The blue glow seen in the water around a nuclear reactor core
Beta particles from fission products outrun light in the coolant.
Science · Physics
Cherenkov radiation
Observed 1934; explained 1937
Physics — particle and nuclear physics
Cherenkov radiation is the eerie blue light emitted when a charged particle travels through a transparent medium faster than light does in that medium. Nothing outruns light in a vacuum, but light in water crawls at about three-quarters of its vacuum speed, and an energetic electron easily beats it. The particle drags a shock front of electromagnetic radiation behind it in a cone, exactly as a supersonic aircraft drags a sonic boom. Pavel Cherenkov noticed the glow in 1934 while working under Sergey Vavilov, and Igor Tamm and Ilya Frank explained it in 1937; the three shared the 1958 Nobel Prize in Physics. It is the blue shimmer seen in the cooling ponds of nuclear reactors, and the working principle of detectors from Super-Kamiokande to IceCube.
How to spot them
- ▸The blue glow in the water of a nuclear reactor pool
- ▸The optical equivalent of a sonic boom, complete with a cone and a shock front
- ▸Happens when something outruns light — legally, because it is only doing so inside water or glass
- ▸How a cubic kilometre of Antarctic ice was turned into a neutrino telescope
- ▸In Russia it carries a second name, that of the discoverer's supervisor
Key facts
The optical analogue of a sonic boom, emitted in a cone behind the particle
The cone angle obeys cos θ = 1/(nβ), so it measures the particle's speed.
Turns a cubic kilometre of Antarctic ice into a neutrino telescope
IceCube watches for the flashes from muons made by neutrinos in the ice.
Spotted in 1934 by a Soviet graduate student and explained in 1937 by Tamm and Frank (1934)
The discoverer observed it by eye after hours of sitting in the dark to adapt his vision.
It is blue because its intensity rises with frequency rather than peaking at a colour
A continuous spectrum weighted to the short-wavelength end, unlike a fluorescence line.
The subject of the 1958 Nobel Prize in Physics, shared by three Soviet physicists (1958)
The first Nobel in physics won by Soviet citizens.
Key ideas
- Light in a medium of refractive index n travels at c/n; a particle exceeding that speed leaves behind a coherent wake of light waves that reinforce along a cone.
- The cone's half-angle depends only on the particle's speed and the medium, so measuring it gives the velocity — the basis of ring-imaging Cherenkov (RICH) detectors.
- There is no violation of relativity: nothing here exceeds the speed of light in a vacuum.
Quick-fire facts
- •Named after Pavel Alekseyevich Cherenkov, who observed the glow in 1934; in Russia it is called Vavilov–Cherenkov radiation after his supervisor Sergey Vavilov
- •Igor Tamm and Ilya Frank produced the theory in 1937; all three shared the 1958 Nobel Prize in Physics
- •Oliver Heaviside had predicted the effect mathematically in the 1880s, and Marie Curie noticed blue light in concentrated radium solutions without explaining it
- •The condition is v > c/n; in water (n ≈ 1.33) that means faster than about 225,000 km/s
- •It powers Super-Kamiokande in Japan, a 50,000-tonne tank of ultrapure water lined with photomultipliers
- •IceCube at the South Pole instruments about a cubic kilometre of Antarctic ice the same way
- •Astronauts report seeing flashes of light with their eyes shut, partly attributed to Cherenkov radiation in the eye from cosmic rays
- •The blue flash seen in criticality accidents, such as the one that killed Louis Slotin in 1946, is Cherenkov radiation in air and in the eye
- •Cherenkov's work was initially done without electronics: he measured the faint light by eye using a photometric method
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Sources
- The Nobel Foundation — The Nobel Prize in Physics 1958
- University of Wisconsin–Madison — IceCube Neutrino Observatory For the use of Cherenkov light in Antarctic ice.