Studies of purification of the Resistive Plate Chamber gas mixture for the Large Hadron Collider experiments

The Resistive Plate Chambers (RPCs) installed as part of the large muon detectors at the Large Hadron Collider (LHC) experiments use a gas mixture of 94.7% C2H2F4, 5% iC(4)H(10) and 0.3% SF6. Based on economical grounds, the design philosophy of the gas systems for the ATLAS and CMS RPC's foresees to recirculate the gas mixture in 90-95% closed loop circulation. At the LHC, RPC chambers are operated in a high radiation environment, conditions for which large amount of impurities in the return gas have been observed in earlier studies. They are potentially dangerous for the stable operation of the detectors, the materials in the detector and the gas system. While several purification stages have been foreseen in the present gas systems, chemical reactions between the absorber and the impurities are yet not well understood. Furthermore, the effects on the gas mixture of the foreseen factor 10 increase of luminosity for the LHC upgraded phase should be studied. We present the results of systematic studies of the impurities produced in the RPC chambers irradiated in the high intense photon flux of the CERN Gamma Irradiation Facility (GIF), in order to understand the properties of possible absorbers in a series of orderly tests. The final objective is a proposed optimization of the filters for the LHC closed-loop gas systems and their operation to ensure that RPC chambers can sustain stable, long-term operation under radiation in a cost-effective manner.