A Ring Imaging Cherenkov detector (RICH 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, $\backslash theta\_c$, i.e. the angle between the emitted Cherenkov radiation and the particle path. This is related to the velocity by $\backslash cos\; \backslash theta\_c\; =\; c/nv$, where $c$ is the speed of light and $n$ is the refractive index of the medium.

The technique of this detector is proposed by T. Ypsilantis and J. Séguinot, working in Max Ferro-Luzzi's group. The first large-scale application was for the DELPHI experiment at LEP (Large Electron-Positron Collider) at CERN.

In a RICH detector a cone of Cherenkov light is produced when a high speed particle traverses a suitable medium, ofttimes called radiator, with a velocity greater than the speed of light in that medium (this is due to the refractive index $n$). This light cone is detected on a position sensitive planar photon detector, which allows reconstructing a ring or disc, the radius of which is a measure for the Cherenkov emission angle. Both focusing and proximity-focusing detectors are in use. In a focusing RICH detector the photons are collected by a spherical mirror with focal length $f$ and focused onto the photon detector placed at the focal plane. The result is a circle with a radius $r=f\backslash theta\_c$, independent of the emission point along the particle track. This scheme is suitable for low refractive index radiators, i.e. gases, due to the larger radiator length needed to create enough photons.

In the more compact proximity-focusing design a thin radiator volume emits a cone of Cherenkov light which traverses a small distance - the proximity gap - and is detected on the photon detector plane. The image is a ring of light the radius of which is defined by the Cherenkov emission angle and the proximity gap. The ring thickness is determined by the thickness of the radiator. An example of a proximity gap RICH detector is the High Momentum Particle Identification (HMPID), a detector currently under construction for ALICE (A Large Ion Collider Experiment), one of the five experiments at the LHC (Large Hadron Collider) at CERN.

Particle detectors