Cherenkov radiation: a standalone construct

Pavel Čerenkov's 1934 experiments recorded a faint blue glow from pure liquids exposed to gamma radiation, light that remained after ordinary fluorescence had been ruled out. Igor Frank and Igor Tamm explained it in 1937: the glow appears when a charged particle moves through a medium faster than the phase speed of light in that medium (the speed of light in vacuum divided by the medium's refractive index), so the particle outruns the electromagnetic waves it excites and they add up into a coherent cone of light. Čerenkov's 1937 measurements confirmed the two signatures the theory predicted, that the radiation is emitted only above a velocity threshold and only along a cone whose half-angle is set by the particle's speed and the medium's refractive index. The effect is a landmark because its geometry carries information: the cone angle fixes the particle's velocity, which is the working principle of ring-imaging Cherenkov detectors used to identify particles in high-energy physics, and the same blue light is what is seen glowing in the water surrounding nuclear reactor cores.[1][2][3]

Unit

A form of electromagnetic radiation, emitted as a coherent cone of light above a velocity threshold

Recovery means

Not applicable

Measured by

Detected optically with photomultiplier tubes and quantified from the emission cone's half-angle, which is fixed by the particle's speed and the medium's refractive index, as in ring-imaging Cherenkov detectors