Performance and longevity of ATLAS RPCs with new lower GWP mixtures

Resistive Plate Chambers (RPCs) are critical components of the muon systems of most HL-LHC experiments. Until 2023, all LHC RPC systems used a so-called standard gas mixture (STD), consisting of 94.7% C 2 H 2 F 4 (Tetrafluoroethane (TFE) - R134a), 5% i-C 4 H 10 (Isobutane), and 0.3% SF 6 (Sulfur Hexafluoride), highly tuned for RPC performance but having very high global warming potential (GWP). The environmental impact and the increasing difficulty in procuring these types of fluorinated gases imposes to pursue a solution for the long-term experiment plans, such as a new mixture having a lower GWP and preserving, as well, the detector performance and longevity. In the last 2 years, ATLAS muons have been following such strategy, progressively replacing TFE (GWP: 1430) with CO 2 (GWP: 1), and validating the choice with extensive ageing tests performed on realistic ATLAS RPC prototypes. This led ATLAS to be the first experiment replacing the RPC gas mixture in July 2023 with a new mixture, where 30% of TFE has been replaced with CO 2 ; the ATLAS RPC system behavior has been since then studied carefully, to spot in vivo any eventual sign of accelerated ageing. More challenging perspectives, presently under validation, prior to apply them in the experiment, include a further reduction of TFE to 40%, and a lowering, or a total replacement of SF 6 , which GWP (23800) is extremely high. We will report the experience of this 2-year-long performance and longevity study, including the results of one full HL-LHC year of the ATLAS RPC system with the new gas.