Development of neutron in-beam Mossbauer spectroscopy

关于核转化的化学效应，即热原子化学（简称HAC），迄今已有许多研究报道。由于核反冲作用，热原子处于高度激发态。弄清楚核事件发生后固体中立即发生了什么是固相HAC的研究目标。然而，传统固相hac法测定的放射化学产物在固体溶解和化学分析过程中的分布受到严重影响。利用发射穆斯堡尔光谱学可以非破坏性地获得与穆斯堡尔跃迁核衰变相关的化学效应信息。如果测量通过核反应供给的源系统的发射穆斯堡尔光谱，即在束内穆斯堡尔光谱功中，则光谱中应该出现核反应和随后的迅速衰减到穆斯堡尔能级的化学效应。由于核反应的反冲能量，假定核反应对HAC效应的贡献要比随后的穆斯堡尔衰变大得多。在本项目中，我们对硫化铁等铁化合物进行了中子束内^<57>Fe穆斯堡尔光谱研究，使我们能够在100 ns的时间尺度上非破坏性地获得信息，这比传统的HAC技术要短得多。我们的研究重点和兴趣集中在^<56>Fe(n, γ)^<57>Fe反应的化学效应，如化学键断裂和铁氧化态的变化作为热原子效应。我们研究的另一个重要目的是在中子设施中建立束内穆斯堡尔谱，通过它我们给出了固体中中子诱导热原子化学行为的实验结果。利用平行板雪崩计数器（PPAC）在日本原子能研究所JRR-3M反应堆中子束厅的快速γ射线分析装置上进行了穆斯堡尔测量。本工作对PPACs进行了制备和改进。黄铁矿的束内发射穆斯堡尔光谱表现为两个重态。黄铁矿对应的一种成分占总峰面积的1/3，另一种主要成分暂定为黄铁矿间隙处的Fe^<3+>。马氏体的发射光谱也由两个重态组成：一个对应于马氏体，另一个对应于马氏体。在这两种二硫化铁中，中子俘获反应发生后，级联γ射线发射的反冲作用迅速产生了一种与原始目标化合物不同的新物质，这是通过束内^<57>Fe穆斯堡尔光谱得到的。少

A number of researches have been so far reported on the chemical effects of nuclear transformations, i.e., hot-atom chemistry abbreviated by HAC. Hot atoms are in highly excited states because of nuclear recoil. It is a research goal of solid-phase HAC to make clear as to what happens in solid immediately after the nuclear event. However, it has been expected that the distribution of radiochemical products measured in conventional solid-phaseHAC should be seriously affected in solid during dissolution and chemical analysis. By emission Mossbauer spectroscopy we could obtain information non-destructively on the chemical effects associated with a nuclear decay of Mossbauer transition. If the emission Mossbauer spectrum is measured for the source system fed through a nuclear reaction, that is, in an in-beam Mossbauer spectroscopic work, the chemical effects of the combination of the nuclear reaction and the subsequent prompt decays to Mossbauerr level should appear in the spectrum. The co … More ntribution to HAC effects is assumed to be much greater from the nuclear reaction than from the following Mossbauer decay because of their recoil energies.In the present project, we did the neutron in-beam ^<57>Fe Mossbauer spectroscopic investigations for iron compounds such as iron sulfide, allowing us to obtain non-destructively the information with the time scale of 100 ns, which much shorter than that in the conventional HAC techniques. Our research attention and interest were paid to the chemical effects associated with the ^<56>Fe(n, γ)^<57>Fe reaction, such as chemical-bond rupture and change of iron oxidation-state as the hot-atom effects. Another important aim of our study was to establish the in-beam Mossbauer spectroscopy in neutron facilities, by which we present the experimental results on the chemical behaviors of neutron-induced hot-atoms in solid. Mossbauer measurements were carried out using a parallel plate avalanche counter (PPAC) at the prompt γ-ray analysis setup at the neutron beam hall of the JRR-3M reactor of Japan Atomic Energy Research Institute. The PPACs were fabricated and improved in the present work. The in-beam emission Mossbauer spectrum of pyrite showed two doublets. One component corresponding to pyrite occupied 1/3 of the total peak area, and the other major component was tentatively assumed as Fe^<3+> at the interstitial site of pyrite. The emission spectrum of marcasite was also composed of two doublets : one corresponding to marcasite, and the other doublet. In both iron disulfides, it was revealed that recoil of the cascade γ-ray emission promptly after the neutron capture reaction produced one new species distinguishable from the original target compound by in-beam ^<57>Fe Mossbauer spectroscopy. Less