The Ring Imaging CHerenkov detector, or RICH, estimates the particles velocity with a high accuracy (0.1%). Velocity derives from pattern recognition of photons distributed over geometrical shapes as circles, ellipses, arcs or crescents produced by the Cherenkov effect.
What is the Cherenkov Effect?
The speed of light in vacuum (c = 300,000 km/s) is the maximum reachable velocity. However, the speed of light in a medium is less than that (v = c/index of refraction). It can happen that very fast particles traverse medium with a velocity that is slower than the speed of light in vacuum but faster than the speed of light in that medium. This particle will emit a cone of light. This process, called the Cherenkov effect, is quite similar to the sonic boom produced by airplanes exceeding the speed of sound in air. The Cherenkov radiation consists of photons emitted along a characteristic cone whose angular aperture is directly related to the particle velocity and with the index of refraction of the material.
How does the RICH work?
The figure below depicts a schematic view of RICH measuring principle. A charged particle passing through the RICH thin radiator causes the emission of a Cherenkov cone. This light cone expands in vacuum and is finally detected over a sensible surface. On this surface, the cone projection appears to be an ellipse or a circle depending on the particle incidence angle. A mirror encloses the RICH volume increasing the photons collection on the sensitive plane. Cones incident on the mirror will project crescents on the sensible surface.
A pattern recognition algorithm reconstructs the particle velocity and the incoming angle from the detected geometrical shape. Charge measurement (Z) derives from the total amount of collected photons. Higher is the charge higher is the number of photons.
Why do we need the RICH?
Particle mass is an indirect measurement in AMS. It is calculated starting from rigidity (R) measured by Tracker, charge (Z) measured by Tracker, ToF and RICH, and velocity (beta = v/speed of light) measured by RICH and ToF.
Mass has a heavy dependence on beta, and the RICH – with its astonishing velocity resolution – allows a good mass resolution. AMS will be able to distinguish light isotopes among the most abundant chemical species.
How is the RICH built?
The AMS-02 RICH consists of a radiator plane, a conical mirror and a photon detection plane.
The radiator is the responsible for the Cherenkov radiation production. It consists of a dodecahedral polygon with a 118.5 cm internal tangent diameter. An array of 2.7 cm thick aerogel tiles with a refractive index between 1.03–1.05 surrounds a central 35×35 cm2 region equipped with 5 mm thick Sodium Fluoride (NaF) radiator (nNaF = 1.335). This radiators combination optimizes the overall counter acceptance since the Cherenkov photons radiated by the NaF in large cones will fall within the detection area. Indeed the detector plane has an empty 64×64 cm2 area in its center, matching the active area of the electromagnetic calorimeter located below.
Outside the ECAL hole, 680 4×4-multi-anode PMTs (gain 106 at 800 V) are arranged to cover the circular 134 cm diameter surface at the basis of the conical mirror.
A 47 cm height conical reflector multi-layer structure on a Carbon fiber reinforced composite substrate encloses the radiator and the detection plane. The mirror increases the RICH acceptance reflecting high-inclination photons and provides the necessary photon drift ring expansion. RICH provides the β measurement with a resolution ∼ 0.1% for Z=1 particles, and ∼ 0.01% for ions (Z>1). It also provides a particle charge measurement with a charge confusion of the order of 10 %.
In Depth: Why is there a hole in the middle of the RICH?
The electromagnetic calorimeter (ECAL) is placed under RICH. To reduce the RICH material effect on the ECAL energy measurement the sensitive area presents a hole. With a clever combination of radiator materials RICH “reduces” the hole. Indeed cones produced in the central radiator are larger than the ones produced in Aerogel. As a consequence, larger cones have an increased probability of being collected on the PMTs plane.