also unlike the sharp peaks of fluorescence or ordinary emission. Cherenkov radiation is continuous. The Frank-Tamm formula describes how much energy is radiated for each unit of distance travelled and each unit of frequency, using the particle's charge and speed along with the permeability and refractive index of the material.
The brightness cannot keep climbing toward ever shorter wavelengths. The refractive index changes with frequency, and past a cut-off the angle equation can no longer be satisfied, even for particles moving very close to the speed of light. At X-ray frequencies the refractive index falls below 1. A phase velocity above the vacuum speed does not break relativity inside a medium, but it does mean no X-ray or gamma-ray emission appears.
There is one loophole. Just below the frequencies tied to core electronic transitions in a material, the refractive index is often greater than 1 again, and X-rays can be generated at those special frequencies.
At the Lebedev Institute in 1934, Pavel Cherenkov noticed a faint bluish light around a radioactive preparation sitting in water. He was working under the supervision of Sergey Vavilov, which is why the effect is also called Vavilov-Cherenkov radiation. His doctoral thesis had examined how solutions of uranium salts glow when excited by gamma rays rather than by the usual, less energetic visible light.
Cherenkov found that the light was anisotropic, meaning it did not spread evenly in every direction. That led him to conclude the bluish glow was not fluorescence.
Others had come close. Marie Curie saw a pale blue light in a highly concentrated radium solution in 1910 but did not look into its source. In 1926 the French radiotherapist Lucien Mallet described the light of radium shining through water and found its spectrum was continuous.
Theory had come close too. The English polymath Oliver Heaviside predicted conical wavefronts in papers published between 1888 and 1889, and Arnold Sommerfeld did the same in 1904. Both predictions were quickly dismissed, because relativity seemed to forbid superluminal particles. They were not taken up again until the 1970s.
The explanation that stuck came in 1937, when Cherenkov's colleagues Igor Tamm and Ilya Frank built a theory within Einstein's special relativity. Tamm, Frank and Cherenkov shared the Nobel Prize in 1958. In their original paper, Tamm and Frank wrote that the radiation "can evidently not be explained by any common mechanism" such as an electron interacting with single atoms. Their answer was 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."
The Smith-Purcell effect shows how far the usual picture can be stretched. Because the phase velocity can change dramatically in a periodic medium, Cherenkov radiation can occur there with no minimum particle velocity at all. In a more complex periodic structure such as a photonic crystal, stranger variants appear, including radiation sent backwards. Ordinary Cherenkov light instead forms an acute angle with the particle's direction.
Negative-index metamaterials produce a reverse Cherenkov effect. These engineered materials have a fine internal structure that gives them negative permittivity and negative permeability. A fast charged particle crossing one emits radiation trailing behind it rather than ahead. The same reversed cone can appear in periodic media whose structure is about the size of the wavelength, which cannot be treated as a uniform metamaterial.
One-dimensional metamaterials go further. They introduce a gradient of phase delay along the particle's path, which lets the emission be reversed or steered to an arbitrary angle.
The effect can even occur in vacuum. In a slow-wave structure such as a traveling-wave tube, the phase velocity drops, so a particle can outpace it while staying below the speed of light. This vacuum Cherenkov radiation is used to generate high-power microwaves.
A collective version also exists. In plasma acceleration setups, density profiles can be shaped so that structures carrying up to nanocoulombs of charge travel faster than light and emit optical shocks at the Cherenkov angle. The individual electrons remain slower than light. The electrons making up the structure at one moment are different from those in it a moment later.
In 2019, researchers from Dartmouth's and Dartmouth-Hitchcock's Norris Cotton Cancer Center found Cherenkov light being generated in the vitreous humor of the eye in patients undergoing radiotherapy. They used a camera imaging system called CDose, designed to view light emitted by biological systems. For decades, patients receiving radiation treatment for brain cancer had reported seeing "flashes of bright or blue light", but no one had observed the effect experimentally. Charged particles striking the vitreous humour can produce the same impression of flashes in cosmic ray visual phenomena, and possibly in some observations of criticality accidents.
The same light now helps with treatment. External beam radiotherapy using electron or photon beams in the 6 MV to 18 MV range induces a substantial amount of Cherenkov light in the tissue being treated. Secondary electrons set off by the high-energy x-rays create it, and it can be imaged at the entry and exit surfaces of the tissue.
The signal is very dim. Specially designed cameras detect it by synchronizing their exposures to the pulses of the linear accelerator. What they capture is the shape of the radiation beam as it strikes the tissue, in real time.
Injected radiopharmaceuticals can be tracked the same way. Positron emitters 18F and 13N, and beta emitters 32P and 90Y, give off measurable Cherenkov light. Both 18F and 131I have been imaged in humans to demonstrate diagnostic value.
In laboratories, phosphorus-32 can be introduced into biomolecules by enzymatic and synthetic means. Cherenkov detection then reveals tiny quantities of them, helping researchers work out biological pathways, affinity constants and dissociation rates.
In open pool reactors, the glow comes from beta particles, which are high-energy electrons released as fission products decay. It does not stop when the chain reaction does. The light continues, dimming as the shorter-lived products decay away. The same principle can characterize the remaining radioactivity of spent fuel rods, and it is used to verify that spent fuel is present in storage pools for nuclear safeguards.
High above the ground, a TeV gamma photon or a cosmic ray striking the atmosphere may produce an electron-positron pair moving at enormous speed. The Cherenkov light those particles give off in the air reveals the direction and energy of the original ray. The Imaging Atmospheric Cherenkov Technique does this, and experiments such as VERITAS, H.E.S.S. and MAGIC rely on it.
Other projects capture the light in tanks of water when charged particles reach the ground. HAWC and the Pierre Auger Observatory work this way, as part of Extensive Air Shower experiments. Very large neutrino detectors use similar methods, including Super-Kamiokande, the Sudbury Neutrino Observatory and IceCube. These observatories help establish the properties of objects that emit very-high-energy gamma rays, such as supernova remnants and blazars.
One past project, STACEE, was a former solar tower in New Mexico refurbished to work as a non-imaging Cherenkov observatory.
Particle physicists use Cherenkov light for identification. A charged elementary particle's velocity can be measured, or limited, from the light it emits in a chosen medium. If its momentum is measured separately, its mass follows from momentum and velocity, and the mass reveals what the particle is.
The threshold counter is the simplest device of this kind. It asks only whether a particle is slower or faster than the value c divided by the refractive index, by checking whether any Cherenkov light appears. With the momentum known, lighter particles can be separated from heavier ones above or below the threshold.
The most advanced type is the ring-imaging Cherenkov detector, or RICH, developed in the 1980s. A fast particle passing through a radiator produces a cone of light, which lands on a position-sensitive planar photon detector. The light forms a ring or disc, and its radius measures the Cherenkov emission angle.
In a focusing RICH, a spherical mirror gathers the photons onto a detector at the focal plane. The resulting circle has a radius independent of where along the track the light was emitted. Because enough photons need a long radiator, this design suits low-index radiators such as gases.
A proximity-focusing RICH is more compact. A thin radiator emits its cone across a short proximity gap before reaching the detector, and the ring's thickness is set by the thickness of the radiator. One example is the High Momentum Particle Identification Detector, or HMPID, described as under construction for ALICE, one of the six experiments at the Large Hadron Collider at CERN.
Updated
Common questions
What is Cherenkov radiation?
Cherenkov radiation is electromagnetic radiation emitted when a charged particle, such as an electron, passes through a dielectric medium, such as distilled water, faster than the phase velocity of light in that medium. A classic example is the blue glow of an underwater nuclear reactor.
Why is Cherenkov radiation blue?
Cherenkov radiation looks blue because its intensity rises with frequency, and most of it is ultraviolet. Human eyes are most sensitive to green and very weak toward violet, so the visible part appears brilliant blue.
Who discovered Cherenkov radiation?
Pavel Cherenkov was the first to detect it experimentally, in 1934 at the Lebedev Institute, under the supervision of Sergey Vavilov. It is therefore also called Vavilov-Cherenkov radiation.
Who explained Cherenkov radiation and won the Nobel Prize for it?
Igor Tamm and Ilya Frank developed the theory in 1937 within Einstein's special relativity. They shared the 1958 Nobel Prize with Pavel Cherenkov.
How is Cherenkov radiation similar to a sonic boom?
A supersonic aircraft outruns its own sound waves, which pile into a conical shock front. A charged particle moving faster than light in a medium likewise creates a conical shock wave of visible light.
How is Cherenkov radiation used in medicine?
Cherenkov light is used to image radioisotopes in the body and to show the shape of radiation beams during external beam radiotherapy in real time. Specially designed cameras synchronized to linear accelerator pulses detect the very faint signal.
How do scientists use Cherenkov radiation in astrophysics and particle physics?
Atmospheric Cherenkov light reveals the direction and energy of cosmic rays and gamma rays in experiments such as VERITAS, H.E.S.S. and MAGIC. In particle physics, threshold counters and ring-imaging Cherenkov detectors use the light to identify particles.
All sources
37 references cited across the entry
- 1BookClassical electrodynamicsJohn David Jackson — Wiley — 1999
- 2JournalVisible emission of clean liquids by action of γ radiationP. A. Cherenkov — 1934
- 4BookOliver Heaviside: The Life, Work, and Times of an Electrical Genius of the Victorian AgeP. J. Nahin — JHU Press — 1988
- 5BookRadioactivity: Introduction and History, From the Quantum to QuarksMichael F. L'Annunziata — Elsevier — 2016
- 6JournalOliver Heaviside: an accidental time travellerPaul J. Nahin — The Royal Society — 2018
- 7BookClassical electrodynamicsP. Sengupta — New Age International — 2000
- 8BookThe Physics of Nuclear ReactorsSerge Marguet — Springer — 2017
- 9For the first time, scientists capture light flashes from human eye during radiotherapyAmerican Association for the Advancement of Science (AAAS) — 7 January 2020
- 10JournalExperimentally Observed Cherenkov Light Generation in the Eye During Radiation TherapyIrwin I. Tendler et al. — 25 October 2019
- 11JournalFlashes of light-radiation therapy to the brainDeborah T. Blumenthal et al. — August 2015
- 12JournalCerenkov Radiation in Photonic CrystalsC. Luo et al. — 2003
- 13JournalCoherent radiation of fast electrons in a mediumI.E. Tamm et al. — 1937
- 14JournalControlled steering of Cherenkov surface plasmon wakes with a one-dimensional metamaterialP. Genevet et al. — 2015
- 15NewsTopsy turvy: The first true "left handed" materialP. F. Schewe et al. — American Institute of Physics — 24 March 2004
- 16JournalCherenkov radiation from the quantum vacuumAlexander J. Macleod et al. — 2019
- 17JournalGeneralized momentum equation of quantum mechanicsZhong-Yue Wang — 2016
- 18JournalRelativistic multiwave Cerenkov generatorS. P. Bugaev et al. — 1983
- 19JournalCoherence and superradiance from a plasma-based quasiparticle acceleratorBernardo Malaca et al. — 2023
- 20JournalExperimentally Observed Cherenkov Light Generation in the Eye During Radiation TherapyIrwin I. Tendler et al. — 2020
- 21JournalVavilov – Cherenkov radiation: Its discovery and applicationB. M. Bolotovskii — 2009
- 22JournalRadiation-luminescence-excited quantum dots for in vivo multiplexed optical imagingH. Liu et al. — 21 May 2010
- 23JournalOptical imaging of reporter gene expression using a positron-emission-tomography probeHongguang Liu et al. — 2010
- 24JournalCerenkov Luminescence Tomography for In Vivo Radiopharmaceutical ImagingJianghong Zhong et al. — 2011
- 25JournalRadical irradiation for carcinoma of the prostateC. L Sinoff — 1991
- 26JournalIn vivo Cerenkov luminescence imaging: A new tool for molecular imagingG. S Mitchell et al. — 2011
- 27BookEmerging Applications of Molecular Imaging to OncologyS. Das et al. — 2014
- 28JournalFirst human CerenkographyAntonello Enrico Spinelli et al. — 2013
- 29JournalCherenkov Video Imaging Allows for the First Visualization of Radiation Therapy in Real TimeLesley A Jarvis et al. — 2014
- 30JournalTechnical Note: Time-gating to medical linear accelerator pulses: Stray radiation detectorM.R. Ashraf — Dec 14, 2018
- 31JournalInitial Clinical Experience of Cherenkov Imaging in External Beam Radiation Therapy Identifies Opportunities to Improve Treatment DeliveryL. A. Jarvis — April 1, 2021
- 32JournalOn Cherenkov light production by irradiated nuclear fuel rodsE Branger et al. — 2017
- 34ALICE2026-01-28
- 35BookElectrodynamics of Continuous MediaL. D. Landau et al. — Pergamon Press — 1984
- 36BookCerenkov Radiation and Its ApplicationsJ. V. Jelley — Pergamon Press — 1958
- 37JournalVisible Light from Localized Surface Charges Moving across a GratingS. J. Smith et al. — 1953