RADIOISOTOPES

We investigated the characteristics of the uranium sample RIU2, which has been preserved at Yamagata University, by inductively coupled plasma mass spectrometry (ICP-MS), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM–EDS), powder X-ray diffraction (XRD), and γ-ray spectrometry with a high-purity germanium (HPGe) detector. The sample is considered to have been associated with a uranium enrichment experiment conducted by Professor Kunihiko Kigoshi in 1945. The ICP-MS analysis showed a ²³⁵U isotopic abundance of 0.72％ in the RIU2 sample. Gamma-ray spectrometry also supported the isotopic composition evaluated by ICP-MS, because the energy dependence of the relative intensities of the photopeaks originating from ²³⁴Th, ^234mPa, and ²³⁵U was consistent with that of natural uranium. The EDS measurement showed that the RIU2 sample primarily consisted of U, F, and O, with a non-negligible amount of Cu. The analysis of the XRD pattern indicated that the RIU2 sample was a multiphase material rich in uranium fluoride, primarily consisting of UF₄. However, several peaks remained unassigned, precluding its identification as a single pure phase. These results indicate that the RIU2 sample contains uranium with a natural isotopic composition and is likely a multiphase mixture primarily composed of uranium fluorides.