15 MeV电子直线加速器驱动的光中子源装置，将用于中国科学院战略性先导科技专项“钍基熔盐堆”中的核数据初步测量工作、中子探测器的研制和反应堆相关材料的辐照研究等。光中子源的中子能谱是连续的，中子能量通过中子飞行时间法测量得到，需要利用吸收片确认中子吸收峰，刻度飞行时间，计算等效飞行距离，扣除实验本底等，而实验本底的扣除对最终总截面计算有很大的影响。因此通过Geant4蒙特卡罗模拟软件构建了包括中子源、吸收片在内的模拟实验环境；研究了不同吸收片的吸收谱和吸收片厚度的关系，同理论计算值进行了比较，给出了推荐的吸收片厚度值；模拟计算了中子飞行时间谱，并和实验测量结果比较，确定中子等效飞行距离为5.70 m。Geant4的理论计算也可以模拟出多吸收片本底函数曲线，可用于实验数据的本底扣除和误差分析。实验测量、模拟分析以及理论公式计算的吸收片厚度和中子飞行时间参数得到了完全一致的结果，验证了实验测量的可靠性。
A photo-neutron source driven by a 15 MeV electron LINAC is built for the “Strategic Priority Research Program” of the Chinese Academy of Sciences - the “thorium-based molten salt reactor” pro ject to conduct the nuclear data measurement work, develop neutron detector and carry out reactor material irradiation studies. Since the neutron energy spectrum is continuous, the neutron energy is measured by the time of flight (TOF) method, and neutron filters are needed to confirm absorption peaks, calibrate the TOF, calculate the equivalent flight distance, and remove the experimental background which has great influence on the calculation accuracy of the total cross section. Based on the Monte Carlo simulation tool, Geant4 a simulation environment is set up, including neutron source and neutron filters, to study the energy absorption spectra and thickness of different filters and recommended data for the thickness are provided. The neutron TOF spectra are simulated and compared with experimental measurement, deciding the equivalent TOF distance to be 5.7 m. Geant4 can also simulate the background curve of multiple filters and be used to remove background and analyze errors for the experimental data. All the experiments, simulation and theoretical calculation show consistent results on filter thickness and neutron TOF parameters, indicating the accuracy of the measurement.
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