Performance studies on Resistive Plate Chambers detectors operated with new environmentally friendly gas mixtures at CERN GIF++ facility

Abstract

Resistive Plate Chamber (RPC) detectors are widely used at CERN LHC experiments as a muon trigger due to their excellent time resolution. They are operated with a Freon-based gas mixture containing $C_2 H_2 F_4$ and $SF_6$, both greenhouse gases (GHG) with a high Global Warming Potential (GWP) and therefore subject to European regulations aiming at reducing the GHG emissions. Alternative gases in the HydroFluoroOlefin (HFO) family have already been identified by the industry as a low GWP replacement of the $C_2 H_2 F_4$. The search of new environmental friendly gas mixtures is advisable for reducing greenhouse gas emissions, costs as well as optimize RPC performance and possible detector aging issues. The aim of this study is to characterize the RPC detector operation using low GWP gas mixtures based on HFO and compare the results with the standard gas mixture ($C_2 H_2 F_4$/$i C_4 H_{10}$/$SF_6$ - 95.2/4.5/0.3) used at the ATLAS and CMS experiments. The RPC detectors were tested in laboratory conditions and at the CERN Gamma Irradiation Facility (GIF++), which provides a high energy muon beam from the SPS combined with an intense gamma source, allowing to simulate the background radiation expected at the High Luminosity LHC Phase (HL-LHC). Firstly, several eco-friendly gas mixtures were tested on a dedicated experimental setup in laboratory. A five component gas mixture (HFO-1234ze/$C_2 H_2 F_4$/$CO_2$/$SF_6$/$i C_4 H_{10}$) was selected as a suitable candidate for its low GWP and for giving good detector performance comparable with the currently used mixture at LHC experiments. The setup was then moved to the GIF++ to validate the operation of RPC at high gamma rate with the selected gas mixture. At the GIF++, the RPC detectors were studied with different muon and gamma background rates in terms of efficiency, streamer probability, induced charge, cluster size, and rate capability. The results indicate that the use of HFO based mixtures lead to a working point shifted of 1000V towards higher voltage and a streamer probability at the efficiency knee higher than the one with the standard gas mixture. The RPC operated with the HFO based gas mixture proved to have efficiency versus rate curves comparable to the standard gas mixture, indicating a rate capability suitable for the background rate expected at HL-LHC. However, the higher streamer probability could induce long term ageing effects that are now under evaluation