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.