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

Abstract

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