Shunsuke Kurosawa

In this study, we examined the corrosion resistance of Ru ₅₇ Mo _14.25 W _23.75 Fe ₅ , Ru ₅₄ Mo _13.5 W _22.5 Fe ₁₀ , and Ru ₄₈ Mo ₁₂ W ₂₀ Fe ₂₀ alloys in inorganic acids. The alloys were fabricated using the dewetting micro-pulling-down and plasma-melt stamping methods. Immersion tests in H ₂ SO ₄ at 300 °C for 24 h indicated that Ru-Mo-W-Fe alloys exhibit better corrosion resistance (10 mm y ⁻¹ ). Immersion tests at 25 °C in 5% HCl for 72 h and at 100 °C in 50% HF for 24 h also indicated that Ru-Mo-W-Fe alloys are highly corrosion resistant. Electrochemical tests in 1 M HCl and 1 M H ₂ SO ₄ revealed an increase in the anodic current and the formation of a low charge transfer resistance passive layer corresponding to alloying Fe. XPS analysis revealed that the primary corrosion product formed during polarization of Ru-Mo-W-Fe alloys in 1 M H ₂ SO ₄ was WO ₃ . Notably, WO ₃ is kept passivated on pure W during the corrosion in H ₂ SO ₄ ; however, it is not kept passivated when present on Ru-Mo-W-Fe alloys.

In this study, we developed and characterized novel scintillators with Ce: LiCaAlF₆–CaF₂–Li₃AlF₆ and Ce: CaF₂–LiF–Li₃AlF₆ ternary systems for thermal neutron detectors. The eutectics were grown by the vertical Stochbarger-Bridgman (VB) technique, and their constituent phases were identified using powder X-ray diffraction and scanning electron microscopy. Radioluminescence spectra irradiated under an Ag-target X-ray tube and confirmed the 5d-4f and self-trapped exciton luminescence derived from Ce³⁺. Scintillation decay and pulse height measurements were performed using ²⁵²Cf and ⁶⁰Co sources. The Ce: CaF₂–LiF–Li₃AlF₆ sample exhibited approximately 5.6 times higher effective neutron sensitivity compared with a Ce: LiCaAlF₆ single crystal. A favorable decrease in the neutron discrimination threshold level (Q_th) due to reduced γ-ray emission was observed. ⁶Li-enriched Ce: CaF-based scintillators hold potential for nuclear decommissioning applications.

Ce-doped Lu₂Si₂O₇ has a high density, high luminescence efficiency even at high temperatures, and a high effective atomic number, making it a promising candidate for use as a radiation detector in medical devices and resource exploration equipment. In this study, we grow and characterize Pr³⁺ and Ce³⁺-doped Lu₂Si₂O₇ single crystals by systematically varying the Ce³⁺ to Pr³⁺ ratio to further improve scintillation properties. The optical characterization results show a bidirectional energy transfer: from the Pr³⁺ 5d levels to the Ce³⁺ 5d levels and from the Ce³⁺ 5d levels to the Pr³⁺ 4f levels. Consistently with this result, the PL decay time of emission from the Pr³⁺ 5d–4f transition tends to become faster as the Ce³⁺/Pr³⁺ ratio increases, due to the energy transfer from the Pr³⁺ 5d levels to the Ce³⁺ 5d levels. Additionally, (Ce_0.0022 Pr_0.0016 Lu_0.9962)₂Si₂O₇ exhibits a high light yield comparable to Ce-doped Lu₂Si₂O₇ and a slightly faster decay time than Ce-doped Lu₂Si₂O₇.

Corundum‐structured Sn‐doped indium oxide (rh‐ITO) is investigated as a novel transparent conductive oxide. Herein, its gamma‐ray tolerance up to a total dose of 77 kGy is examined for potential applications in harsh environments, such as space. The investigations are conducted on rh‐ITO with Sn concentrations of 0 and 5 at%. X‐ray diffraction 2θ‐ω scan analysis reveals that no phase separation occurs due to gamma‐ray irradiation. The carrier concentration in undoped rh‐In ₂ O ₃ increases after irradiation, indicating that the gamma rays displace the oxygen atoms and form oxygen defects that generate donors. The high visible light transparency of rh‐In ₂ O ₃ and rh‐ITO is maintained after irradiation. This study demonstrates the high stability of rh‐ITO to gamma‐ray irradiation until 77 kGy dose. This work contributes to the application of rh‐ITO as an electrode in high‐radiation environments.

Aluminum garnets display exceptional adaptability in incorporating mismatching elements, thereby facilitating the synthesis of novel materials with tailored properties. This study explored Ce³⁺-doped Tb₃Al_5−xSc_xO₁₂ crystals (where x ranges from 0.5 to 3.0), revealing a novel approach to control luminescence and photoconversion through atomic size mismatch engineering. Raman spectroscopy confirmed the coexistence of garnet and perovskite phases, with Sc substitution significantly influencing the garnet lattice and induced A_1g mode softening up to Sc concentration x = 2.0. The Sc atoms controlled sub-eutectic inclusion formation, creating efficient light scattering centers and unveiling a compositional threshold for octahedral site saturation. This modulation enabled the control of energy transfer dynamics between Ce³⁺ and Tb³⁺ ions, enhancing luminescence and mitigating quenching. The Sc admixing process regulated luminous efficacy (LE), color rendering index (CRI), and correlated color temperature (CCT), with adjustments in CRI from 68 to 84 and CCT from 3545 K to 12,958 K. The Ce³⁺-doped Tb₃Al_5−xSc_xO₁₂ crystal (where x = 2.0) achieved the highest LE of 114.6 lm/W and emitted light at a CCT of 4942 K, similar to daylight white. This approach enables the design and development of functional materials with tailored optical properties applicable to lighting technology, persistent phosphors, scintillators, and storage phosphors.

病態に基づく早期診断は，早期の治療開始のみならず，同時に，臓器機能損失を最小限にとどめることも可能とする。核医学は機能の可視化により早期診断を可能とする手法の一つであり，中でも，single photon emission computed tomography（SPECT）による画像診断が国内では広く普及し，日本の医療を支えてきた。しかし，近年の画像診断のニーズにSPECT装置の性能は追いついていない。本稿では，これを解決する方法としてSPECTの一部をpositron emission tomographyに置き換えることの展望や課題について議論する。

Real-time dose-rate monitoring system under the high dose-rate situation is required for the decommissioning of Fukushima Daiichi Nuclear Plants to remove the debris remaining inside the plants. We have proposed a dose-rate monitor consisting of a scintillator, over 100-m long optical fiber and CCD spectrometer. Some noises with a dominant emission wavelength of below 550 nm originating from the optical fiber itself must be separated from the signal from the scintillator, and the signal region has been defined as 650–1000 nm to use significant data, up to now. In this paper, we investigated if the noise region defined as below 550 nm can be supporting data for the monitoring system, because such noises are expected to include position information through the fiber. In the emission spectrum, the intensities of the noise emission or absorption band are expected to depend on the position of the fiber, and we demonstrated such dependence, “one-dimensional dose-rate distribution”, with 20 m-long optical fiber and CCD spectrometer. Optical fiber was placed around a ⁶⁰Co source with an activity of approximately 60 TBq, and emission spectra of the fiber were acquired with different dose-rate conditions from 10 to 700 Gy/h. The noise intensities were well fitted with the power function as a function of the product of fiber length and dose rate, and we found that fiber noise has rate information in one-dimensional imaging.

The ³He gas is commonly used for the detection of thermal neutrons. However, with the depletion of ³He gas, there is a need to develop new solid scintillators for thermal neutron detection. Solid scintillators containing ⁶Li, which have large neutron capture cross-sections and a large amount of energy released by transmutation reactions, are commonly used as alternative candidates. However, only single-crystal scintillators are currently used, and their ⁶Li concentration is limited by their chemical composition. In this study, we designed, grew, and evaluated a new eutectic scintillator, Rb₂CeCl₅/LiCl, which can improve the ⁶Li concentration compared with single-crystal scintillators. Rb₂CeCl₅, which was selected as the scintillator phase, has excellent scintillator properties (light yield: 36,000 photons/MeV, decay time: mostly 24 ns, slightly 153 ns), and is less deliquescent than other halide scintillators. The crystal grown using the vertical Bridgman method exhibited a eutectic phase composed of Rb₂CeCl₅ and LiCl. The eutectic crystals exhibited Ce³⁺ 5d-4f emissions, with a peak between 360 and 370 nm. The Rb₂CeCl₅ phase was identified as the luminescent phase via cathodoluminescence mapping, and 16,000 photons/neutron of the light yield and 56.1 ns of the decay time were observed. This study indicates that the Rb₂CeCl₅/LiCl eutectic scintillator is a promising candidate for use in thermal neutron detectors.

Internal exposure for decommissioning workers at the Fukushima Daiichi Nuclear Power Plant must be prevented, and we have developed a monitoring system for alpha-ray emitting dust (alpha dust). When the dust size is less than several tens of micrometers, a dust protection mask does not work effectively to prevent internal exposure. Since no devices have been operated to observe alpha-dust images in real-time up to now, we have developed an alpha-ray imaging detector consisting of a scintillation material and imaging detector. Scintillators are required to have high light output and chemical stability. Cs₃Cu₂I₅ (CCI) scintillator was found to be one of the candidates, and an imaging test was operated with this material. The CCI crystal was grown by the Bridgman-Stockberger method, and a scintillation sheet for alpha-ray imaging was prepared by CCI microcrystals with a thickness of approximately 70 μm on a 200-μm thickness transparent polyethylene terephthalate film as a first imaging test. The sheet was irradiated with 5.5-MeV alpha rays from an ²⁴¹Am source, and scintillation photons were detected with a CMOS camera through an optical lens. We succeeded in visualizing alpha rays, and a position resolution of our system achieved approximately 16.2 ± 2.6 μm (10–90 %). This resolution was similar value to other imaging devices with Ag:ZnS powder scintillator, and a CCI single crystal sheet was expected to reach better resolution.

In this study, Ce:6LiCl/LaCl3 with a high 6Li concentration was developed as a novel thermal neutron scintillator, in which Ce3+ serves as an activator for the LaCl3 phase. The Ce:6LiCl/LaCl3 eutectic was grown in a quartz ampoule (inner diameter: 4.0 mm) using the Bridgman–Stockbarger technique. The grown eutectic contained 0.038 mol/cm3 6Li, which is greater than that found in commercially available scintillators such as LiCaAlF6 single crystals and Li glass. The Ce:6LiCl/LaCl3 eutectic has a lamellar eutectic structure and is optically transparent. A 350 nm emission due to Ce3+ 4f-5d transition was observed. The light yield under neutron irradiation was estimated to be 5.0 times higher than that of Li-glass. In this eutectic, the 6Li (n, a) 3H reaction made the 6LiCl phase to act as the neutron capture phase and LaCl3 to act as the scintillator phase.