Outgassing and leak test of INO-ICAL RPC detectors

Abstract

Glass Resistive Plate Chambers (RPCs) of size 2 m × 2 m, operated in avalanche mode will be used as an active detector element at the INO-ICAL experiment. The RPC system of the ICAL experiment will operate with a gas mixture of C2H2F4 (94.5%), isobutane (5%) and SF6 (0.5%). To fulfil the physics goals, about 29,000 RPCs will be used which are expected to last for 20 years or more. The quality and purity of the gas plays a vital role in the stable operation of RPC detectors. The presence of impurities in a gas mixture contribute towards the degradation of detector performance. The various assembly materials like glues, button spacers, frames, etc. used in the construction of the chamber may outgas and contaminate the gas mixture. We have performed the very first study to estimate the outgassing due to various materials used in the construction of INO RPCs. The present study includes the results obtained from gas chromatography showing the generation of impurities and dangerous radicals produced due to outgassing when RPC are operated in the cosmic stand. The study also includes a test of purity and effectiveness of the gas mixing system. Since a large amount of gas mixture is to be circulated inside the RPC during the operation of the ICAL detector, a proper leak test will help to minimize gas leaks reducing operational costs and atmospheric air pollution. A proper quantitative leak test is also performed on RPC after assembly by monitoring the absolute pressure inside the chamber, along with the atmospheric pressure and temperature to estimate the leakage rate.