放射性氙叠层闪烁探测器的设计与初步实验

摘要

Several isotopes of xenon are sufficiently produced in fission and a few of them have suitable half‐lives and radiation to be detected .These are 131Xem ,133Xe , 133Xem and 135Xe and they have been selected for continuous monitoring in CTBTO inter‐national monitoring system .Employing the phoswich technology with β‐γ coincidence counting capability accompanied by digital signal processing of PM T pulse ,the current coincidence detectors were simplified . The radiation transport simulations were performed using MCNP toolkit to determine the optimum design and estimate the detection efficiency of the β and γ rays in phoswich detector .The results show that almost all of the beta particles deposite in plastic scintillator BC 404 as its thickness is set to 1.5 mm . The self‐absorption of radioxenon sample was also observed . The absorption of 346 keV β ray increases by 45.6% as the pressure from 0.1 M Pa to 0.25 MPa .The data acquisition and analysis system was developed as the phoswich detector for radioxenon detection was constructed recently . The preliminary experi‐ments demonstrate that the phoswich detector is efficient in reducing the background and the inhibiting factor is about 96.4 ± 1.0 .%放射性氙的4种同位素131 Xem 、133 Xe、133 Xem 和135 Xe是地下核试验最重要的示踪气体，是目前CTBTO全球监测系统监测的主要核素。本文设计并研制了一种针对低水平放射性氙测量的叠层闪烁探测器，可实现放射性氙4种同位素的β‐γ符合测量，简化了探测器结构。利用MCNP5工具包模拟了两种结构的探测器，获得了β闪烁层的最佳厚度，1.5 mm厚塑料闪烁体BC404几乎完全屏蔽4种氙同位素的β信号，135 Xe的910 keV特征β射线仅0.8％沉积在γ闪烁层CsI（Tl）中，而对133 Xe的81 keV特征γ射线吸收低于6.5％；观察到明显的氙气样品自吸收效应，氙气气压由0.1 M Pa增加到0.25 M Pa时，氙气对133 Xe的346 keV β射线吸收增加45.6％。基于模拟结构设计，研制了用于放射性氙测量的叠层闪烁探测器，初步实验结果表明叠层闪烁探测器能有效抑制本底计数，抑制因子约96.4±1.0。