中文摘要：

利用编制的快中子照相数值模拟程序（FNRSC）模拟计算了入射中子能量为14MeV时,厚度5—300mm闪烁体内次级中子对快中子图像质量的影响,结果表明闪烁体厚度d〈50mm时,次级中子对图像的影响强烈依赖于闪烁体厚度,而当d〉50mm时,次级中子对图像的影响趋于饱和.将文献中利用蒙特卡罗中子-光子输运程序（MCNP）计算的次级中子对图像影响和文中计算结果进行了对比,给出了二者存在差异的主要原因：次级中子分布对入射中子空间分布的强烈依赖性;能量沉积和荧光输出这两种计算方法对快中子图像的贡献形式差异.计算了入射中子能量不同时,次级中子对图像对比度的影响变化,结果表明入射中子与碳核反应截面有较大值时,图像对比度降低.

英文摘要：

The effects of secondary neutrons on the image quality is simulated using FNRSC （fast neutron radiography simulation code） for a series of scintillator thickness values ranging from 5 mm to 300 mm. The calculated results show that with an incident neutron energy of 14 MeV, the effects of secondary neutrons on the image quality strongly depends on the thickness d of the scintillator for d smaller than 50 mm. When d is higher than 50 mm, this effect is almost saturated. By comparing the image contrast calculated by MCNP （Monte Carlo neutron and photo transport code） with that by FNRSC, we find that the main reasons for the discrepancy is that, the distribution of secondary neutrons strongly depends on that of incident neutrons in the scintillator; contributions of secondany neutrons resulting from energy deposit and fluorescence photons to fast neutron images are different. The results also indicate that a drop occurs in the contrast of neutron images when the elastic scattering cross section between incident neutrons and carbon nucleus is larger.