Report dei test condotti sui rivelatori RPC del Muon IDentifier dell'esperimento ALICE e sul R&D mirato all'individuazione di una miscela ecologica per le RPC

A Large Ion Collider Experiment (ALICE) is one of the four Large Hadron Collider (LHC) large experiments at CERN and its main task is the study of the Quark Gluon Plasma (QGP). The latter is a state of matter where quarks and gluons are deconfined and it is thought to have been the state of matter a few micros after the Big Bang. In order to study QGP physics, collisions of Pb-Pb ions at ultra-relativistic energy are performed and studied. This means that QGP cannot be studied directly and it is necessary to use some probes, sensitive to the decofined medium, which can provide information on the QGP. In particular, the Muon Spectrometer of ALICE studies the muonic decays of the so-called hard probes (quarkonia and open heavy flavour) and it is located in the forward rapidity region of ALICE, covering the pseudo-rapidity range 2.5<|eta|<4. The Muon spectrometer consists of a silicon tracking detector (Muon Forward Tracker, MFT), formed by an array of monolithic silicon sensors, followed by a composite material hadron absorber to stop most of the hadrons produced in the collision entering the spectrometer, a magnetic dipole to bend the muon tracks, five stations of multiwire proportional chambers for muon tracking (Muon Chambers, MCH), an iron wall to eliminate hadron contamination and, lastly, two station of Resistive Plate Chambers to provide muon identification (Muon IDentifier, MID). During my master thesis, I focused on the MID, which is composed by four planes of 18 Resisitve Plate Chambers each and located downstream of the iron wall. MID RPCs are gaseous detectors, with resisitive electrodes made of high pressure laminate (HPL). These detectors have been operated since the start of the LHC physics program, 13 years ago. After all these years, some RPCs started to show signs of ageing, mainly as an increase in absorbed dark current. In order to operate the system at the best of its performance (and for a further 10 years during the next physics runs), it was decided to substitute the detectors showing signs of ageing in ALICE. A new RPC production was launched in 2021 and, before performing in-situ substitution, it was necessary to assess the performance of each of the new detector. During the first part of my thesis I tested 6 RPCs of the new production (18 in total), using a cosmic ray test station, located in the INFN Torino laboratory. This thesis will describe in details the experimental setup, the analysis procedure as well as the main results obtained from the tests, in terms of efficiency, dark rate and working voltage. The second part of my thesis is focused on an R&D campaign aimed to devise an eco-friendly RPC gas mixture. The current gas mixture contains C2H2F4 and SF6, which are fluorinated greenhouse gases (GHG). European Union regulations, issued in 2014, imposed a progressive phase-down of the production and consumption of such gases. CERN has consequently adopted a policy of reduction in the consumption of these gases and, since 80% of the whole CERN detectors greenhouse gases emission is due to RPCs (mainly due to leaks at the detector level), a whole research line devoted to the search for alternative, more eco-friendly gas mixtures for RPCs was opened. To this end, during my thesis work, I participated to the upgrade of the cosmic-rays test station previously used in the INFN Torino laboratory to perform the studies on eco-friendly alternative gas mixtures.