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Ring Imaging Cherenkov Detectors
Ring Imaging Cherenkov (RICH) detectors are photosensitive position devices realized to identify particles thanks to the Cherenkov light they emit.
They are commonly used in accelerator experiments (i.e. LHCb, ALICE, FAIR/CBM) and they consist of a radiator and a photosensitive surface made by a matrix of photosensors. When a particle crosses the radiator, with a very high velocity, it forms a light cone (Cherenkov light cone).
The photosensitive surface is designed to capture the Cherenkov emission of the particles crossing the radiator. By measuring the angle of emission, θ, of the Cherenkov cone, RICH detectors can go back to the particle velocity. Combining this information with the direction of the particle (obtained matching the RICH information with those of the tracking detector) it is possible to calculate the mass and charge of the particle. The knowledge of the mass and charge allows the identification of the particle.
This detection technique, proposed for the first time in 1977 by Jack Séguinot and Tom Ypsilantis, is based on the principle that the speed of light can be exceeded when particles move in a medium (air, water, ice, quartz, …), also called radiator. Crossing a medium a particle faster than the light produces a shock wave in the form of light. This phenomenon, called Cherenkov effect, is very similar to the sonic boom produced by an aircraft breaking the sound barrier.
The light emission occurs continuously while the particle is crossing the radiator. Considering four possible consecutive instant of time it is possible to “capture” the relative light emission and it is easy to understand that at the end it will result in a cone shape and the angle of emission θ strictly depends on the particle velocity.
Scheme of the Cherenkov light emission by a particle
Particle tagging is one of the most challenging aspects of high energy physics experiments. In modern particle accelerators the high rates and the background conditions give binding constraints to the detectors’ characteristics.
The use of a RICH-like detector is necessary for all those high energy physics experiments which aim to identify charged hadrons in the final state.
NA62 is a kaon factory located at CERN, it was proposed to measure the very rare kaon decay K+-> π+-nu-nubar to extract a 10% measurement of the CKM parameter |Vtd|.
To achieve this result, NA62 exploits a RICH detector which allows the identification of pions with respect to muons between 15 and 35 GeV/c momentum providing a muon suppression factor of at least 10-2.
It consists of a cylindrical vessel (Length ~18m, Ø=3.2 – 3.8m ) with the beam pipe passing in its center. The vessel is filled with Neon (1 atm), at the downstream end a mosaic hexagonal mirrors focuses the Cherenkov light onto 2x1000 photo detectors situated at the upstream end of the vessel.
The reflected light is collected by about 2000 PMTs (PhotoMultiplier Tubes) Hamamatsu type R7400U-03 photomultipliers. These devices were chosen for their compactness and high speed.
The NA62 RICH can also measure the pion crossing time with a resolution of about 100 ps, producing the L0 trigger for a charged track.
The LHCb experiment aims to study heavy-flavour hadron production. It will measure the CKM parameters of the Unitarity Triangle with high precision, and search for physics beyond the Standard Model.
This experiment boasts two RICH-like detectors: RICH1 and RICH2.
FIG. 2 The LHCb RICH detector: RICH1 (Left) RICH2 (Right).
copyright by CERN
RICH-1 is the upstream detector with silica aerogel and C4F10 gas radiators (FIG. 2). It is set up to detect low-momentum particles. RICH-2 is the downstream detector that uses CF4 gas as radiator. It has an acceptance which is limited to the low-angle region where there are mostly high-momentum particles.
In their first versions, both RICH1 and RICH2 used HPDs (Hybrid PhotoDetectors). When they upgraded the detectors, instead the LHCb collaboration adopted new MAPMTs (8x8 pixels each) from Hamamatsu, to collect the Cherenkov light emitted in the radiator.
For RICH1 and the inner region of RICH2 the R13742 (2700 MAPMT with a 23x23 mm2 active area) was selected.
FIG. 3 The RICH2 photosensitive surface.
For the outer regions of RICH2 instead the R13743 (400 MAPMT with a 48.5x48.5 mm2) was selected (FIG. 3).
The current choice of using PMT-like detectors (photomultiplier and multianode photomultiplier tubes) has the advantage of a very low dark current, whilst they are very sensitive to the strong magnetic fields present in the particle accelerators.
Hamamatsu Photonics is following some of the R&D projects to find alternative solutions to PMTs. It has been proved that Silicon PhotoMultipliers (SiPMs) can be used as single photon detectors in a RICH counter. They have the great advantage of being insensitive to the magnetic fields, but the disadvantage of a high dark current.
Recently, Hamamatsu made commercially available SiPMs with a rather low dark current and a larger active area (MPPCs Multi Pixel Photon Counters), offering a possibility for a much better signal-to-noise ratio.
Up to now, Hamamatsu 8×8 channel S11834 MPPCs module has been successfully trialed in a test beam with good results.
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