放射性氙在塑料闪烁体表面的吸附机理与抑制方法研究

Radioxenon detection systems are used worldwide in the International Monitoring System (IMS), which is part of the verification regime of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). The beta-gamma coincidence detector, based on the plastic scintillator, has the advantages of high detection efficiency and multi-isotopes recognition in radioxenon monitoring. However, part of the xenon sample diffuses into the plastic scintillator material during the measurement. The result is residual activity left in the detector, even after the sample has been removed by pumping and flushing with helium. The residual activity, called memory effect, increase the radioactive background of the detector, especially if a strong sample is measured. Experimental data have shown that the residual radioxenon in the plastic scintillator is reduced while coated with a thin metal, SiO2 and Al2O3 film. We suggest that a dense and smooth film, which have better adsorption and desorption properties, can prevent the diffusion of xenon atom into the plastic scintillator. As a result, less radioxenon was adsorped by the surface and the residual xenon atoms are easier to flush out with carrier gases. It is necessary to study on the microscopic mechanism of adsorption and desorption of xenon atoms on the plastic and the coating surfaces. In addition, we can establish a correlation model between micro-adsorption energy and macro-adsorption amount. At the same time, the adsorption and desorption rules under different experimental conditions such as membrane material, film thickness and surface roughness were studied by the soaking of radioactive gas and neutron activation analysis method. Exploring the optimal method for the suppression of the memory effect of plastic scintillator can enhance the performance of the radioxenon detection system and strengthen the monitoring of clandestine nuclear explosion.

放射性氙探测系统是“全面禁止核试验条约”（CTBT）组织建立的全球监测系统中违约核爆监测的重要组成部分。基于塑料闪烁体的β-γ符合探测器具有探测效率高和多核素识别等优点，但放射性氙与塑料闪烁体接触后可吸附于探测材料表面，清洗后仍有部分放射性的残留，称之为“记忆效应”。这种“记忆效应”将导致探测器放射性本底升高，严重时数周无法恢复，成为限制放射性氙β-γ符合探测器使用的瓶颈问题。实验数据表明，在塑料闪烁体表面镀金属、SiO2和Al2O3等膜材料，可减少放射性氙的残留。项目组认为致密光滑的镀层可有效抑制放射性氙向塑料闪烁体内部的扩散，同时具有更佳的吸附与脱附性能。为此需开展表面吸附机理研究，建立微观吸附能与宏观吸附量的关联模型，同时研究膜材料、厚度和粗糙度等实验条件下的吸附与脱附规律。探索抑制塑料闪烁体“记忆效应”的最优方法，可进一步增强放射性氙探测系统的性能，加强对违约核爆监测能力。

为有效解决惰性气体核素在塑料闪烁体表面的“记忆效应”导致的应用瓶颈，进一步增强内充气放射性氙、氪探测系统性能。本项目开展了氙在塑料闪烁体和镀膜材料表面吸附机理研究、放射性氙吸附与脱附规律实验研究、镀膜塑料闪烁体探测性能研究等研究工作。研究结果表明，Kr/Xe原子在不同材料表面的内部扩散系数及扩散深度是影响其吸附性能的根本原因，通过在塑料闪烁体表面添加致密的透明镀层如Al2O3可大幅降低Kr/Xe的扩散深度，是一种优良的改性材料。在模拟研究的基础上，支持开展了新型镀膜塑料闪烁体探测性能实验研究，相关技术已成功用于新型放射性氙叠闪探测器的研制，其综合吸附性能降低至0.7%，可广泛用于军用级民用核设施的环境流出物监测。研制了具有优良吸附抑制性能的内充气Kr-85探测器，技术指标满足核电站监测使用条件。此外，本项目提出芪晶体、CeBr3等致密闪烁材料，由于其自身具有极佳的吸附抑制性能，特别适合用于放射性氪氙的直接测量，可在后续项目支持下开展进一步研究工作。