Refurbishment of CLS System for Mini-ICAL Detector

Resistive Plate Chamber (RPC) is a gaseous detector, which is going to be used as an active detector element for the Iron CALorimeter (ICAL) experiment, which is planned to be built by the India-based Neutrino Observatory (INO). A gas mixture consisting of R134a (95.2%), Iso-Butane (4.5%) and SF6 (0.3%) is used to operate the RPCs in the Avalanche mode. Composition of gas mixture plays a crucial role in the RPC detector performance. Since the past four years, a closed loop gas system (CLS) is in operation with 20 RPCs in the mini Iron CALorimeter (miniICAL) detector at Madurai. In the CLS system, the gas mixture flowing out of the RPCs is routed back to the inlet after a suitable purification. Top-up of the fresh gas maintain the differential pressure at the outlet header within the range of −3 mmWC to +8 mmWC. The amount of gas top-up is the measure of the leakage in the gas circulation path. During the CLS operation, the top-up of the gas was observed in the intervals of 50 to 90 min, which has indicated an increase in the leakage. Extensive leak testing and mitigation operations were carried out on the system. The number of gas pipe joints have been reduced to a minimum in the CLS path and faulty pneumatically operated solenoid valves have been replaced or repaired. These refurbishing operations in CLS system have resulted in achieving the top-up cycle of three and half days. Residual Gas Analyser (RGA) has been used to find out the quality of gas mixture, which is flowing in the RPC detectors. After a brief introduction of the CLS system, the refurbishment operations of the CLS system will be presented in detail in this paper.