[论文解读] Evaluation of the in-situ Performance of Neutron Detectors based on EJ-426 Scintillator Screens for Spent Fuel Characterization

H. Perrey, Linus Ros+4 more|arXiv (Cornell University)|Jul 2, 2021

Radiation Detection and Scintillator Technologies参考文献 11被引用 4

一句话总结

本研究评估了在瑞典CLAB乏燃料储存设施附近高剂量混合场辐射环境中，基于EJ-426闪烁体与滨松光电倍增管（PMT）的中子探测器的原位性能。在为期八个月的监测中，探测器累积吸收剂量达35 kGy，由于PMT窗口变黄及增益损失，中子探测效率下降约30%，但通过提高高压可部分补偿该损失，证明了该系统在长期非破坏性乏燃料表征中的可行性。

ABSTRACT

The reliable detection of neutrons in a harsh gamma-ray environment is an important aspect of establishing non-destructive methods for the characterization of spent nuclear fuel. In this study, we present results from extended in-situ monitoring of detector systems consisting of commercially available components: EJ-426, a $^6$Li-enriched solid-state scintillator material sensitive to thermal neutrons, and two different types of Hamamatsu photomultiplier tubes (PMT). Over the period of eight months, these detectors were operated in close vicinity to spent nuclear fuel stored at the interim storage facility CLAB, Oskarshamn, Sweden. At the measurement position the detectors were continuously exposed to an estimated neutron flux of approx. 280 n/s $\cdot$ cm$^2$ and a gamma-ray dose rate of approx. 6 Sv/h. Using offline software algorithms, neutron pulses were identified in the data. Over the entire investigated dose range of up to 35 kGr, the detector systems were functioning and were delivering detectable neutron signals. Their performance as measured by the number of identified neutrons degrades down to about 30% of the initial value. Investigations of the irradiated components suggest that this degradation is a result of reduced optical transparency of the involved materials as well as a reduction of PMT gain due to the continuous high currents. Increasing the gain of the PMT through step-ups of the applied high voltage allowed to partially compensate for this loss in detection sensitivity. The integrated neutron fluence during the measurement was experimentally verified to be in the order of $5 \cdot 10^9$ n/cm$^2$. The results were interpreted with the help of MCNP6.2 simulations of the setup and the neutron flux.

研究动机与目标

• 评估商用EJ-426闪烁体中子探测器在真实乏燃料储存环境中的长期抗辐射能力。
• 评估在高γ射线剂量率（约6 Sv/h）和中子通量（约280 n/s cm²）条件下中子探测性能的退化情况。
• 区分混合场辐射中闪烁体与PMT的退化机制。
• 验证非3He中子探测器在未来的封装设施中用于长期原位乏燃料表征的可行性。

提出的方法

• 将EJ-426闪烁体探测屏与两种类型滨松PMT（熔融石英和硼硅酸盐玻璃）耦合，部署在CLAB乏燃料附近。
• 在高剂量混合场条件下（γ射线：约6 Sv/h，中子：约280 n/s cm²）连续进行原位监测，持续八个月。
• 利用离线软件算法基于脉冲波形和幅度识别并表征中子脉冲。
• 对组件进行辐照后分析，包括光学透明度测量和PMT增益测试。
• 在辐照前后使用Am/Be源进行控制测量，对比信号幅度分布。
• 通过MCNP6.2模拟验证总中子注量。

实验结果

研究问题

• RQ1 长期暴露于混合场辐射环境（γ射线与中子）对基于EJ-426闪烁体的探测器中子探测效率有何影响？
• RQ2 在高达35 kGy的高剂量γ射线辐照下，EJ-426闪烁体和PMT的主要退化机制是什么？
• RQ3 通过调节PMT所加高压，能否缓解中子探测器的性能退化？
• RQ4 辐照后闪烁体和PMT窗口材料的光学透明度降低程度如何？其对信号幅度有何影响？
• RQ5 该探测器系统能否在典型乏燃料封装设施中高本底、高剂量环境下长期保持可靠的中子探测能力？

主要发现

• EJ-426/QRZ探测器（熔融石英玻璃PMT）在吸收35 kGy剂量后，中子探测效率降低约30%，主要由于阳极电流过高导致PMT增益损失。
• EJ-426/BSG探测器（硼硅酸盐玻璃PMT）退化更为严重，信号幅度降低主要由于PMT窗口变黄及增益损失。
• 提高PMT所加高压可部分恢复探测灵敏度，尤其在EJ-426/BSG配置中效果显著。
• 辐照后EJ-426闪烁体因变色导致光学透明度下降，影响光收集效率，进而降低信号幅度。
• 通过实验验证，总中子注量约为5 × 10⁹ n cm⁻²，与MCNP6.2模拟结果一致。
• 系统实现了超过95%的运行时间，通过USB闪存盘实现远程操作，证明了其在自主长期部署中的鲁棒性。