Influence of CO2 as quench gas on the characteristics of the proportional counter in MARDS

The Movable 37Ar Rapid Detection System (MARDS) was first developed by the Institute of Nuclear Physics and Chemistry (INPC) of the China Academy of Engineering Physics (CAEP) in 2006, and it has been continually upgrading and serving for On-site Inspections (OSI) under the Comprehensive Nuclear Test Ban Treaty (CTBT) to this day. To make MARDS more suitable for OSI, we have tried to use CO2 as the quench gas of the proportional counter. The feasibility was confirmed by using a 1 L proportional counter and energy spectrum analysis method. The characteristics influence of the proportional counter in MARDS has been studied with different CO2 concentration and different operating voltage. The results show that the 37Ar peak shape can be improved when the CO2 concentration is about 2% to 6%, and the resolution of 37Ar energy peak becomes worse when the CO2 concentration is above 6%. Meanwhile, the operating voltage almost linearly increases with the CO2 concentration, and it is proportional to the gas pressure when the CO2 concentration is constant.