NaI闪烁体的快中子辐照损伤

Fast neutron radiation damage to NaI scintillator detector

中子瞬发γ活化分析(Prompt Gamma Neutron Activation Analysis,PGNAA)是工业物料成分检测的最主要方法之一,该方法通过大体积NaI闪烁体探测器测量物料被中子活化后的瞬发γ能谱来分析物料成分。测量过程中,NaI晶体处于较强的快中子场中,该快中子场对晶体的辐照损伤是影响NaI探测器性能的最主要因素,而NaI探测器的稳定性直接决定了PGNAA整体设备的性能。本文基于中国工程物理研究院CFBR-Ⅱ(China Fast Burst Reactor Ⅱ)型反应堆,精确地测量了NaI闪烁体在裂变中子场的10~8 cm^(-2)、10~9 cm^(-2)、10^(10) cm^(-2)、10^(11)cm^(-2)、10^(12) cm^(-2)、10^(13) cm^(-2)、10^(14)cm^(-2)共7个中子注量照射后,NaI闪烁体组装的闪烁体探测器的性能变化。测量结果表明,NaI探测器除有较强的中子活化效应外,能量分辨率未发生显著变化,这为PGNAA设备中探测器的中子屏蔽设计和寿命估计提供了重要参数。

Background： Prompt gamma neutron activation analysis （PGNAA） is an most important method to monitor the chemical compositions for bulk industrial materials. NaI detectors with large volume are often used for energy spectrum measurements of the prompt gamma rays in these cases. In the measurement, the NaI detectors are under a fast neutron field with large flux which is the primary factor of radiation damage to Nal crystal. Purpose： This study aims to estimate the lifetime of the NaI crystal in the PGNAA quantificationally, and investigate the energy resolution evolution for the NaI crystal caused by the fast neutrons damage. Methods： Based on the pulsed reactor CFBR-II （China Fast Burst Reactor II） in Chinese Academy of Engineering Physics （CAEP）, two pieces of NaI crystal were irradiated with the fast neutron flux of 108 cm^-2, 109 cm^-2, 10^10 cm^-2, 10^11 cm^-2, 10^12 cm^-2, 10^13 cm^-2, 10^14 cm^-2, respectively. Meanwhile, the energy resolution was measured tinder each condition. Results： The energy resolution of NaI does not show obvious changes with the fast neutron irradiation flux upto 10^14 cm^-2. Conclusion： The energy resolution performance of NaI crystal does not deteriorate in fast neutron irradiation, except the strong neutron activation gamma rays. Therefore, to evaluate the lifetime of NaI crystal in a fast neutron field, a reliable lower limit can be figured out.