Evaluating the performance and long-term stability with LHC-like background irradiation of RPC detectors with CO <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si14.svg" display="inline" id="d1e244"> <mml:msub> <mml:mrow/> <mml:mrow

Resistive Plate Chamber (RPC) detectors at CERN’s LHC experiments traditionally use a Freon-based gas mixture containing C 2 H 2 F 4 (R-134a) and SF 6 , both of which are high global warming potential (GWP) gases. To reduce greenhouse gas (GHG) emissions, operational costs, and optimise RPC performance, one possible mid-term solution is the gas mixture with a substitution of 30% of R-134a with CO 2 in the standard RPC gas mixture. This gas mixture was successfully adopted in the ATLAS RPC system since August 2023. This study investigates the detector’s performance when CO 2 is introduced into the standard gas mixture to minimise emissions while maintaining compatibility with current CERN RPC systems. Conducted at the CERN Gamma Irradiation Facility (GIF++), this research uses a 12 TBq 137 Cs source and a muon beam to simulate the LHC experiment’s background radiation. The setup includes 2 mm single-gap High-Pressure Laminate (HPL) RPCs placed at different distances from the gamma source. Since March 2023, the detectors inside the bunker have been continuously irradiated to assess long-term performance, targeting the integrated charge expected for ATLAS RPC detectors in LHC Run 3 and the High-Luminosity LHC phase. Monitoring is performed using various metrics such as gas analysis, oxygen content, humidity, dose, environmental parameters, and flow measurements to ensure the gas system operates correctly. Several periodic test beam periods are conducted to assess muon performance parameters, including: efficiency, current, streamer probability, mean prompt charge, cluster size, time resolution. Results from the ageing studies, using the proposed 30% CO 2 gas mixture, are presented.