Ring-imaging Cherenkov detector

A ring-imaging Cherenkov detector (RICH detector) is a particle detector that can determine the velocity, $v$ , of a charged particle. This is done by an indirect measurement of the Cherenkov angle, $\theta_c$ , i.e. the angle between the emitted Čerenkov radiation and the particle path. This is related to the velocity by $\cos \theta_c = c/nv$ , where $c$ is the speed of light and $n$ is the refractive index of the medium.

The technique of this detector has been 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.

A cone of Cherenkov light is produced when a high speed particle traverses a suitable medium, often called radiator, with a velocity greater than the speed of light in that medium ( $v > c/n$ , $n$ being the refractive index of the medium). In a RICH detector 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 $r=f\theta_c$ , independent of the emission point along the particle track. This scheme is suitable for low refractive index radiators, i.e. gases, with their 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 mainly 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.

In a DIRC (Detection of Internally Reflected Cherenkov light), another design of a RICH detector, light that is captured by total internal reflection inside the solid radiator reaches the light sensors at the detector perimeter, the precise rectangular cross section of the radiator preserving the angular information of the Cherenkov light cone. One example is the DIRC of the BaBar experiment at SLAC.

The LHCb experiment on the Large Hadron Collider uses two RICH detectors for differentiating between pions and kaons. The first (RICH-1) is located immediately after the Vertex Locator (VELO) around the interaction point and is optimised for low-momentum tracks and the second (RICH-2) is located after the magnet and tracker layers and optimised for higher-momentum tracks.

The Alpha Magnetic Spectrometer device AMS-02, recently mounted on the International Space Station uses a RICH detector in combination with other devices to analyze cosmic rays.