Speaker
Description
The trigger system in the barrel region of the ATLAS experiment at CERN relies on about 3700 2 mm gas-gap Resistive Plate Chambers (RPCs), which will be upgraded during Long Shutdown 3 with an additional system of about 1000 new 1 mm gas-gap RPCs to be installed in the muon Inner Barrel. Since 2008 and until 2023, these detectors have been operated in avalanche mode with a gas mixture based on R134a, i-C₄H₁₀, and SF₆. While this mixture has provided relatively stable operation and good performance, it presents two major limitations for the future: 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. Indeed, since 2023, when the LHC reached a stable luminosity of 2 × 10^34 cm^-2 s^-1, this effect has become evident.
To address both issues, a stepwise strategy for reducing fluorinated gases 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, replacing 30% of the R134a with CO₂, was adopted by ATLAS for the 2024 run, achieving a ~17% GWP reduction while preserving performance. A further optimization in 2025 consisted of reducing the SF₆ fraction, leading to a total GWP reduction of about 25%. In both cases, a significant and increasing reduction of the ageing impact was measured on the ATLAS RPCs during the 2024 and 2025 runs, respectively.
The ongoing validation activity is focused on replacing SF₆ with an alternative component, aiming to achieve a total GWP reduction of about 36% with respect to the standard mixture. A dedicated long-term ageing campaign has started to validate the compatibility of the new mixture with the expected Run 4 luminosity of up to 7.5 × 10^34 cm^-2 s^-1 and beyond.
A recent spinoff study presents an even more ambitious perspective, based on the experimental observation that the efficiency of RPCs decreases less than expected at higher CO₂ concentrations, opening the possibility of a further substantial reduction in GWP while increasing the resilience of the RPCs.