钍是重要的核能材料。天然钍只有232Th一种同位素；232Th在中子作用下既可发生裂变释放核能，又可通过钍-铀循环转换成易裂变核素233U。精度足够高的快中子诱发232Th裂变反应截面，是设计建造先进核能系统的基础。然而，迄今该反应截面实验数据不足、误差大，且不同实验方法结果间存在系统差异。本项目拟基于北京大学4.5MV静电加速器以及原子能院HI-13串列加速器，在1.5-20MeV能区对快中子诱发232Th裂变反应截面进行系统的实验研究。用双屏栅电离室作裂变碎片探测器，采用相对测量法，以235U和238U裂变截面为标准；用飞行时间法测量源中子能谱，用蒙卡方法模拟散射中子能谱，进而将低能中子对测量结果的干扰进行扣除。结合理论分析，得到系统而准确的裂变反应截面。本项目的研究，对正确认识裂变反应规律、完善核反应模型理论、提高我国核物理基础研究(尤其是核反应精确测量)水平有重要作用。

Thorium is an important nuclear material. 232Th is the only isotope of thorium in nature. When irradiated by the neutron, 232Th can release tremendous nuclear energy through nuclear fission reaction, and also can be transformed into fissile nuclide 233U through the thorium-uranium fuel cycle. High accurate fission cross sections of 232Th induced by fast neutrons are the basis for the design and construction of the advanced nuclear energy systems. However, measurement cross sections of the 232Th(n,f) reaction are not enough in both quantity and quality up to now, and there are systematic deviation between the experimental data measured using different methods. The present project is plan to systematically measure the cross sections of the 232Th(n,f) reaction in the 1.5-20 MeV neutron energy region, based on the 4.5 MeV van de graaff accelerator of PKU and the HI-13 tandem accelerator of CIAE. Using relative measurement method, A twin-gridded ionization chamber will be used as the detector of the fission fragments, and the cross sections of the 235U and 238U (n,f) reactions will be used as the standard cross section. To correct the interference of the low energy neutrons to the results of the experiment, the energy spectrum of the source neutron will be measured using the time-of-flight method, and that of the scattered neutrons will be simulated to obtain the proportion and energies of the low energy neutrons. Combining the experiment data and the theoretical analysis results, the accurate fission cross section excitation function of the 232Th(n,f) reaction will be obtained. The present research is very important for the understanding of the fission reactions, for the development of the nuclear reaction theory, and for the improvement of the basic research in nuclear physics of China (especially for the accurate measurement of the nuclear reactions).