Mélanges gazeux respectueux de l'environnement et systèmes de surveillance pour les détecteurs du LHC

Greenhouse gas emissions from particle detectors at the CERN LHC pose significant environmental and operational challenges due to their reliance on fluorinated gases with high global warming potential. This thesis addresses these challenges through a comprehensive investigation of eco-friendly gas mixtures and gas monitoring strategies for Resistive Plate Chambers (RPCs) and other gaseous detectors. Two primary research directions are explored. The first involves the development and validation of sustainable gas mixtures. A short-term solution using CO2 as a partial replacement for R134a was validated up to 30% substitution, demonstrating comparable detector performance and stable long-term behavior under irradiation. The ATLAS experiment has adopted this mixture since August 2023. A longer-term solution based on HFO-1234ze was studied within the RPC ECOgas@GIF++ collaboration, showing promising results in ageing campaigns and achieving over 75% reduction in CO2-equivalent emissions. Additionally, a novel MRPC prototype was constructed to test alternatives to SF6, including Novec extsuperscript{TM} 4710, demonstrating operational feasibility with lower environmental impact. The second part of the thesis focuses on gas system monitoring when operated with gas recirculation and recuperation. Detailed studies on the ALICE MID system confirmed the stability of operation under high recirculation rates, up to 87% and revealed correlations between integrated charge, LHC luminosity and fluoride production. Similarly, monitoring in the CMS CSC system using Single Wire Proportional Chambers provided real-time impurity detection and enabled quantification of oxygen contamination. A new setup was also developed to measure fluoride ion production for alternative CF4-based mixtures. The findings of this thesis contribute significantly to the ongoing effort to reduce the environmental impact of LHC detectors while preserving performance and reliability.