Signal shape studies and rate dependence of HFO-based gas mixtures in RPC detectors

The RPCs employed at the LHC experiments are currently operated in avalanche mode, with a mixture containing a large fraction of C₂H₂F₄ ($\approx$90% or more) with the addition of i-C₄H₁₀ and SF₆ in different concentrations. However, C₂H₂F₄ and SF₆ are fluorinated greenhouse gases (F-gases) with Global Warming Potential (GWP) of $\approx$1400 and $\approx$22 800, respectively. EU regulations imposed a progressive phase-down of C₂H₂F₄ production and consumption, aiming at strongly reducing its emission. This is already resulting in an increase of its price and reduction in availability.

The most desirable long-term solution to this problem is to find an alternative, F-gases-free gas mixture, able to maintain similar detector performance. To study lower-GWP gas mixtures for RPC detectors, the RPC ECOGasas@GIF++ collaboration (including RPC experts of ALICE, ATLAS, CMS, SHiP/LHCb, and the CERN EP-DT group) was created in 2019 and it is currently investigating a gas from the olefine family, the C₃H₂F₄ (or simply HFO, with GWP $\approx$6), to be used, in combination with CO₂, as a substitute for C₂H₂F₄.

This contribution will focus on the signal shape studies that have been carried out by the collaboration during dedicated beam test periods. The methodology used in the data analysis will be presented, together with the results obtained with several HFO-based gas mixtures, and with the currently employed one. Furthermore, results on the counting-rate dependence of the RPC performance, obtained by combining the muon beam with the GIF++ ¹³⁷Cs source with different attenuation factors, will also be presented.