Akihiro Yamaji

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.

The effect of the difference in the thickness ratio of the double-layered thickness-shear resonator on the temperature characteristics of the resonance frequency was investigated using a Ca ₃ TaGa ₃ Si ₂ O ₁₄ (CTGS) single crystal. Three specimens with thickness ratios of ${x} =0.25$ , 0.33, and 0.50 were prepared using $122^{\circ } ~{Y}$ - and $171^{\circ }~{Y}$ -cut CTGS substrates. For the specimens with ${x} =0.25$ and 0.33, the temperature characteristics varied depending on the order of the resonance mode. For the specimen with ${x} =0.50$ , on the other hand, almost the same temperature characteristics were observed regardless of the order of the resonance mode. To interpret this phenomenon, a new equation for predicting the temperature characteristics of the fundamental mode (first mode) for the double-layered resonator was created using the electric flux density ratio generated in the two substrates. The expected values using this equation were in good agreement with the result of the first mode temperature characteristics.

A novel single-crystal growth method was developed, using arc plasma and metal melt, for a quick survey of high melting point materials. Single crystals of Yb-doped Lu₂O₃, Lu_0.388Hf_0.612O_1.806, and Lu_0.18Hf_0.82O_1.91, with melting points of 2460, 2900, and 2840 °C, respectively, were grown by an indirect heating method using arc plasma. We refer to this indirect heating growth method as the core heating (CH) method. The CH-grown Yb1%-doped Lu₂O₃ sample showed a full width at half maximum of 286 arcsec in the X-ray rocking curve. This value is better than the 393 arcsec obtained for the crystal grown by the micro-pulling-down (μ-PD) method. The Yb charge transfer state (CTS) emission was observed at 350 nm in the Yb1%-doped Lu₂O₃ and Lu_0.18Hf_0.82O_1.91. In the case of the μ-PD method, using a rhenium (Re) crucible, absorption due to Re contamination and a resulting reduction in the Yb CTS emission were confirmed. However, contamination did not influence the properties observed in the crystals grown by the CH method.

Ce-doped LaBr₃/AEBr₂ (AE = Mg, Ca, Sr, Ba) eutectics were grown using the Bridgman–Stockbarger (BS) method in quartz ampoules. The eutectics (AE = Mg and Ca) showed optical transparency like optical fiber bundles. A grown Ce-doped LaBr₃/MgBr₂ eutectic shows a 355 nm emission ascribed to Ce³⁺ 4f-5d transition under X-ray excitation. The smaller the ionic size of AE, the higher the light yield of the sample was. The light yield of Ce:LaBr₃/MgBr₂ was 34,300 photon/MeV, which is higher than Ce:LYSO standard. Scintillation decay time under 662 keV gamma-ray excitation was 18.8 ns.

The scintillation characteristcs of Lu0.8Gd2.2(Al5- xGax)O12:Ce,Mg (x = 2.6 and 3) single crystals grown by the micro-pulling down method are investigated. The light yield of 27,600 photons/MeV obtained for Lu0.8Gd2.2Al2Ga3O12:Ce,Mg is higher than that of 25,400 photons/MeV obtained for Lu0.8Gd2.2Al2.4Ga2.6O12:Ce,Mg. The photoluminescence and scintillation decays measured at room temperayture (RT) for Lu0.8Gd2.2Al2Ga3O12:Ce,Mg are faster than those for Lu0.8Gd2.2Al2.4Ga2.6O12:Ce,Mg. Temperature dependence of radioluminescence (RL) intensity indicates a lower quenching temperature for the Lu0.8Gd2.2Al2Ga3O12:Ce,Mg. Coincidence time resolution of Lu0.8Gd2.2Al2Ga3O12:Ce,Mg detector in coincidence experiment with a BaF2 detector was measured to be 410 ps. The Afterglow and thermally stimulated luminescence (TSL) characteristics below RT are also measured.

Carbazole crystals were grown using self-seeding vertical Bridgman technique, and the scintillation properties for neutron detection were investigated. Results of powder XRD and FTIR analyses indicated the single phase of the grown carbazole crystal with orthorhombic structure. Photo- and radio- luminescence measurements were carried out, and maxima of both emission peaks were at 420 nm. The light yield under 5.5 MeV alpha-ray irradiation was estimated to be 1.87 times higher than that of lithium glass GS-20. The scintillation decay time under neutron irradiation was estimated to be 11.4 ± 0.3 ns. It follows from the temperature dependence measurements thatthe light output under 662 keV gamma-ray excitation decreases with increasing temperature.

This article presents the development of diced SrI2:Eu arrays and their scintillation properties when coupled with a multipixel photon counter (MPPC) array. Using a dicing technique specific to halide scintillators, a high-energy resolution scintillator array is developed that is cost-effective and widely applicable. The developed SrI ₂ :Eu arrays are of 3 x 3 x 3 mm3/pixel 4 x 4 matrix (TYPE1) and 1 x 1 x 1 mm ³ /pixel 9 x 9 matrix (TYPE2) structures. We fabricated test gamma-ray detector modules consisting of the TYPE1 and TYPE2 arrays coupled with an MPPC array. The mean peak to valleys ratio is 85.3 ± 14.9 for the TYPE1 array and 38.5 ± 16.6 for the TYPE2 array detector. The average energy resolutions obtained for the TYPE1 and TYPE2 gamma-ray detectors were 5.4 ± 0.4% full-width at half-maximum (FWHM) and 5.2 ± 0.6% FWHM, respectively, for 662-keV gamma-rays. The variation in the signal pulse height of the TYPE1 and TYPE2 detectors was only 10.6% and 10.4% in FWHM, respectively. These results demonstrated that the dicing technique proposed in this article could be applied to hygroscopic halide scintillators.

We measured the scintillation light yield of ZnWO4 crystal using a quasi-monochromatic neutron beam of 865 keV. Irradiating the neutron beam on two surfaces which are almost perpendicular to the crystal axis, the light yield of oxygen recoil was measured. The obtained light yields ratio of a neutronrecoil to gamma-ray induced electron recoil for two surfaces was measured to be 0.235 ± 0.026 and 0.199 ± 0.020, respectively, which corresponds to the 15.3% of anisotropy for ∼200 keV nuclear recoils of this crystal. This property can be applied to a direction sensitive dark matter detector.

The microstructure and mechanical properties of platinum (Pt) fibers fabricated by unidirectional solidification using the alloy-micro-pulling-down (A-μ-PD) method were investigated using a Universal Testing Machine and Electron Backscattered Diffraction (EBSD). The Pt fiber fabricated at a growth rate of 10 mm/min was composed of relatively large grains with crystal orientation along the growth direction. The crystal orientation was consistent with the easy axis of the crystal growth on the face-centered-cubic (f.c.c.) structure. On the other hand, the adjacent grains of the Pt fiber fabricated at 50 mm/min were randomly oriented owing to a faster growth rate. In tensile tests, the Pt fibers fabricated by the A-μ-PD method indicated extremely different stress–strain curves compared to the commercial Pt wire. The maximum tensile stress of the Pt fiber reached ~100 MPa, and the Pt fiber ruptured after 58% nominal strain.

!A new red-emitting halide scintillator Rb2HfI6 was developed using the vertical Bridgman furnace and quartz ampoule. Under X-ray irradiation, Rb2HfI6 had a broad emission band around 725 nm. The light output and energy resolution were determined to be ~40,000 photons/MeV and ~7.4% (full width at half maximum at 662 keV), respectively. Since red photons are transmitted with smaller losses than blue or yellow photons inside an optical fiber, long-fiber-reading radiation detection system using the high light-output red scintillator can be established for severe or hazardous environments. We fabricated Rb2HfI6 scintillation crystal and performed the gamma-ray detection test under high dose irradiation condition from a 60Co source with activity of ~100 TBq. The dynamic range of the Rb2HfI6 gamma-ray detection with a 20-m optical fiber was evaluated and compared with the previously-developed Cs2HfI6 crystal and conventional deep-red Cr:α-Al2O3 (ruby) scintillator. The gamma-ray doses from ~0.4 to ~1 kSv/h were successfully measured using Rb2HfI6 through the 20-m fiber.