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 $C_2{H}_2{F}_4/i-C_4{H}_{10}/{\mathrm{SF}}_6$, 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 aging. However, the mixture has a high Global Warming Potential (GWP $\sim$1430), primarily due to the presence of $C_2{H}_2{F}_4$. The $C_2{H}_2{F}_4$ and ${\mathrm{SF}}_6$ 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 within the RPC community. CERN is also driving efforts to reduce these gases, as they contribute significantly to the LHC greenhouse gas emissions. Within the ATLAS experiment, the search for an environment-friendly gas mixture involves both the legacy system and the new generation of RPC detectors for the HL-LHC [1]. The thin 1 mm gas gap 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 compared with the standard one. 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 $C_2{H}_2{F}_4$ with a mixture of $C_3{H}_2{F}_4/{\mathrm{CO}}_2$, which has a significantly lower Global Warming Potential (GWP$\sim$ 200). In order to achieve an efficiency greater than 90%, the concentration of $C_2{H}_2{F}_4$ must be maintained above 50%. Meanwhile, the addition of ${\mathrm{CO}}_2$, improves the time resolution, reaching approximately 285 ps. The second approach consists in adding a modest fraction of ${\mathrm{CO}}_2$ in the standard gas, with the aim to reduce the $C_2{H}_2{F}_4$ emissions. In this case, the currents are lower compared to those observed with the $C_3{H}_2{F}_4/{\mathrm{CO}}_2$-based gas mixtures, and no significant impact on detector aging is expected, as the overall composition of the mixture remains similar to the standard gas. In both approaches, the concentration of ${\mathrm{SF}}_6$ must be maintained at a minimum of 1% to prevent premature streamer formation and the associated increase in current. The paper provides a detailed study of efficiency, time resolution, and current under different irradiation backgrounds.