A closed-circuit gas system for RPC detectors

The Neuland detector for R3B at FAIR will detect high-energy neutrons and is planned to be based on timing resistive plate chambers (RPC). Current RPC detectors for timing purposes often use a common gas mixture, composed of 85% Reclin-134a, 10% sulfur hexafluoride, and 5% isobutane, which allows the operation of the detector under optimal conditions. Each gas has a series of advantages and disadvantages, which are briefly listed. Reclin-134a and sulfur hexafluoride are potent greenhouse gases with high to very high global warming potentials, respectively. The long-term release of these gases to the atmosphere must therefore be avoided. Due to the considerable gas volume involved in the Neuland detector, a closed-circuit gas recirculation system is proposed for this purpose, consisting of a main circuit with a gas scrubber for the removal of eventual impurities in the gas, a condensation circuit for the recovery of the gas mixture, an injection circuit for the supply of fresh gas, and an on-line gas-analysis system based on a quadrupole mass spectrometer. This closed-circuit system is presented, and its various subsystems are described in detail.