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

本项目的目标是在10 - 600 keV的入射中子能量范围内，同时精确测量非常重要的长寿命核废料Tc-99和I-129的中子俘获截面和俘获γ射线谱，并研究快中子俘获截面和俘获γ射线谱。从理论上分析了中子俘获反应机理，并结合19种稳定裂变产物和长寿命次锕系元素的实验数据，讨论了中子俘获反应机理。第237章也是我的在本项目期间取得了以下成果。（1）Tc-99和I-129瞬发γ射线的测量用高灵敏度γ射线探测器测量了Tc-99或I-129中子俘获反应产生的瞬发γ射线，采用中子飞行时间法，脉冲keV中子来自^7Li（p，n）^7Be反应，1.5-纳秒脉冲质子束从Pelletron加速器的研究实验室的核反应堆在东京工业大学。在10 ~ 600 keV的中子能量范围内，计算了Tc-99和I-129的俘获截面，误差分别为5%和10%左右。Tcb-99和I-129的俘获γ能谱是世界上首次获得。（2）对实验结果的理论分析利用核反应理论中的统计模型，对Tc-99和I-129的俘获截面和俘获γ能谱进行了计算分析。结果表明，除了γ能谱中的高能部分和相应的低能部分外，统计模型计算可以再现Tc-99和l-129的俘获截面和俘获γ能谱。因此，用统计模型可以解释Tc-99和I-129快中子俘获反应的主要机理。

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.