Positronium is a bound state of an electron and its anti-matter counterpart, the positron, and plays a crucial role in a wide range of fields, including fundamental physics, materials science, and medical applications. Although positronium, composed of a particle and its antiparticle, annihilates within a short lifetime, it behaves as an electrically neutral particle until annihilation and exhibits diverse interactions with matter. To investigate these interactions in detail, an energy-tunable positronium beam is highly effective. We have been developing a novel method for producing positronium beams by utilizing the photodetachment of positronium negative ions. In this article, we describe the principle of this production method, techniques for improving beam quality, and the momentum control approach using laser polarization.

Many cells have been reported to exhibit a unique cellular response to low doses of radiation (hyper-radiosensitivity: HRS) that cannot be expressed using conventional mathematical models. To describe the cellular response to low-dose radiation, we proposed a repair response model (r′ model) that assumes that repair responses change over time and with dose, in contrast to the repair response model (r model) that assumes that repair of DNA damage progresses as a first-order response. The model was verified by measuring the survival rate and analyzing gene expression levels of budding yeast cells irradiated with soft X-rays under culture conditions. The results suggested that the r′ model can reproduce cell responses that occur in the low-dose range, which cannot be expressed by the r model.

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.