采用飞行时间技术测量了氚氚（DT）源中子穿过不同厚度板状聚乙烯样品后40。方向的泄漏中子时间到达谱，样品的长和宽均为100cm，厚度分别为4．5，9，18和27CM。本底谱测量采用了无样本底测量和无样堵孔本底测量2种方案，利用MCNP4C程序模拟了相同实验条件下的泄漏中子时间到达谱，模拟过程中考虑了源中子的能谱与角分布、脉冲中子束宽度、探测器的效率以及样品的有效面积。通过比较发现，采用无样测量谱作为本底时，计算值／实验值（C／E）值大于1，并且随着样品厚度的增加而偏离1；而本底谱采用无样堵孔测量谱时，C／E小于1。并且随着样品厚度的增加而接近1。通过对两套本底谱的分析，并结合蒙特卡罗模拟，计算求得了相应样品厚度下比较接近实际的本底谱，采用该模拟计算本底谱后，C／E值有了明显的改善。
The neutron leakage spectrum were measured at 40~ by time-of-flight method for polyethylene slabs with the thicknesses of 4.5, 9, 18 and 27 cm, respectively. The experimental results were compared with the MCNP-4C simulations, which carefully considered many effects, such as the angle and energy dis- tributions of the source neutrons, the width of the beam pulse, the detection efficiency and the effective measured area. The sample out and collimator-filled spectra were measured as the background, and the re- sults showed that C/E values were larger than 1 when using the sample out background spectra, but smal- ler than 1 when using the collimator-filled background spectra. Combine these two different background spectra, a new method was used to calculate the background spectrum, and the results have a better agree- ment with the simulations.
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