This research topics is based on the studies published in “Scientific Reports“on August 21, 2015.

Assist Prof. Masai, H.

(Division of Materials Chemistry, Inorganic Photonics Materials)

Assist Prof. Masai, H. (Division of Materials Chemistry, Inorganic Photonics Materials), Prof. Yanagida, T. (NAIST), Prof. Mizoguchi, T. (The University of Tokyo), Dr. Ina, T. (JASRI), Dr. Miyazaki, T. (Tohoku Univ.), Dr. Kawaguchi, N. and Dr. Fukuda, K. (Tokuyama Corporation) examined local coordination state of rare earth in fluoride eutectics for neutron detector applications. 

Detector of ionizing radiation, including medical imaging, particle physics, security, astrophysics, and searching for natural resources, is one of the practical applications of phosphor containing emission center. Since ³He gas suffers a serious shortage by a large demands or security applications, the effort toward the fabrication of novel thermal neutron scintillators containing ⁶Li or ¹⁰B to replace the present ³He-based systems has been heightened. Recently, lithium-containing fluorides have attracted attention across the world as a  high conversion efficiency attained by low phonon energy. Among these Li-containing fluorides, the activator–doped LiF/CaF₂eutectic prepared by a simple solidification method is reported as a candidate for neutron scintillator applications. Although the actual distribution of the activator in solid state matter is a critical factor in attaining a high performance, it is not easy to observe atomic distribution of activator cations in fluorides because of low chemical durability of fluoride against electron beam observation.
By observation of LiF-CaF₂ structure, the research team demonstrates that lamella structure at micron scale range (Figure 1 (a)) was changed depending on the Eu concentration, and the resulting transparency decreased with increasing Eu concentration. It is notable that the LiF-CaF₂ eutectic containing low Eu concentration exhibits higher detection performance compared with commercially available Li-glass. On the other hand, X-ray absorption fine structure (XAFS) analysis at SPring-8 reveals that valence state of Eu is affected by the doping concentration and that emission intensity depends on the Eu²⁺/Eu³⁺ ratio. Figure 1(b) shows spherical aberration corrected Scanning Transmission Electron Microscope (STEM) images of a LiF-CaF₂ eutectic obtained by optimization of observation condition. From results of the EDS (Figure 1(c) and (d)), the high-angle annular dark-field (HAADF) image (Figure (e)), and the cation distribution of Eu and Ca (Figure (f)), we find that Eu cation replaces Ca sites in the CaF₂ layer in the high-doping sample, because Eu cation exists homogeneously in CaF₂ layer not in LiF layer. The result indicates that the origin of concentration quenching is due to the shortening of interatomic distance of Eu, which substitutes Ca site in the CaF₂ layer.
Emission of Eu in the CaF₂ layer is generated by activation of the secondary particles from neutrons interaction with a ⁶LiF layer. In the study, we have examined the emission of Eu cation homogeneously located in Ca site in the CaF₂ by energy transfer from the LiF layer. This result demonstrates that high light yield can be attained by separation of absorber of neutrons and the emitting regions. Since neutron-induced luminescence spectra show the maximum light yield from the low-doping sample, it is expected that the ordered lamella structure and the resulting transparency is important factor for detector applications.

The present work is expected to accelerate the study on fabrication of solid state neutron detector, and leads to the development of the future high sensitivity detector.

Figure 1. Schematic image of Eu-doped LiF-CaF2 eutectics and the STEM images. Schematic image of the eutectic structure (a), STEM image (b) and the EDS mappings of Eu (c), and Ca (d). In Figure 1 (b), 1(c), and 1(d), (Ca,Eu)F₂ layer exists between (Ca,Eu)F₂ layers. The elemental mappings of each cation show that Eu cations are homogenously dispersed in CaF₂ region without remarkable aggregation at the interface. (e) HAADF image of (Ca,Eu)F₂ layer, and (f) comparison of the distribution of Eu and Ca along the dashed line in Figure 1 (c) and (d). Eu cations exist at Ca site in CaF₂.

Thermal Neutron Scintillator: For detection of thermal neutrons, ⁶Li or ¹⁰B possessing high interaction probability is needed in solid state matter. Because of lack of ³He gas, alternative thermal neutron scintillators have been studied all over the world.

X-ray Absorption Fine Structure (XAFS) Measurement: One of the spectroscopic analysis using high-energy synchrotron radiation beam of a large synchrotron radiation facility, such as SPring-8. By using this method, information about atomic distance or valence state can be obtained.

High-angle Annular Dark-field (HAADF) Image: One of the microscopy techniques for observation of constituent atom and atomic arrangement, with a resolution of 0.1 nm scale at the present equipment.

This work was partially supported by the Collaborative Research Program of ICR., Kyoto University (grant #2014-31, #2015-40), and the SPIRITS program, Kyoto University.

Masai, H.; Yanagida, T.; Mizoguchi, T.; Ina, T.; Miyazaki, T.; Kawaguchi, N.; Fukuda, K., Local Coordination State of Rare Earth in Eutectic Scintillators for Neutron Detector Applications,Scientific Reports, 5, 13332 (2015).