Speaker
Description
The ALICE Muon IDentification (MID) system consists of 72 single-gap Resistive Plate Chambers (RPC), operated with a gas mixture composed of: 89.7% C2H2F4, 10% i-C4H10 and 0.3% SF6, with a 37% relative humidity content. To reduce gas consumption and emissions, the MID gas system is operated in closed loop mode with a recirculation fraction of 88%. The high concentration of fluorinated gases in the mixture, combined with the background radiation and the high voltage applied to the detectors, leads to the production of F- ions and F-based impurities. Among these, hydrofluoric acid is especially dangerous since it could lead to long-term deterioration of the detector or the gas system surfaces.
During the LHC Run 2, a preliminary setup was installed to monitor the formation of these impurities, successfully demonstrating their production as well as the capability of the purifying cartridges employed in the closed-loop gas system to trap them, further validating the operation of the gas system in recirculation mode.
During Long Shutdown 2, the set up was improved and commissioned at the restart of LHC Run 3. It now features a gas chromatograph to monitor the correct composition of the gas mixture and F-based impurities and an Ion Selective Electrode (ISE) station to quantify the F- concentration.
This contribution provides an overview of the production and accumulation of F- impurities within the MID system for the whole duration of the LHC Run 3, as well as a comparison to what was measured during LHC Run 2.