Cherenkov radiation

The blue glow of Cherenkov radiation during a fuel change at the HFIR research reactor, Oak Ridge
The blue glow of Cherenkov radiation during a fuel change at the HFIR research reactor, Oak RidgePhoto: Jason Richards, Oak Ridge National Laboratory, CC BY 2.0, Wikimedia Commons

Cherenkov radiation is the bluish glow that arises when a charged particle traverses a transparent medium faster than light propagates within it. It is the detection principle of Super-Kamiokande, IceCube, KM3NeT and the Sudbury Neutrino Observatory.

Faster than light — but not faster than c

The sentence sounds like a contradiction and is none. The only thing that cannot be exceeded is the speed of light in a vacuum. In water light moves at only about three quarters of that, because it is constantly absorbed by the molecules and emitted again.

An electron, by contrast, is not slowed down by this. It can therefore quite well be faster than light in water — without violating the theory of relativity.

The picture of the bow wave

What then happens is familiar from water and from air.

A ship travelling more slowly than its own waves pushes a circular wave ahead of it. If it travels faster, the waves are left behind and superimpose into a wedge-shaped bow wave. With a supersonic aircraft the same phenomenon is called a Mach cone.

With a charged particle in water it is the electromagnetic equivalent: a cone of light that follows the particle.

The opening angle of this cone depends solely on the speed. From it one can therefore read off how fast the particle was — and in water there is a sharp threshold: below about three quarters of the vacuum speed of light no cone arises at all.

Why it glows blue

The radiation is not distributed evenly across the colours. Its intensity grows towards the short-wavelength end — the bluer, the more.

This is why the glow in reactor pools appears deep blue. Anyone who has ever seen a photograph of a research reactor knows this colour: it comes from electrons from the decay of the fission products, which traverse the cooling water faster than light does within it.

What a detector reads from it

On the wall of a water tank the cone draws a ring. Three things can be read from it:

The direction. The position of the ring shows where the particle came from — and thus from which direction the neutrino arrived. Precisely this allowed Masatoshi Koshiba to demonstrate for the first time that the measured neutrinos really do come from the Sun.

The energy. The brighter the ring, the more energetic the particle.

The kind of particle. Here lies the decisive point. An electron is deflected in the water and produces a small shower; its ring is blurred. A muon runs almost straight ahead; its ring is sharply outlined.

On this distinction rests the discovery by Takaaki Kajita in 1998. He was able to show that muon neutrinos are missing and electron neutrinos are not only because the detector could tell the two apart.

The limit of the method

The drawback is the yield. A water detector gains only a few hundred photons per megaelectronvolt; a scintillator delivers around ten thousand.

From this follows a higher detection threshold. Neutrinos of low energy — those from the Sun's main reaction, for instance — remain invisible to a water detector. For those one needs facilities such as Borexino.

The two methods therefore complement each other: water supplies direction and large volumes, scintillator supplies sensitivity.

Discovery

Pavel Cherenkov observed the glow in 1934 in Moscow. Ilya Frank and Igor Tamm explained it theoretically in 1937. All three received the Nobel Prize in Physics together in 1958.

Sources

  • P. A. Cherenkov: Visible emission of clean liquids by action of γ radiation, Doklady Akademii Nauk SSSR 2, 451 (1934).
  • Nobel Foundation: Nobel Prize in Physics 1958.