自然界で生じる核反応がもたらす元素同位体変動に基づく宇宙・地球化学的研究

抄録

Nuclear reactions are one of major causes of isotopic variations of several elements in natural materials. The Oklo uranium ore in the Francevillian Basin at Republic of Gabon is known as a fossil of natural fission reactor, because large-scaled fission chain reactions spontaneously occurred in the ore two billion years ago. It is of major concern to characterize the properties of the nuclear reactors and to investigate the behavior of fission products in and around the reactors. In this paper, firstly, geological and physicochemical characteristics of the Oklo natural fission reactors and their implications for radioactive waste disposal are shown. In the second, isotopic variations of several elements observed in meteorites and lunar surface materials are reviewed from my previous studies. Spallation reactions and neutron capture reactions induced by the interaction of cosmic rays produce the isotopic variations of some elements in the surficial materials of solar planets without atmospheric layers. Neutron capture-produced isotopic shifts of ¹⁴⁹Sm-¹⁵⁰Sm and ¹⁵⁷Gd-¹⁵⁸Gd have been usefully applied to understand the cosmic-ray exposure histories of meteorites and lunar surface materials. Additionally, several elements of solar primitive materials like carbonaceous chondrites show isotopic variations affected from the input of nucleosynthetic components out of the early solar system. As one of examples, Ba isotopic variations found in chemical separates of the Murchison meteorite are shown.