Cerenkov counter(chĕ-renk -off) An instrument for detecting and measuring the velocity of very energetic particles such as cosmic rays or the cascades of electrons generated in the Earth's atmosphere by gamma rays. Faint blue light – Cerenkov radiation – is emitted by the particles as they pass through a transparent nonconducting (dielectric) material at a speed greater than the speed of light in that material. The radiation is emitted at a fixed angle, θ, to the direction of motion of the particle, such that
in which v is the particle velocity, n the refractive index of the transparent medium, and c the speed of light in a vacuum. This cone of light is focused on to a photomultiplier so that an amplified electric pulse is registered. The velocity of the particle is determined from the angle of the cone.