CERN Accelerating science

ALICE (A Large Ion Collider Experiment) studies heavy-ion (mainly Pb-Pb), proton-ion and proton-proton (pp) collisions at the CERN Large Hadron Collider (LHC). In central heavy-ion collisions, a deconfined state of matter, known as Quark-Gluon Plasma (QGP), is formed. In the forward rapidity region, ALICE is equipped with a muon spectrometer, detecting quarkonia (e.g. J/$\Psi$, $\Upsilon$) and open heavy flavour via their muonic decays. Quarkonia are key probes of the QGP properties: the suppression of J/$\Psi$ has been one of the first proposed signatures for the formation of QGP in heavy-ion collisions. The identification of muons in ALICE is performed by two stations of Resistive Plate Chambers (RPCs) placed downstream of two hadron absorbers. Particles tagged as muons are tracked by a set of ten stations of multi-wire proportional chambers. The correct operation of RPCs is ensured by the choice of the proper gas mixture and currently the ALICE RPCs are operated with a mixture of \ce{C_{2}H_{2}F_{4}}, \ce{i-C_{4}H_{10}} and \ce{SF_{6}}. Starting from 2016, new European Union regulations enforced a progressive phase-out of \ce{C_{2}H_{2}F_{4}} and, although its use for scientific purposes may still be allowed, CERN has adopted a policy of reduction of its greenhouse gases consumption, to which RPC operation by the LHC experiments contributes for about 80\%. Most importantly, the ban on industrial usage of \ce{C_{2}H_{2}F_{4}} will make it difficult and costly to purchase. The work described in this thesis is devoted to the detailed study and characterization of alternative eco-friendly gas mixtures for the ALICE muon RPCs. Preliminary tests with cosmic rays, reported in literature, have indicated that mixtures where \ce{C_{2}H_{2}F_{4}} is fully replaced by a combination of \ce{CO_{2}} and \ce{C_{3}H_{2}F_{4}} (HFO-1234ze) in variable proportions are promising, hence mixtures of this type were chosen for this work. The next crucial steps are the detailed in-beam characterization of such mixtures, the study of their performance under increasing irradiation levels, and their stability over time and as a function of the integrated charge. The Gamma Irradiation Facility (GIF++) at CERN, equipped with a 12.5 TBq~\ce{^{137}Cs} source able to induce counting rates up to several~kHz/cm$^{2}$ and located along a secondary beam line of the CERN accelerator complex, is an ideal tool for these studies, allowing one to to study the RPC performances in a high background radiation environment and to accumulate the integrated charge expected in a few years of operation at the LHC in a much shorter time span. This thesis describes the methodology and results of a set of beam and aging tests carried out at the GIF++ with ALICE-like RPC prototypes, operated with several mixtures with varying proportions of \ce{CO_{2}} and \ce{C_{3}H_{2}F_{4}}. In both cases, the RPC standard gas mixture has been used to establish the baseline behavior of the RPC. The tested mixtures have been fully characterized, in terms of absorbed current, efficiency, prompt charge, cluster size and time resolution, using the muon beam provided to the GIF++ in dedicated beam time periods. Both digitized (for detailed shape and charge analysis) and discriminated signals (using the same front-end electronics as employed in ALICE) were analyzed. Throughout the test periods, the irradiation from the GIF++ source was used to simulate a high background radiation on the RPCs, to test their rate capability. The role of the new components in the RPC mixture has been analyzed and, in light of the obtained results, the most promising mixtures have been pinpointed. Outside of the allocated beam time, a (still ongoing) long-term irradiation campaign at GIF++ was started, to study the stability of the detector response (so far only in terms of absorbed current) if exposed to intense radiation for a prolonged period of time, thus obtaining the first important indications on the long-term behavior of RPC detectors operated with HFO-based mixtures.