Rate-capability study of a four-gap phenolic RPC with a $^{137}$Cs source

In this paper, we report on the systematic rate-capability study of four-gap phenolic resistive plate chambers (RPCs), with cosmic muons and gamma rays emitted from a $^{137}$Cs source. A prototype four-gap RPC with a gap thickness of 1.06 mm has been constructed with 2-mm thick phenolic high-pressure-laminated (HPL) plates. A 32-channel front-end-electronics board, which has been developed for the operation of the current double-gap RPCs in the CMS experiment, was used to digitize the detector signals of the prototype RPC, with charge thresholds of 80, 130, and 170 fC. The cosmic muons were reliably measured with efficiencies of higher than 95\%, at a maximum gamma-hit rate of 5 kHz cm$^{-2}$. The present research confirms that the use of the current four-gap phenolic RPCs is advantageous to the high-$\eta$ triggers in CMS, by virtue of the high rate capability.