Correlation of Streamer Pulse with Contaminants in the RPC Detector

The INO’s ICAL experiment will be instrumented with glass-based Resistive Plate Chambers (RPCs) as an active detector element. The mini-ICAL (Mini Iron CALorimeter) is a 600-times scaled-down version of the ICAL. The prototype is currently in operation at the IICHEP, a transit campus in Madurai, mainly to study the engineering aspects of its ambitious ICAL detector. The mini-ICAL employs a Closed Loop gas System (CLS) that will supply a gas mixture of R134a (95.2%), isobutane (4.5%), and \(SF_6\) (0.3%) and purify it after flowing through the different filters. A contamination of any other gas may change the characteristics of the detector, e.g., increasing the current, and increasing the background noise rates and consequently, a deterioration of the performance of RPC. The composition of the gas mixture can influence the RPC pulse shape. As the electron multiplicity reaches an extreme value, the avalanche mode has a chance to transform into a streamer mode. This can also happen due to leakage of atmospheric gas. A dedicated study was done by inserting different fractions of atmospheric air in the RPC system with and without removing the vapour. The percentage of impurities are monitored by the Gas Chromatograph (GC) and studied the RPC dark current, noise rates and also the strip multiplicity, and efficiency factor, as well as the fraction of streamer pulses in cosmic muon events. This paper will present the correlation of all these variables using RPCs in a mini-ICAL setup.