Rich Detector Principles

Emission of light by charged particles moving faster than the speed of light was discovered by Pavel Cherenkov(1) and explained theoretically by Frank and Tamm (2). A recent theoretical review is provided by Tom Ypsilantis and Jacque Seguinot (3). They also discuss techniques of constructing and using Ring Imaging detectors to identify particles.
The basic idea is that light is emitted in a cone around the direction of the charged particle. The emission angle of the light is given by the formula cos(theta)=1/(n*beta), where n is the index of refraction and beta is the particles velocity as a fraction of the speed of light in vacuum, c. If the momentum of the particle is measured independent and the angle of Cherenkov light emission also reconstructed, then it is possible to derive the mass of the particle. Thus Cherenkov detectors are mostly used to identify charged particles.
A Ring Imaging Cherenkov Detector consists of a radiator, an expansion volume, which allows the photons to separate and a photon detector. Often the light is focused with a spherical mirror to reduce the error from the uncertainty in the emission point of the light.
More information is available in reference (3).

P. A. Cherenkov, Phys. Rev. 52 (1937) 378.
I. E. Tamm and I. M. Frank, Doklady AN SSSR 14 (1937)107.
T. Ypsilantis and J. Seguinot, in the Proceeding of the First Workshop on Ring Imaging Cherenkov Detectors Bari, Italy, June 1993, which is published by Nuclear Instruments & Methods in Physics Research Vol. 343, No. 1. These proceedings have a wealth of information not only on Cherenkov radiation, but also on techniques of constructing and using Ring Imaging detectors to identify particles.

The figure below shows a sketch of the CLEO III RICH structure. The radiator is LiF, 1 cm thick, followed by a 15.7 cm expansion volume and photon detector consisting of a wire chamber filled with a mixture of TEA and CH4 gas. TEA is photosensitive. The resulting photoelectrons are multiplied by the HV on the wires and the resulting signals are sensed by a rectangular array of pads coupled with highly sensitive electronics.