放射性惰性气体实时测量装置的设计与模拟研究

Design and simulation study of an online activity measuring detector for radioactive noble gases

核反应堆在运行过程中或核应急情况下会产生^(85)Kr、^(133)Xe、^(135)Xe和^(41)Ar等放射性惰性气体,准确测量不同惰性气体的放射性活度对了解反应堆的运行状况和核应急预警均有重要意义。根据各种核素衰变发射的β、γ射线,设计并优化了可用于放射性惰性气体活度实时测量的4π双叠层闪烁体探测器。探测器的内层塑料闪烁体用于测量β射线,外层碘化铯闪烁体(CsI)用于测量γ射线,并通过β-γ的符合测量实现不同放射性核素的分辨及活度测量。针对核电放出的4种主要放射性惰性气体,基于GEANT4(GEometry ANd Tracking 4)模拟库包,研究了塑料闪烁体、CsI厚度及气体采样腔尺寸对不同核素发射的β、γ探测效率的影响;并给出该4π型双叠层闪烁体探测器的优化几何尺寸和相应探测器性能,为后续探测器的制作与测试提供参考。

[Background] In normal operation or emergency, nuclear reactor can produce radioactive noble gases, the main components are 85Kr, 133Xe, 135Xe and 41Ar. It is of great significance to monitor the radioactivity of these mixed noble gases accurately for the operation of nuclear power plant.[Purpose] This study aims to develop a 4π phoswich scintillator detector to measure the radioactivity of noble gases in online model.[Methods] The 4π phoswich scintillator detector consists of a sampling chamber, an inner plastic scintillator chamber for β measurement, and a CsI scintillator chamber for γ measurement. The pulse shaping discrimination (PSD) method is developed to distinguish the events between β,γ and β-γ coincidence events. Based on GEANT4 simulation toolkit, the performance of the detector, including the detection efficiency is investigated.[Results] The simulation results show that both 0.3 mm plastic scintillator layer and a 20 mm CsI layer are necessary for measuring the radioactivity of noble gases. The size of sampling chamber should not be larger than Ф50 mm×50 mm. Under these conditions, the detection efficiency for the above nuclides can be up to 90%.[Conclusions] According to the simulation results, a composed 2-layer scintillation detector is designed, and its detection and recognition capability for different inert gases is studied. The comparison of these characteristics with actual calibration is the crucial step for this device.