EXFOR-based simultaneous evaluation for fast neutron-induced fission cross section of thorium-232

The ²³²Th neutron-induced fission cross section was evaluated from 500 keV to 200 MeV. The experimental ²³²Th fission cross sections and their ratios to the ^235,238U fission cross sections in the EXFOR library were reviewed and analysed by using the least-squares method. The newly published ²³²Th/²³⁵U fission cross-section ratios from the time-of-flight measurements at the CERN n_TOF and CSNS Back-n facilities were compiled in EXFOR. Additional simultaneous evaluation was performed by including the experimental ^233,238U and ^239,240,241Pu fission cross sections and their ratios. The new evaluation provides the ²³²Th fission cross section systematically lower than the JENDL-5 cross section. The reduction is 4% in the plateau region between 2 and 6 MeV and more significant in the subthreshold fission region. The present evaluation reduces the ²³²Th fission cross section averaged over the ²⁵²Cf spontaneous fission neutron spectrum from the JENDL-5 evaluation by 4%, which is closer to the other general purpose libraries but underestimates Grundl et al.’s measurement by 11%.本研究对500 keV至200 MeV能区的钍-232（²³²Th）中子诱发裂变截面开展了评价。本研究采用最小二乘法，对EXFOR数据库中的钍-232（²³²Th）裂变截面实验数据及其与铀-235（²³⁵U）、铀-238（²³⁸U）裂变截面的比值进行了综述与分析。新近发表的、基于欧洲核子研究中心（CERN）n_TOF装置与中国散裂中子源（CSNS）Back-n谱仪的飞行时间测量得到的²³²Th/²³⁵U裂变截面比值数据，已被收录至EXFOR数据库。此外，本研究还纳入了铀-233（²³³U）、铀-238（²³⁸U）以及钚-239（²³⁹Pu）、钚-240（²⁴⁰Pu）、钚-241（²⁴¹Pu）的裂变截面实验数据及其比值，开展了同步评价。本次评价得到的钍-232裂变截面系统低于JENDL-5库的对应结果，在2~6 MeV的坪区降幅约为4%，在亚阈裂变能区的降幅更为显著。基于锎-252（²⁵²Cf）自发裂变中子能谱的平均结果显示，本次评价将钍-232裂变截面相较于JENDL-5库的结果降低了4%，该结果与其他通用评价核数据库的结果更为接近，但较格伦德尔（Grundl）等人的测量结果低11%。