The RICH

 

Background

RICH_Exploded-300-1.jpgThe speed of light in vacuum (c = 300.000 km/s) is the maximal reachable velocity. However, in a medium the speed of light is slower (v = c/index of refraction). It can therefor happen, that very fast particles enter the medium at a velocity higher than the speed of light in that medium. Such a particle will emit a cone of light. This is called the Cherenkov effect, which is similar to the sonic boom produced by aircraft moving faster than the speed of sound. The Cherenkov radiation consists of photons emitted along a cone whose angular aperture is directly related to the velocity of the particle.

This emitted light cone can be detected by a light-sensitve surface. On this surface, the projected cone looks like a circle or an ellipse, depending on the incident angle. The RICH volume is inclosed with a reflective surface, maximizing the number of photons detected.

By analyzing the geometrical shape of the cones projection, the particle velocity and incoming angle can be reconstructed. Additionally, the particles charge can be detected, as the number of photons is a function of the charge.

Construction

The AMS-02 RICH is made up of a radiator plane, a conical mirror and a photon detection plane. Passage through the radiator plane causes the emission of Cherenkov radiation. The radiator plane is built as a dodecahedral polygon with an internal tangent diameter of 118.5 cm. An array of 2.7 cm thick Aerogel tiles with a refractive index of 1.03-1.05 surround a central 35x35cm2 region which consists of 5 mm thick NaF radiator. This second radiator type produces larger light cones, that are still capable of reaching the photon detection plane which as an "ECAL hole" in it.

This ECAL hole has an area of 64x64cm2, matching the active surface of the ECAL. This allows for a better ECAL energy measurement, because particles would otherwise deposit some of there energy in the PMTs that make up the photon detection plane.

This plane consists of 680 4x4 multi-anode PMTs which cover the 134 cm diameter surface at the bottom of the mirror.

The RICH allows for a beta measurement with a resolution of 0.1% for Z=1 particles and 0.01% for ions.