【摘要】 ⁴¹Ar是反应堆和加速器运行释放到环境中的主要放射性核素之一，测量其活度浓度对于确保公众健康具有重要意义。由于β-γ符合法能显著降低本底，提高探测灵敏度，为此设计了由BC404和CsI（Tl）组成的β-γ符合闪烁体探测器，用于进行⁴¹Ar高灵敏度测量，并建立了以最小可探测活度浓度（MinimumDetectable Concentration,MDC）为优化目标的探测器结构优化方法。首先，运用Geant4对β射线穿过BC404和铝膜沉积到CsI（Tl）中的穿透率进行模拟计算，选择BC404厚度为3 mm，此时穿透率为0.78%；其次，对不同的气体腔尺寸进行模拟，分析计算β射线探测效率、γ射线全能峰效率，以及气体腔体积和取样时间对MDC的综合影响，完成探测器的结构优化；最后，分析了本底计数率和测量时间对MDC的影响。当测量时间为200 min、冷却处理时间为30 min、本底计数率为5×10^-3 s^-1时，优化后的探测器对⁴¹Ar的MDC估算值为1.7 Bq·m^-3。更多还原

【Abstract】 [Background] ⁴¹Ar is one of the main radionuclides released into the environment by the operation of reactors and accelerators. The measurement of ⁴¹Ar activity concentration is of great significance for ensuring public health. In the field of radioactive gas measurement, the β-γ coincidence method is widely used because it can significantly reduce the background and improve the sensitivity of the detector. However, there is currently little research on β-γ coincidence detectors for ⁴¹Ar measurements in literature. [Purpose] This study aims to optimize the β-γ coincidence detector composed of plastic scintillator（PS） and CsI（Tl） scintillator to realize high sensitivity measurement of ⁴¹Ar. [Methods] An optimization method of detector structure based on minimum detectable concentration（MDC） was proposed. The optimization process of the detector was realized based on Geant4simulation. Firstly, the penetration percentage of β-rays depositing energy into CsI（Tl） through different thicknesses of BC404 and 0.5 mm aluminum film was simulated. Secondly, the variation of full-energy peak efficiency of 1 293.6 keV γ-ray in CsI（Tl） scintillator with different thicknesses of CsI（Tl） was simulated. Thirdly, simulating with different gas chamber volume, the comprehensive effects of β-rays detection efficiency, γ-ray full-energy peak efficiency, gas chamber volume and sampling time on MDC were analyzed with the assumption that the sampling time of argon was proportional to the volume of the detector’s gas chamber. Finally, the influence of measurement time on MDC under different background counting rates was analyzed. [Results] Simulation results show that the penetration percentage of β-rays emitted from the decay of ⁴¹Ar is about 0.78% when the thickness of BC404 is 3 mm. The γ-ray full-energy peak efficiency increases with the increase of CsI（Tl） scintillator thickness. With the argon sampling rate of 600 mL · h-1, the optimal detector size parameters that minimize MDC are completely determined. When the background count rate is 1×10-3～1 s-1, the recommended measurement time for ⁴¹Ar is about 200 min. The MDC of the optimized β-γ coincidence detector for ⁴¹Ar measurement is about 1.7 Bq·m-3 under the condition of measurement time of 200 min, background count rate of 5×10-3 s-1 and sample cooling time of 30 min.[Conclusions] The detector structure optimization method used for β-γ coincidence detectors in this study can be applied to the structural design of other radioactive gas detectors, and the impact of detector structure on background counting rate needs to be considered in further enhancement study.更多还原