A Ring Imaging Cherenkov radiation (RICH) detector uses the width of the cone of Cherenkov radiation
created by a high energy elementary particle
to determine the speed of the particle as it passes through the detector. The wider the cone of emitted radiation, the faster the particle is traveling, as dictated by the relation
cos A = c / (n*v)
where A is the angle enclosed within the cone, c is the speed of light
, n is the index of refraction
of the medium, and v is the velocity of the particle.