Speaker
Description
The muon trigger system of the ATLAS experiment at the CERN Large Hadron Collider relies on about 3700 Resistive Plate Chambers (RPCs) operated in avalanche mode with a gas mixture based on R134a, i-C₄H₁₀, and SF₆. While this mixture has provided stable operation, high efficiency, and excellent time resolution since 2009, it presents two major limitations for long-term operation: the increasing difficulty in procuring R134a and SF₆ due to their high global warming potential (GWP), and the production of chemically aggressive fluoride radicals that negatively affect RPC longevity, particularly under high-intensity running conditions.
To address these issues, a progressive strategy of fluorinated gas reduction has been pursued since 2022, targeting both environmental impact and detector ageing. Following extensive validation at the Gamma Irradiation Facility (GIF++), a first new mixture was deployed in 2023, replacing 30% of R134a with CO₂, achieving a ~17% GWP reduction while preserving performance and lowering the operating voltage. A further optimization in 2025, with a reduced SF₆ fraction, led to a total GWP reduction of about 25%.
These changes were validated with Run 3 collision data and dedicated scans, showing stable operation, full efficiency at reduced voltage, and improved resilience under irradiation, in agreement with ageing studies at GIF++. This stepwise approach demonstrates a viable path to extend RPC lifetime and sustainability toward HL-LHC conditions.
As a next step, further efforts will focus on replacing SF₆ with an alternative component, aiming to reach a total GWP reduction of about 36% with respect to the standard mixture. A dedicated long-term ageing campaign will be carried out to validate the new mixture and assess its compatibility with sustained operation in the ATLAS RPC system.