Measurement of ³⁷Ar Activity by Low Background Internal Gas Proportional Counter

³⁷Ar is one of the important radionuclides for on-site inspection compliance with Comprehensive Nuclear-Test-Ban Treaty. High sensitive detection of ³⁷Ar is a key step for underground nuclear test monitoring. A low background internal gas proportional counter system was established in Beijing Radionuclide Laboratory. The background of proportional counter caused by cosmic-ray was actively shielded by plastic scintillator detectors which were equipped on the top and the bottom of the lead shielding. The background count rate of ³⁷Ar peak region in anti-cosmic spectrum was reduced to 7.3% of which in the original spectrum. Within 24 h measurement, the minimum detectable activity of ³⁷Ar of 2.5 mBq was achieved.