Enabling high-performance RPCs with semi-insulating GaAs: from material characterization to beam tests

Resistive Plate Chambers are widely used in elementary particle physics owing to their fast time response, high intrinsic efficiency, and low cost per unit area, making them particularly suitable for large-scale trigger systems. In particle accelerator experiments, a major challenge is to increase their rate capability by at least one order of magnitude. Large-area single-gap RPCs currently demonstrate rate capabilities of about 10 k H z / c m 2 , as achieved with bakelite electrodes developed for the ATLAS Phase-2 upgrade. This remains significantly below the performance of other gaseous detector technologies such as MPGDs. Reducing electrode resistivity is one of the most effective approaches to increasing rate capability, although new materials require characterization in terms of detector performance, technological feasibility, radiation tolerance, and long-term stability. Recent developments include low-resistivity doped glasses, polymer-based materials, and Diamond-Like Carbon (DLC) coatings. Thin multigap configurations using low-resistivity glass or Diamond-Like Carbon have shown promising performance, with some studies indicating potential rates in the M H z / c m 2 range, although for architectures and irradiation conditions not directly comparable with conventional single-gap RPCs. This work investigates Semi-Insulating Gallium Arsenide (SI-GaAs) as an Resistive plate chamber electrode material, reviewing previous results, advantages, critical issues, and unresolved aspects. SI-GaAs, widely used as a substrate for epitaxial growth, has attracted interest since the 1990s owing to its bulk resistivity of approximately 1 0 8 Ω ⋅ c m , about two orders of magnitude below conventional bakelite. Combined with ε r ≃ 13 , this gives a dielectric relaxation time τ ≃ 0.1 m s , compared with several milliseconds for bakelite, suggesting substantially improved rate capability. Previous SI-GaAs RPC prototypes demonstrated excellent performance, but also revealed stability issues. Advances in sensitive front-end electronics have reduced RPC operating voltages and average charges, while decreasing GaAs wafer costs and commercially available diameters up to approximately 15 c m have improved scalability to medium-area detectors. Previous detector and material characterization is complemented here by new measurements under intense 137 Cs photon irradiation and Monte Carlo simulations of photon conversion efficiency. Stable operation is demonstrated at counting rates approaching 40 k H z / c m 2 , while maintaining high detection efficiency. The combined results are used to discuss the main limitations of SI-GaAs electrodes and possible physical mechanisms underlying the observed stability behaviour.