Study on Fast-Neutron Capture Cross Sections of Long-Lived Nuclear Wastes

本项目的目的是在10 ~ 600 keV的入射中子能量范围内，对重要的长寿命核废料Tc-99和I-129的中子捕获截面和捕获伽马能谱进行准确、同步的测量，并通过理论分析本课课组获得的19种稳定裂变产物和长寿命小锕系元素Np-237的测量数据，探讨快中子捕获反应机理。在本项目期间取得了以下成果：(1) Tc-99和I-129的测量采用高灵敏度伽马射线探测器，采用中子飞行时间法，利用来自东京工业大学核反应堆研究实验室Pelletron加速器的1.5 ns脉冲质子束产生^7Li(p,n)^7Be反应的脉冲keV中子，测量了Tc-99或I-129中子捕获反应的提示伽马射线。结果表明，Tc-99和I-129在入射中子能量10 ~ 600 keV范围内的俘获截面误差分别为5%和10%左右。在捕获伽马能谱方面，Tcb-99和I-129的捕获伽马能谱在国际上是首次获得。(2)实验结果的理论分析利用基于核反应理论统计模型的计算机程序，同时分析了Tc-99和I-129与其他核素的捕获截面和捕获伽马能谱。结果发现，统计模型计算可以再现Tc-99和l-129的捕获截面和捕获伽马能谱，但伽马能谱中的高能部分和相应的低能部分除外。因此，发现Tc-99和I-129的快中子捕获反应的主要机理可以用统计模型来解释。

The objectives of this project are to accurately and simultaneously measure the neutron capture cross sections and capture gamma-ray spectra of the very important long-lived nuclear wastes of Tc-99 and I-129 in the incident neutron energy region from 10 to 600 keV and to investigate the fast-neutron capture reaction mechanism by theoretically analyzing these measured data and those of 19 stable fission products and the long-lived minor actinide of Np-237 which were also obtained by our group. The following results were obtained in the term of this project.(1)Measurements for Tc-99 and I-129Prompt gamma rays from the neutron capture reaction of Tc-99 or I-129 were measured with a high-sensitive gamma-ray detector, adopting a neutron time-of-flight method with pulsed keV neutrons form the ^7Li(p,n)^7Be reaction by a 1.5-ns pulsed proton beam from the Pelletron accelerator of the Research Laboratory for Nuclear Reactors at the Tokyo Institute of Technology. As a result, the capture cross sections of Tc-99 and I-129 were obtained with the errors of about 5% and about 10%, respectively, in the incident neutron energy region from 10 to 600 keV. As for capture gamma-ray spectra, those of Tcb-99 and I-129 were obtained for the first time in the world.(2)Theoretical analysis of experimental resultsThe measured capture cross sections and capture gamma-ray spectra of Tc-99 and I-129 together with those of the other nuclides were simultaneously analyzed, using a computer program based on the statistical model in the nuclear reaction theory. As a result, it was found that the statistical model calculation could reproduce both capture cross sections and capture gamma-ray spectra of Tc-99 and l-129 except for the high-energy part and the corresponding low-energy part in the gamma-ray spectra. Therefore, it was found that the main mechanism of the fast-neutron capture reaction of Tc-99 and I-129 could be explained with the statistical model.