Optimization of the Gas System in the CMS RPC Detector at the LHC

The Resistive Plate Chambers (RPC) are a part of the muon spectrometer for the Compact Muon Solenoid (CMS) experiment at Large Hadron Collider (LHC). RPCs are ionizing-particle detectors consisting of two gaps filled up with gas. They are used as dedicated trigger detectors but they also contribute to the muon reconstruction at CMS. The CMS RPC detectors are divided in three regions which are two endcaps and a central barrel, in total there are 480 RPCs in the barrel region, and 432 RPCs in the endcap regions covering more than 3000 $m^2$. Such a large RPC system was successfully operated with a closed loop gas system in the first three-running-years of the LHC. Annual campaigns were done during 2011 and 2012 to measure the gas leak rates of the RPCs and the results were compared to the flow-cell readout data. The total leak rate on the barrel increased between 2011 and 2012 while the total rate on the endcaps was negligible during the same period. Consequently, the CMS gas consumption increased, but the gas leaks could not be repaired during the whole period of data taking at the LHC that lasted more than two years. Here, we present the results of the extensive tests for the first long shutdown, including the leak detection in the chambers (internal leak) or gas pipes and connectors (external leak), details on the new leak measurement, and the status of a leak repair.