Studies of eco-friendly gas mixtures for RPC detectors at CERN LHC experiments

Several gaseous detectors are operated at the CERN LHC experiments. In particular, Resistive Plate Chambers (RPCs) can be found in the ALICE, ATLAS, and CMS Muon Systems. RPCs detectors are operated with a freon-based gas mixture containing C2H2F4 and SF6, both greenhouse gases (GHGs) with a very high global warming potential (GWP), 1430 and 22800 respectively. These gases, together with CF4 and C4F10, are responsible for 70\% of CERN operation’s direct greenhouse gas emission. Moreover, the European Union defines a set of regulations aiming at reducing the GHG emissions from fluorinated gases. This regulation could bring to have lower availability and higher costs in the future. The present work is related to one of the strategies delineated by the CERN gas group to reduce GHG emissions: the search for eco-friendly gas mixtures for RPCs detectors. The first part of the thesis is focused on the study and characterization of RPC performances with these new gas mixtures in the laboratory environment. Some alternative to C2H2F4 were tested, such as CO2, He and HFO1234ze. This last one is a molecule in the family of HydroFluoroOlefins with a GWP of 7. Furthermore, Novec 4710 was tested as a substitution of SF6 since it presents a good electronegativity and a GWP of 2100. The RPC performances were evaluated by studying the detector’s efficiency, currents, streamer probability, prompt charge, cluster size, and time resolution. The second part of this thesis deals with RPC detectors tested at the CERN Gamma Irradiation Facility with some of the alternative gas mixtures. Here the RPC performances were studied under a similar LHC experiment radiation background, thanks to a 12 TBq 137Cs source and a muon beam, provided by the SPS accelerator. This studies also allow to investigate the behavior of the single gas component of the mixtures. The main outcome is that substituting 30\% R134a concentration with He or CO2 does not imply any change in RPC performance with respect to the standard gas mixture. Moreover, it is possible to see that an increase of the SF6 concentration together with a decrease of the ratio between R134a and HFO1234ze brings to an increase in the currents and to a drop in the efficiency with respect to the standard gas mixture, in a condition of gamma rates of about 600 Hz/cm2. The Novec 4710 seems promising as a SF6 substitute with 0.1\% concentration although it is known that it can react with water. Studies of the production of possible compounds is ongoing as well as validation tests both in laboratory condition and in test beams with high particle rate environment.