Characterization of RPC detectors operated with new eco friendly gas mixtures

Resistive plate Chambers (RPCs) are gaseous particle detectors widely used at CERN LHC experiments thanks to their excellent time resolution and production cost. They are operated with a Freon-based gas mixture containing C2H2F4 and SF6, both greenhouse gases (GHG) with a high Global Warming Potential (GWP) and therefore subject to European regulations aiming at reducing the GHG emissions. The search for new environmentally friendly gas mixtures that are compatible with the current ATLAS and CMS RPC systems is therefore determined by the need of lowering GHG emissions and, obviously, of finding alternatives to gases that could disappear in the future. The goal of this work is to characterize the RPC performance using low GWP mixtures. In the first part of the work, C2H2F4 was partially replaced either by inert gases or by C3H2F4 (also known as R-1234ze), a gas in the family of the HydroFluoroOlefin (HFO), which has already been identified by the refrigerant industry as a suitable low GWP alternative to the R-134a. Recently developed high electronegativity and electrically insulating gases, 3MTM NovecTM 4710 and 3MTM NovecTM 5110, were tested as SF6 replacements. Several gas mixtures were tested in laboratory conditions using cosmic muons, and the obtained results were compared to the standard gas mixture currently used at CMS and ATLAS experiment (C2H2F4/i-C4H10/SF6 95.2/4.5/0.3). Novec gases have proven to be promising alternatives to SF6. The second part of this work is related to long term tests and validation of low GWP gas mixtures for RPC detectors under irradiation condition at the CERN Gamma Irradiation Facility (GIF++), where a 12.2 TBq 137Cs source provides an irradiation background similar to the one foreseen for the High Luminosity LHC (HL-LHC) phase. The RPC detectors under irradiation were kept at the working point in order to investi- gate the performance stability over time. The gas mixture which underwent the validation test was a five compo- nents mixture where part of the C2H2F4 has been replaced by R-1234ze and Helium (C2H2F4/HFO/He/iC4H10/SF6 37.45/37.45/20/4.5/0.6). Currents measured at the gas mixture’s working point were approximately larger by a fac- tor two with respect to the standard gas mixture, but they showed good stability over a six-month operation period. Long term performance studies were started and are being carried on.