A RICH (Ring Imaging Cherenkov) detector is a particle detector that can determine the velocity, v, of a fundamental particle. This is done by an indirect measurement of the Cherenkov angle, θc, i.e. the angle between the emitted Cherenkov radiation and the particle path. This is related to the velocity by cosθc = c / nv, where c is the speed of light and n is the refractive index of the medium.
The detector consists of a ring of material in which the radiation is produced. The outer radius of the ring is a mirror which reflects the radiation unto the inner radius, a radiation detector. The distance on the inner radius, r, between the original particle path and the detected radiation is related to the Cherenkov angle by r = fθc, where f is the focal lenght of the mirror. Combining this with the above relation, the velocity can be determined.