Tests of multigap RPCs for high-η triggers in CMS

In this paper, we report a systematic study of multigap Resistive Plate Chambers (RPCs) for high-η triggers in CMS. Prototype RPC modules with four- and six-gap structures have been constructed with phenolic high-pressure- laminated (HPL) plates and tested with cosmic muons and gamma rays irradiated from a 200-mCi ¹³⁷Cs source. The detector characteristics of the prototype multigap RPCs were compared with those of the double-gap RPCs currently used in the CMS experiment at LHC. The mean values for detector charges of cosmic-muon signals drawn in the four- and six-gap RPCs for the efficiency values in the middle of the plateau were about 1.5 and 0.9 pC, respectively, when digitized with charge thresholds of 150 and 100 fC, respectively. They were respectively about one third and one fifth of that drawn in the current CMS double-gap RPC with a charge threshold of 200 fC. We concluded from the current R&D that use of the current phenolic-HPL multigap RPCs is advantageous to the high-η triggers in CMS in virtue of the smaller detector pulses.