Hirokazu Masai

X-ray-induced Scintillation Governed by Energy Transfer Process in Glasses

The efficiency of X-ray-induced scintillation in glasses roughly depends on both the effective atomic number Zeff and the photoluminescence quantum efficiency Qeff of glass, which are useful tools for searching high-performance phosphors. Here, we demonstrate that the energy transfer from host to activators is also an important factor for attaining high scintillation efficiency in Ce-doped oxide glasses. The scintillation intensity of glasses with coexisting fractions of Ce3+ and Ce4+ species is found to be higher than that of a pure-Ce3+-containing glass with a lower Zeff value. Values of total attenuation of each sample indicate that there is a non-linear correlation between the scintillation intensity and the product of total attenuation and Qeff. The obtained results illustrate the difficulty in understanding the luminescence induced by ionizing radiation, including the energy absorption and subsequent energy transfer. Our findings may provide a new approach for synthesizing novel scintillators by tailoring the local structure.

Validity of Valence Estimation of Dopants in Glasses using XANES Analysis

X-ray absorption near edge structure (XANES) measurement is one of the most powerful tools for the evaluation of a cation valence state. XANES measurement is sometimes the only available technique for the evaluation of the valence state of a dopant cation, which often occurs in phosphor materials. The validity of the core excitation process should be examined as a basis for understanding the applicability of this technique. Here, we demonstrate the validity of valence estimation of tin in oxide glasses, using Sn K-edge and L-edge XANES spectra, and compare the results with 119Sn Mössbauer analysis. The results of Sn K-edge XANES spectra analysis reveal that this approach cannot evaluate the actual valence state. On the contrary, in LII-edge absorption whose transition is 2p1/2-d, the change of the white line corresponds to the change of the valence state of tin, which is calculated from the 119Sn Mössbauer spectra. Among several analytical approaches, valence evaluation using the peak area, such as the absorption edge energy E0 at the fractions of the edge step or E0 at the zero of the second derivative, is better. The observed findings suggest that the valence state of a heavy element in amorphous materials should be discussed using several different definitions with error bars, even though L-edge XANES analyses are used.

Luminescence of Sn 2+ center in oxide glass with a tendency toward phase separation

Photoluminescence (PL) of Sn2+ in oxide glasses is affected by the local coordination field because of the electrons in the outermost shell. Not only the emission energy varies depending on the local coordination, but also the transparency, and it is worth examining the local distribution of Sn2+ cations in oxide glasses, which have a tendency toward phase separation, especially in an ionic (phosphate)–covalent (borate) binary system. Here, we report the structural changes in zinc borophosphate glass and the PL properties of Sn2+ in oxide glasses. The building blocks of the main glass network vary depending on the chemical composition. We found that the Sn2+ species in B2O3-rich glasses differ from those in P2O5-rich glasses, as observed in the optical absorption, PL peaks, and PL decay constants. 119Sn Mossbauer spectra indicate that isomer shifts of Sn2+ also affect the local coordination change depending on the chemical composition. According to these results for the Sn2+ center, we conclude that Sn2+ centers are homogeneously dispersed in the borophosphate network without localization around the phosphate region.

Photoluminescence and radioluminescence in Pr-doped lanthanum aluminoborate glasses

Abstract Photoluminescence (PL) and radioluminescence properties of Pr-doped lanthanum aluminoborate glasses with the composition xPr-20La2O3-30Al2O3-50B2O3 (0 ≤ x ≤ 1.0) were studied. In the PL spectra, the characteristic emission bands due to both f–f and the d–f transitions of Pr3+ were confirmed for the glass system. In X-ray induced scintillation spectra, emission from the host matrix was observed in addition to emission bands from d–f and the f–f transitions of Pr3+; the peak area ratio of the host to the f–f transition depended on the Pr content. It is significant that emission from the host was confirmed in both X-ray-induced scintillation and thermally stimulated luminescence, and the emission intensity from the host decreased with an increase in the Pr content. It is suggested that doping with Pr decreases the number of trap sites in the host glass.