Th-U燃料循环中关键反应233U(n,2n)的截面测量

233U(n,2n) is a key reaction in the Th-U fuel cycle. Because of the difficulties of direct measurement in neutron reaction, there exist no experimental data up to now, and exist large differences in the theoretical data among the evaluated libraries. Surrogate reaction by charged particle is a valuable method to solve this difficulty. The peripheral reactions like inelastic, transfer reactions were traditionally employed as surrogate reactions. The angular momenta bring into reaction systems by the peripheral reactions are much larger than those bring by the neutrons in the direct reactions, resulting in the difficulty to make theoretical correction. We hereby propose a new way for surrogate reaction, i.e. utilize the capture reaction as surrogate reaction, to measure the neutron reaction cross sections of interested systems through the bridge of compound nuclei. From the reaction mechanism point of view, the angular momenta bring into systems by capture reactions are quite low, which avoids the difficulty to make spin corrections. Concretely speaking, using 230Th(a,2n) to replace 233U(n,2n), they form a same compound nucleus 234U. The spin differences are very small with the same excitation energy of compound nuclei. The a+230Th capture cross sections and the (a,2n) cross sections can be determined by the offline measures of the 232U alpha activity and the online measures of the fission cross sections and the elastic angular distributions. Comparing with the calculated n+233U capture cross sections, the 233U(n,2n) cross sections can be derived with error less than 15% and overall error less than 20%.

233U(n,2n)是Th-U燃料循环中一个关键反应。由于中子反应测量上的困难，至今没有实验数据，各评价库的理论数据存在巨大差异。带电粒子的替代反应是解决这个困难一个有价值的方法。传统上用非弹散射、转移反应等周边反应作替代反应。周边反应带入体系的角动量远大于直接反应中子带入的角动量，理论修正存在困难。我们提出替代反应的一条新途径，即用俘获反应作为替代反应，通过复合核做桥梁，测量感兴趣体系的中子反应截面。从反应机制上看，俘获反应带入体系的角动量很低，避免自旋修正的困难。具体而言，用230Th(a,2n)替代233U(n,2n)，生成同一复合核234U。激发能相同时，复合核自旋差别很小。离线测量232U的a衰变活度，在线测量裂变和弹散角分布，确定a+230Th的俘获截面和(a,2n)的截面。与n+233U俘获截面做比较，得到233U(n,2n)的截面，峰值区域误差<15%，总体误差<20%。

在最近几十年核能快速发展，不断对关键核的反应截面数据提出要求。中子诱导核反应截面测量困难，特别是(n,2n)反应截面的测量是极端困难的。利用带电粒子的替代反应法可有效解决这一困难。我们提出了替代反应的一条新途径，即用俘获反应作为替代反应，通过复合核做桥梁，测量感兴趣体系的中子反应截面。该方法已在235U(n,2n)反应截面的替代反应测量中得到验证。本项目中，在实验上我们改为采用236U靶作为实验靶，在线测量了共16个能点的α弹性散射角分布和熔合裂变碎片角分布，通过对裂变碎片角分布拟合分析，获得了236U(α,f)反应激发函数。另外选取了18块236U同位素靶进行17.9~36.0 MeV不同能量的α束流辐照，并通过238Pu的α放射性的离线测量，获得了236U(α,2n)反应激发函数。以复合核240Pu为桥梁，我们利用替代俘获反应法导出了239Pu核的(n,f)和(n,2n)反应截面。在En=10~16 MeV的2n蒸发反应目标能区，利用替代反应法得到的239Pu(n,2n)截面与评价核数据库ENDFB8.0的(n, 2n)数据在误差范围内符合较好。这证明了替代反应方法是可行的，可以应用于锕系核的(n, 2n)等中子反应截面测量。