Yasuo Iwabuchi

Mechanism of photostimulated luminescence process in BaFBr:Eu2+ phosphors

The formation of photostimulated luminescence (PSL) centers in BaFBr:Eu2+ single-crystal phosphors is studied in the 5–9 eV vacuum ultraviolet region. The centers are efficiently formed by the Eu2+ photoionization absorption at 6.6 eV. The PSL excitation spectrum after the photoionization excitation is almost the same as that after X-ray excitation. Photoconductivity is generated in the PSL process after the photoionization excitation. These results present further confirmation of the PSL mechanism of Takahashi et al.. [J. Lumin. 31 & 32 (1984) 266]. The numerical analysis of the dependence of the PSL intensity on X-ray excitation intensity, assuming bimolecular kinetics in accord with this mechanism, gives the linear dependence over a very wide range of X-ray intensity under the practical conditions in the PSL applications. Thus the mechanism of Takahashi et al.. is definitely established.

Interface effects on the properties of confined excitons in CuCl microcrystals

Abstract Optical properties of confined excitons were studied in nanometer-size CuCl microcrystals prepared by three different methods; by annealing in crystalline NaCl and in glass matrices, and by gas evaporation. Interface effects concerning the shape of microcrystals, the effect of finite barrier height and the photo-generated exciton trapping centers were discussed.

Quantum confinement of excitons and their relaxation processes in CuCl microcrystals

Abstract Semiconductor microcrystals of CuCl with different mean sizes of an order of nm are grown inside transparent matrices of NaCl. Since the effective Bohr radius of excitons in CuCl is about 0.7 nm, the excitons in such microcrystals keep their bulk-like character even under the strong influence of confinement effect and the size-quantization occurs mainly in their translational motion. The blue shift and the splitting of the exciton levels are found with a decrease of the crystal size. Although there exists some size distribution of the microcrystals, exciton luminescence under the size-selective excitation with picosecond laser light and the analysis including strong reabsorption effects makes possible the determination of the quantum efficiency of exciton radiative decay. The size-dependent radiative decay time of excitons is found to be inversely proportional to the volume of the microcrystals as long as the energy separation between the size-quantized sublevels is larger than the thermal energy. This fact is quantitatively interpreted as the result of the change in the coherence volume of excitons whose motion is restricted by the microcrystal size.

Photostimulated luminescence process in BaFBr:Eu2+ containing F(Br−) and F(F−) centers

Abstract The excitation spectra of photostimulated luminescence (PSL) and photoconductivity (PSPC) in a BaFBr:Eu 2+ single crystal are in good agreement with each other at 300 and 90K. The temperature dependences of the intensities of PSL and PSPC are measured precisely. It is found that the photostimulation process has a thermal activation energy of about 37 meV for F(Br − ) centers and about 1.3 meV for F(F − ) centers. It is concluded that in the PSL process, electrons trapped in F centers are first raised to the excited level of F centers by visible light and then thermally released to the conduction band, and that these electrons return to Eu 3+ ions and the Eu 2+ luminescence is stimulated.

Comparison in Image Quality and Noise Component of Columnar Phosphor Plate and Powder Phosphor Plate

For Computed Radiography (CR) systems that use a columnar phosphor plate (CPP) and a powder phosphor plate (PPP), we designed the systems to obtain the best image quality. To determine the optimum phosphor layer thickness for each phosphor plate, the relationship between the intensity and spatial spread of photo-stimulated luminescence (PSL), and the phosphor layer thickness of the phosphor plate is quantitatively clarified. Next, to determine the stimulation light intensity, we measured PSL, modulation transfer function (MTF) and detective quantum efficiency (DQE) by varying the stimulation light intensity, using the determined optimum phosphor layer thickness. We also investigated the noise components of each phosphor plate. Results show that, compared to the PPP, the CPP is more favorable in allowing thicker phosphor layer without reduction in MTF. As the result of the relationship between the layer thickness and the PSL, noise analysis, it was confirmed that the CPP could detect PSL in the deep region of the phosphor layer without reducing the intensity of PSL. This suggests that in comparison to the PPP, the CPP can make efficient use of X-ray information, thereby promising to enhance image quality and to reduce exposure dose.

Quantum size effect on excitons in microcrystals of cuprous halides embedded in alkali halide matrices

Abstract Exciton luminescence spectra of CuCl, CuBr and CuI microcrystals embedded in alkali halide matrices are studied experimentally and are explained in terms of the size-quantized excitons with the idea of exciton confinement. From the analysis of the splittings of the exciton states into several components, exciton masses and k-linear terms are derived in these compounds.