Threshold-dependence study for narrow-pitch-strip CMS endcap RPCs

We report on a systematic study of double-gap and four-gap phenolic resistive plate chambers (RPCs) for high-η RPC triggers in CMS. Prototype double-gap and four-gap RPCs with gap thicknesses of 1.6 and 0.8 mm, respectively, have been constructed with 2-mm thick phenolic high-pressure-laminated (HPL) plates. Two different type front-end-electronics (FEE) were used for the digitization of the detector signals; charge-sensitive FEE for the operation of the current double-gap CMS RPCs and higher sensitive voltage-sensitive FEE dedicatedly developed for the fundamental study of RPCs. The prototype RPCs were measured for cosmic muons and gamma rays emitted from a 5.5 GBq 137Cs source. The gamma-signal rates induced in the double-gap and four-gap RPCs installed at a distance of 36 cm from the cesium source ranged from 0.6 to 1.5 kHz cm-2. The muon cluster sizes and the probabilities of occurring large multiplicity were sensitive to the choice of digitization threshold for both type RPCs while the efficiencies were relatively less sensitive. For the double-gap RPCs, better detector characteristics were obtained at the higher digitization threshold. For the four-gap RPCs, on the other hand, choice of the low thresholds will be fairly conducive to suppress the detector current induced by high-rate backgrounds.