Studies on RPC detectors operated with environmentally friendly gas mixtures in LHC-like conditions

Resistive Plate Chambers (RPC) are gasesous detectors employed at CERN LHC experiments thanks to their trigger performance, timing capabilities and contained production costs. High Pressure Laminate RPCs are operated with a three-component gas mixture, made of 90–95% of C$_2$H$_2$F$_4$, around 5% of i-C$_4$H$_{10}$ and 0.3% of SF$_6$. Due to the presence of leaks at detector level and to the greenhouse characteristics of C$_2$H$_2$F$_4$ and SF$_6$, RPCs in ATLAS and CMS were accounting for about 87% of CO$_2$ equivalent emissions during LHC Run 2. To address this, several alternative gases were studied, including R-1234ze as a possible substitute for R-134a. Furthermore, the addition of some amount of CO$_2$ into the RPCs gas mixture was explored as a possible short-to-medium term solution to lower the total greenhouse gas emissions and reduce the usage of C$_2$H$_2$F$_4$. A dedicated data taking campaign was performed at the Gamma Irradiation Facility at CERN, where RPCs detectors performance were studied with muon beam and gamma background. The detectors were operated with the addition of 30% and 40% and 50% of CO$_2$ to the standard gas mixture, together with an increased fraction of SF$_6$. Two different amounts of i-C4H10 were also evaluated to assess compatibility with the CMS and ATLAS requirements. Results from these beam tests with the above-mentioned gas mixtures are reported in this work.