Rate capability of large-area triple-GEM detectors and new foil design for the innermost station, ME0, of the CMS endcap muon system

To extend the acceptance of the CMS muon spectrometer to the pseudorapidity region (2.4,2.8), stations of triple-GEM chambers, called ME0, are planned for the CMS Phase 2 Upgrade. These large-area, micro-pattern gaseous detectors must operate in a challenging environment with expected background particle fluxes up to 150 kHz/cm2. Unlike traditional non-resistive gaseous detectors, the rate capability of large-area triple-GEM detectors is limited not by space charge effects, but by voltage drops on the chamber electrodes due to avalanche-induced currents flowing through the resistive protection circuits. We present a study of the irradiation of large-area triple-GEM detectors with moderate fluxes to obtain a high integrated hit rate. The results show drops as high as 40\% of the nominal detector gas gain, which would result in severe loss of tracking efficiency. We discuss possible mitigation strategies leading to a new design for the GEM foils with electrode segmentation in the radial direction, instead of the "traditional" longitudinal segmentation. The advantages of the new design include maintenance of a uniform hit rate across different sectors, minimization of gain-loss without the need for voltage compensation, and independence of detector efficiency on background flux shape.