Performance and longevity of CO2 based mixtures in CMS improved Resistive Plate Chambers in the HL-LHC environment

Abstract

Resistive Plate Chamber (RPC) detectors are widely used in high-energy physics experiments. In the Compact Muon Solenoid (CMS), the RPC gas mixture is composed of 95.2% C2H2F4, which generates a large number of ion-electron pairs, 4.5% iC4H10 to suppress photon feedback effects, and 0.3% SF6 as an electron quencher to ensure operation in streamer-free mode. Given the high global warming potential (GWP) of C2H2F4 at 1430 and the recent reduction in the emission of F gases imposed by the European Union, efforts have intensified in recent years to explore environmentally friendly gas alternatives. A promising short- to mid-term solution for the upcoming years of Large Hadron Collider (LHC) operations is to lower the GWP of the RPC gas mixture by partially substituting C2H2F4 with CO2. The performance tests of the alternative gas mixtures are conducted at the CERN Gamma Irradiation Facility (GIF++) in the North Area of the Super Proton Synchrotron (SPS), where a 13.6 TBq radiation source and an SPS muon beam simulate the High-Luminosity (HL) Phase II conditions of the LHC. This paper reports on the performance of a 1.4 mm gap RPC using three different CO2-based mixtures under intense gamma radiation, with the first results on the longevity campaign.