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). More than 3000 m2 of 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 the 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 period of LHC, 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.