RPC performances and gas quality in a closed loop gas system for the new purifiers configuration at LHC experiments

In the ATLAS and CMS experiments the RPC detector covers alarge surface of about 4000 m2 equivalent to 14 m3 of gasvolume for each system. RPCs are operated using a C2H2F4(R134a) based humidified gas mixture. A flow of the order of onevolume exchange per hour is needed for the detector operation. Thesecharacteristics make the closed-loop circulation unavoidable.Nowadays the gas systems are operated with a 90–95% re-circulationfactor.Results from tests performed over the past few years have shown howthe molecules in the gas mixture are broken up under the action ofthe high electric field and the high radiation background during LHCoperation.Several RPCs were operated at the CERN Gamma Irradiation Facility ina high radiation environment in order to observe the production oftypical impurities and to find an optimal purifiers configurationfor their absorption. During the test, the detector performanceswere monitored in terms of current stability and HPL resistivity.