240Pu自发裂变中子角关联研究

Quantified characterization of plutonium materials has become an important work in the nuclear non-proliferation monitoring. The 240Pu spontaneous fission neutron angular correlation distributions can be used for simplified determining the 240Pueff mass of plutonium materials. However, the relevant theoretical calculations need to be verified, also there are two problems in the existing measurement results based on the liquid scintillation detectors: (1) the correlated neutron energy range have not been covered below 650 keV yet; (2) the data above the threshold is only the coincidence counts distribution of different angles, which not applicable to other detection systems due to including the contribution of the detector response. Therefore, this subject intends to use the CLYC detectors and the EJ299-33A plastic scintillation detectors with pulse shape discrimination capability to measure the neutron angular correlation of plutonium samples with low multiplication factor in the wide energy range for the first time. By means of Monte Carlo simulations, the accidental coincidences from cross-talking between detectors will be corrected. The intrinsic probability distributions of neutron angular correlation in 240Pu spontaneous fission will be obtained by the normalization of detection efficiency and solid angle at different thresholds, and then compared with the calculation results of the FREYA program. The research results could provide supports for the improving the fission theoretical model, and can also provide technical reference for the non-destructive monitoring of plutonium.

钚材料的量化表征已成为防核扩散监测的一项重要工作。240Pu自发裂变中子角关联分布可用于钚材料等效240Pu质量的简化求解，目前相关的理论计算有待验证和完善，而现有基于液闪探测器的测量结果存在以下问题：(1) 尚未覆盖到650keV以下的中子能区；(2) 阈值以上的数据只是不同互成角度的符合计数分布，其包含了探测器响应的贡献，不适用于其他探测系统。为此，本课题拟首次采用具有脉冲形状甄别能力的CLYC探测器和EJ299-33A塑闪探测器，开展低增殖钚样品出射中子的宽能区角关联测量，并通过蒙卡模拟程序对中子在探测器间发生交叉散射引起的偶然符合事件进行修正，经效率和立体角归一后获得不同能量阈值的240Pu自发裂变本征中子角关联概率分布，进而与裂变理论程序FREYA的计算结果进行比对检验。研究结果可为健全准确的裂变模型提供支撑，也可为发展钚的无损监测技术提供参考。

240Pu易发生自发裂变，出射的中子具有多重性。本课题对240Pu的自发裂变中子从出射到探测的物理过程进行了详细的研究。理论方面主要基于FREYA程序迭代计算了关联中子的空间概率分布；实验方面建立了自发裂变的中子角关联测量平台，发展了效率约化的角关联不对称度测量技术，获取了0.5MeV以上不同能量阈值的240Pu自发裂变中子角关联曲线。可有助于深入理解自发裂变中子出射的物理过程，也可对相关的蒙卡MC输运模拟程序进行检验。另外，本工作在不同探测阈值获得的中子的角度、时间关联特征可用于军控核查及防核扩散监测领域，可对密闭容器内的240Pu和其他替代材料如252Cf进行有效的甄别和表征。