RICH - JLab12

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The new CLAS12 spectrometer that will be installed in the Hall B is made of a forward detector for measuring the higher energy particles produced in the hard scattering of quarks inside the nucleon target and of a central detector for recoil particle measurements. The forward detector is based on a toroidal field and is divided in 6 azimuthal sectors. In its present configuration, it allows good identification of pions and nucleons. However, it is not able to correctly identify kaons in the range of momenta between 3 and 8 GeV/c. For this reason, the realization of a Ring Imaging CHerenkov (RICH) detector has been proposed by the INFN physicists. Such detector is basically composed of a radiator where the particles produce a cone of Cherenkov light, a gap where the photons propagate and an array of photodetectors. Since the opening angle of the cone depends on the speed of the particle, measuring the radius of the ring allows the identification of the particle itself. The constraints imposed by the geometry of CLAS12 forced to choose a hybrid configuration, in which part of the photons are directly detected, while part of them are detected only after a double reflection on a mirror system. Thus, depending on the number of reflections, the pattern of the photons measured by the photodetectors may show full circles as well as pieces of circles. The construction of one RICH sector is currently underway.

It uses a layer of aerogel of several squared meters and different thickness as radiator material, about 400 Multi-Anode PhotoMultiplier Tubes (MAPMT) as photodetectors and two large mirrors, one planar and one elliptical. A large scale prototype of the detector has been build and tested at CERN using the hadron beam of the T9 experimental Hall. The studies of new techniques for large area photon detectors at affordable costs (for example: SiPM, Large-Area Picosecond Photodetectors or GEM) and of highly integrated fast electronics (the 400 MAPMTs require readout of 25600 channels) are also ongoing in view of the possible extension of the RICH detector to the remaining CLAS12 sectors.The project foresees the collaboration between many INFN groups (LNF, Fe, Ge, Sanità/RM1, Ba/Le) and international institutions (JLab and Argonne National Laboratory in USA, Budker Institute for Nuclear Physics in Russia, Universidad Tecnica Federico Santa Maria de Valparaiso in Chile, University of Glasgow in UK, Institut fur Kernphysik in Mainz, Germany).

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