Study of environment friendly gas mixtures for the Resistive Plate Chambers of the ATLAS phase-2 upgrade

The standard gas mixture for the Resistive Plate Chambers (RPC), composed of C2H2F4/i-C4H10/SF6, allows the detector operation in avalanche mode, as required by the high-luminosity collider experiments. The gas density, the low current and the comfortable avalanche-streamer separation guarantee high detection efficiency, rate capability and slow detector ageing. The mixture has a high Global Warming Potential (GWP ~1430), primarily due to the presence of C2H2F4. The C2H2F4 and SF6 are not recommended for industrial uses anymore, thus their availability will be increasingly difficult over time and the search for an alternative gas mixture is then of absolute priority. CERN is also driving efforts to reduce these gases, as they contribute significantly to the LHC greenhouse gas emissions. The thin 1 mm gas gap foreseen for the ATLAS upgrade of the latter requires a high-density in order to achieve high efficiency, due to the less active target available for the primary ionization. The mixture should also guarantee good timing performance and ensure the detector longevity. In this paper, the results obtained on a RPC operated with alternative gas mixtures are shown, following two different approaches. The first study consists of the replacement of the C2H2F4 with a mixture of C3H2F4/CO2 (GWP~200). The second approach consists in adding a modest fraction of CO2 in the standard gas, with the aim to reduce the C2H2F4 emissions. The paper provides a detailed study of efficiency, time resolution, and current under different irradiation backgrounds.