R&D studies on eco-friendly gas mixtures for the ALICE Muon Identifier

The gas mixture for the Resistive Plate Chambers (RPCs), used for the Muon Spectrometer of the ALICE experiment at CERN LHC, is made up of C2H2F4, SF6 and iC4H10. Since the first two gases have high Global Warming Potentials (GWPs), they will be phased out of production in the next years due to the recent European Union regulations; meanwhile their cost is progressively increasing. Therefore, finding a new eco-friendly gas mixture has become extremely important. In order to identify a gas mixture suited to cope with the requirements of the ALICE Muon Identifier in the forthcoming High-Luminosity runs, R&D studies have been performed on promising gas mixtures with small-size RPCs. Hydrofluoroolefins (HFOs) may substitute C2H2F4 thanks to their very low GWPs, especially HFO1234ze. Several tests on HFO-based mixtures are ongoing and encouraging results have already been obtained. Furthermore, the use of CO2 as a quencher has been studied, along with medium-term stability of detectors exposed to the cosmic-ray flux.