Naoki Kawano

Aging Behavior of Ni-Electrode Multilayer Ceramic Capacitors with X7R Characteristics

Capacitance aging under dc field has been studied on BaTiO3-based dielectrics with X7R characteristics for multilayer ceramic capacitors with Ni internal electrodes. Capacitance change strongly depends on additives and firing conditions as well as microstructure. Aging behavior under no dc field is well explained by Richters relaxation curve. On the other hand, aging under dc field cannot be fitted by Richters curve. Therefore, a new model which assumes two factors that affect relaxation is proposed. According to the new model, capacitance changes in two stages, short-term and long-term relaxations. Capacitance change in short-term relaxation strongly depends on applied dc field. On the contrary, that of long-term relaxation does not depend on dc field.

Luminescent and scintillation properties of Ce-doped Tb3Al5O12 crystal grown from Al-rich composition

A Ce-doped Tb3Al5O12 single crystal was successfully synthesized via the floating zone method in an Al-rich composition. In both photoluminescence and scintillation, the emissions were predominantly due to the 5d–4f transitions of Ce3+ peaking around 550 nm, but emissions due to defects and the 4f–4f transitions of Tb3+ were revealed by the time-resolved analyses. The crystal quality and luminescence intensity were sufficiently high such that the crystal was applicable for pulse-height spectrum measurement. Under 137Cs γ-ray exposure, a clear photoabsorption peak was observed, and the estimated light yield was 57,000 photons/MeV.

Scintillating Organic–Inorganic Layered Perovskite-type Compounds and the Gamma-ray Detection Capabilities

We investigated scintillation properties of organic–inorganic layered perovskite-type compounds under gamma-ray and X-ray irradiation. A crystal of the hybrid compounds with phenethyl amine (17 × 23 × 4 mm) was successfully fabricated by the poor-solvent diffusion method. The bulk sample showed superior scintillation properties with notably high light yield (14,000 photons per MeV) under gamma-rays and very fast decay time (11 ns). The light yield was about 1.4 time higher than that of common inorganic material (GSO:Ce) confirmed under 137Cs and 57Co gamma-rays. In fact, the scintillation light yield was the highest among the organic–inorganic hybrid scintillators. Moreover, it is suggested that the light yield of the crystal was proportional with the gamma-ray energy across 122–662 keV. In addition, the scintillation from the crystal had a lifetime of 11 ns which was much faster than that of GSO:Ce (48 ns) under X-ray irradiation. These results suggest that organic–inorganic layered perovskite-type compounds are promising scintillator for gamma-ray detection.

Mixed-crystal effect on the scintillation properties of organic–inorganic layered perovskite-type compounds

We investigated mixed-crystal effects on the luminescence and scintillation properties of organic–inorganic layered perovskite-type compounds. The luminescence and scintillation decay accelerated as the iodine content increased. The radiative and non-radiative decay rates under both ultraviolet and X-ray radiation were calculated; quantum efficiency was used under ultraviolet radiation. The radiative decay rate was almost constant with increasing iodine content under both types of radiation; however, the non-radiative decay rate increased with increasing iodine content. These results indicate that the mixed-crystal effects on the luminescence and scintillation properties are governed by non-radiative processes. Besides, the scintillation decay time was longer than the luminescence decay time although the rise times for both profiles were similar, suggesting that the energy transfer from the organic to the inorganic layer had no effect on the scintillation properties. Moreover, these time profiles became similar with increasing iodine content in the inorganic layer. These results suggest that the difference between the two types of decay can be associated with the scintillation process in the inorganic layer.

Reflective Microoptical Element Array Fabricated by Photofabrication Technique

The authors present reflective microoptical element arrays fabricated by a photofabrication technique. The three-dimensional base of the microoptical element array is photofabricated by exposing photopolymerizable resin to a light beam through a grayscale mask. Reflective film on the photopolymerized base of the optical element is introduced to prevent serious problems in photofabricated refractive optical elements, such as when the optical characteristics of the elements fundamentally depends on the spectroscopic property of the material of the photofabricated optical element. The relationship between the pixel value in the grayscale bitmap image used for making the grayscale transparency mask and the cure depth of the photofabricated base is experimentally clarified for a photopolymerizable resin. 15×15 unit convex and concave micromirror arrays, in which each optical element has a diameter of 930 µm, have been batch fabricated. The focusing capability of the fabricated concave micromirror array is demonstrated.

Resonance effects of Wannier–Frenkel excitons in luminescence properties of layered perovskite compounds (C6H5CH2NH3)2PbClxBr4−x

To analyze the luminescence properties under resonance between inorganic–organic layers, (C6H5CH2NH3)2PbClxBr4−x thin films was fabricated. The Wannier exciton level in inorganic layer has been tuned to triplet excited levels in benzylamine in layered perovskite-type compounds by changing the halogen composition in the inorganic layer. When the Wannier exciton level was higher than the triplet state of benzylamine, energy transfer from the former to the latter occurred. The phosphorescence peak at 3.59 eV was sharpened and enhanced in the films with x = 3.75 and 3.80, and this effect can be ascribed to the enhancement of an electronic transition due to the resonance between the organic and inorganic layers.

Comparative Studies of Optical and Scintillation Properties between LiGaO 2 and LiAlO 2 Crystals

We have investigated optical and scintillation properties of LiGaO2 and LiAlO2 crystals. From the transmittance spectra, the absorption edges were found to be around 200 nm but wider band gap was confirmed for LiAlO2 than LiGaO2. Both photoluminescence and scintillation spectra showed a broad emission band peaking at 340 nm in the both crystals. The scintillation decay times of LiGaO2 were 460 ns and 2.8 µs while that of LiAlO2 was 2 µs. Under neutron irradiation from 252Cf, the absolute scintillation light yields of LiGaO2 and LiAlO2 were 3300 ± 300 and 7200 ± 700 ph/n, respectively.

Effect of organic moieties on the scintillation properties of organic–inorganic layered perovskite-type compounds

The effects of organic moieties on the scintillation properties of organic–inorganic layered perovskite-type compounds have been investigated. Three kinds of single crystals were fabricated, namely, (C4H9NH3)2PbBr4 (C4), (C6H5CH2NH3)2PbBr4 (Ben), and (C6H5C2H4NH3)2PbBr4 (Phe). Among the single crystals, the light output of Phe was found to have the greatest value when exposed to X-ray radiation (67.4 keV). The light output of Phe was 0.62 times that of YAP:Ce. The relative values of the light outputs among the fabricated single crystals under X-ray radiation correlated well with those of the quantum efficiencies and the luminescence intensity under ultraviolet radiation.

Luminescence properties of organic–inorganic layered perovskite-type compounds under vacuum ultraviolet irradiation

We investigated the luminescence properties of organic–inorganic layered perovskite-type compounds under vacuum ultraviolet irradiation. A crystal of (C6H5C2H4NH3)2PbBr4 was fabricated by the poor-solvent diffusion method. Exciton emissions from the inorganic layer were observed at 410 nm under ultraviolet irradiation (excitation wavelengths: 180 and 300 nm). The rise time behavior observed in the luminescence decay curve showed no difference among the excitation wavelengths of 60–300 nm. In addition, no excitation peak of benzene such as an intense peak at 180 nm (1A1g → 1E1u) in the vacuum ultraviolet region was observed in the excitation spectra measured while monitoring the exciton emissions from the inorganic layer. These results indicate that the effect of energy transfer from the organic layer to the inorganic layer has negligible contribution to the luminescence properties of organic–inorganic layered perovskite-type compounds.

Scintillation Characteristics of Pr:CaF2 Crystals for Charged-particle Detection

The scintillation properties of Pr:CaF2 were studied in comparison with those of Eu:CaF2 under α-ray excitation using a sealed source of 241Am. The relative light yields of the Pr:CaF2 crystals were approximately 0.6–13% of the Eu:CaF2 crystal that is known as a conventional scintillator for charged-particle detection. The scintillation decay times of the Pr:CaF2 crystals were significantly faster than that of the Eu:CaF2 crystal. Therefore, the Pr:CaF2 crystals can be used for charged-particle measurements with high count rates.