Investigation of Neutron Cross Section for Iron in the ENDF Library with Pulsed Sphere Measurements

This work investigates the quality of the ENDF 56Fe cross-section libraries for describing the transport of fast neutrons in iron. We have used the D(d,n)3He reaction with a pulsed 7-MeV deuteron beam energy as a neutron source and analyzed the neutrons transmitted through two natural iron spheres of thicknesses 3 and 8 cm. The experimental neutron time-of-flight transmitted spectra for various angles are compared with MCNP simulations. Our result indicates the possibility of an underestimation of the nonelastic cross section and an overestimation of the elastic cross section for 56Fe in the ENDF/B-VII.1 library for the neutron energy range of 7.2 to 10.2 MeV. Our result agrees qualitatively with the Ramsauer model and optical model calculations. This discrepancy in the library cross section might lead to an underestimation/overestimation of material damage in nuclear reactor calculations. A newer evaluation, ENDF/B-VIII.0, was released subsequent to the completion of the majority of this project. The new evaluation has a decreased elastic cross section and an increased inelastic cross section for 56Fe in our energy range of interest, which agrees qualitatively with our result.本研究针对ENDF评价核数据库（Evaluated Nuclear Data File, ENDF）中56Fe的截面库在描述快中子在铁中输运过程的准确性展开了评估。我们以能量为7 MeV的脉冲氘束引发D(d,n)3He反应作为中子源，对两块厚度分别为3 cm和8 cm的天然铁球透射的中子进行了实验分析。将不同角度下测得的实验中子飞行时间透射谱与MCNP模拟结果进行比对后发现：在7.2至10.2 MeV的中子能区内，ENDF/B-VII.1数据库中56Fe的非弹性截面存在被低估的可能，而弹性截面则存在被高估的情况。该结论与拉姆绍尔模型（Ramsauer model）及光学模型的计算结果定性相符。数据库截面的这类偏差可能会在核反应堆计算中导致材料损伤的低估或高估。在本项目大部分工作完成后，更新的评价数据库ENDF/B-VIII.0正式发布；在我们关注的能区内，该新版数据库对56Fe的弹性截面进行了下调，非弹性截面则进行了上调，这与本研究的结果定性一致。