【公開日：2024.07.25】【最終更新日：2024.07.03】

課題データ / Project Data

課題番号 / Project Issue Number

23UT0110

利用課題名 / Title

光導波型シンチレータに関する研究

利用した実施機関 / Support Institute

東京大学 / Tokyo Univ.

機関外・機関内の利用 / External or Internal Use

外部利用/External Use

技術領域 / Technology Area

【横断技術領域 / Cross-Technology Area】（主 / Main）計測・分析/Advanced Characterization（副 / Sub）-

【重要技術領域 / Important Technology Area】（主 / Main）高度なデバイス機能の発現を可能とするマテリアル/Materials allowing high-level device functions to be performed（副 / Sub）-

キーワード / Keywords

センサ/ Sensor,電子顕微鏡/ Electronic microscope,光デバイス/ Optical Device

利用者と利用形態 / User and Support Type

利用者名（課題申請者）/ User Name (Project Applicant)

Kamada Kei

所属名 / Affiliation

東北大学 未来科学技術共同研究センター

共同利用者氏名 / Names of Collaborators in Other Institutes Than Hub and Spoke Institutes

佐々木　玲

ARIM実施機関支援担当者 / Names of Collaborators in The Hub and Spoke Institutes

利用形態 / Support Type

（主 / Main）機器利用/Equipment Utilization（副 / Sub）-

利用した主な設備 / Equipment Used in This Project

UT-102：高分解能走査型分析電子顕微鏡

報告書データ / Report

概要（目的・用途・実施内容）/ Abstract (Aim, Use Applications and Contents)

The demand for thermal neutron detectors is increasing for application in the medical sector, as well as to non-destructive inspection, structural analysis, astronomical observation, water imaging in plants, and resource exploration. In an environment using a neutron detector, radiation in the form of γ-rays exists in addition to neutrons, making it necessary to efficiently distinguish neutron rays. Therefore, a scintillator with high sensitivity to neutron rays and low sensitivity to γ-rays is required. Thermal neutron detectors are composed of gas, liquid, and solid scintillators containing ³He, ⁶Li, ¹⁰B, and ¹⁵⁷Gd, which offer a large neutron capture cross-section. Recently, solid scintillators for detecting thermal neutrons have been developed and applied to replace conventional ³He gas detectors, the resources for which are depleting. In recent years, scintillation detectors using inorganic solid scintillators containing ⁶Li have been increasingly employed owing to their ease of handling and radiation resistivity. In these detectors, the scintillators absorb the α-rays generated by the ⁶Li (n, α) ³H reaction and convert them into visible light. In the past decade, inorganic solid scintillators containing Li, such as Ce or Eu:LiCaAlF₆ (LiCAF) [1] and Ce:Cs₂LiYCl₆ (CLYC) [2], have been developed in addition to the traditional 20Al(PO₃)₃-80LiF (Li-glass) scintillator for thermal neutron detection. To improve the performance of a thermal neutron detector, it is necessary to develop novel scintillators containing ⁶Li at a higher concentration for high neutron sensitivity and a low density for low γ-ray sensitivity. In compound crystals, the Li content is limited by the chemical composition. In contrast, eutectic scintillators containing a high concentration of Li, such as LiF/CaF₂, LiF/SrF₂, LiF/LiGdF₄, LiSrI₃/LiI, LiF/BaCl₂, Li₃AlF₆/CaF₂, LiF/LaF₃, and LiF/CaF₂/LiBaF₃ [3,4] have been reported. Eutectics are composed of neutron-capturing phases containing Li and scintillator phases. As a result, a much higher ⁶Li content and higher neutron sensitivity can be achieved compared to single-crystal scintillators such as CLYC and LiCAF. In addition, scintillators must offer a high light yield and be transparent to the generated scintillation. A higher transparency can be achieved by combining crystal phases with closer refractive indices. At the point view of scintillation properties, high light yield scintillators such SrCl₂, BaCl₂, LaCl₃, CeCl₃, BaBr₂, LaBr₃, CeBr₃ with Eu²⁺ or Ce³⁺ doping were selected as the scintillator phase (Table 1). We searched the phase diagram of each scintillator material and Li-containing halides and systematically examined possible combinations that could constitute eutectics.

実験 / Experimental

In this research, eutectics with the combination of  LiBr/(NaI, CsI) and LiCl/(LaCl₃,Rb₂CeCl₅)  were fabricated. Each powder material at the corresponding eutectic compositions were set in the quartz amples after baking process at 100°C under high vacuum (~10^-4 Pa) to eliminate water and oxygen, and the quartz ampoules was sealed in a quartz tube with an inner diameter of 4 mm. The eutectics were prepared by the vertical Bridgman method at a pull-down rate of 0.1-0.2 mm/min. Figure 1 shows photographs, backscattered electron images (BEIs) and refractive index values of each eutectic. It can be seen that the transparency of the wafer changes due to the difference in refractive indices of each crystal phase in each eutectic. Powder XRD and BEI observations confirmed the expected crystal phases in each eutectic. 

結果と考察 / Results and Discussion

For the LiBr-NaI and LiBr-CsI systems for which no phase diagram exists in the database,  phase diagrams were created by DSC measurement and the eutectic points were found.　 The light yield was determined based on the energy spectra under α-ray (²⁴¹Am) and thermal neutron (²⁵²Cf) excitation using a PMT (R7600U-200, Hamamatsu). The decay time was measured using the same setup as for the PMT and a digital oscilloscope. The LiBr/Tl:CsI, LiCl/LaCl₃ and LiCl/Rb₂CeCl₅ eutectic scintillators showed high light yields of 110,000, 7,000 and 16,000 photons/neutron under ²⁵²Cf neutron irradiation, respectively . The LiBr/Tl:CsI, ample wafer showed sufficient optical translucency for evaluation of scintillation properties. This is thought to be due to the close refractive index of the crystals phases of LiBr (n = 1.79 @550 nm) and CsI (n = 1.79 @550 nm).

図・表・数式 / Figures, Tables and Equations

Table 1. The selected scintillators and their scintillation properties

Fig. 1.  The photographs, BEIs and refractive index values of the fabricated eutectics

その他・特記事項（参考文献・謝辞等） / Remarks(References and Acknowledgements)

References[1]      A. Yoshikawa, et al., Single crystal growth, optical properties and neutron response of Ce³⁺-doped LiCaAlF₆, IEEE Trans. Nucl. Sci. 56 (2009) 3796–3799, [2]      J. Glodo, et al., Development of Cs₂LiYCl₆ scintillator, J. Cryst. Growth, 379 (2013) 73–78, [3]      T. Yanagida, et al., Eu-doped ⁶ LiF-SrF₂ eutectic scintillators for neutron detection, Opt. Mater. 34 (2012) 868–871, [4]      K. Kamada, et al., Growth and scintillation properties of Tb-doped LiGdF₄/LiF eutectic scintillator. Opt. Mater. 61 (2016) 134–138,

成果発表・成果利用 / Publication and Patents

論文・プロシーディング（DOIのあるもの） / DOI (Publication and Proceedings)

1. Rei Sasaki, Investigation of the phase diagram of the CsI-LiBr system and fabrication of the eutectic scintillator for thermal neutron detection, Journal of Crystal Growth, 628, 127543(2024).
   DOI: 10.1016/j.jcrysgro.2023.127543
   
2. Rei Sasaki, Growth and Scintillation Properties of ⁶Li Containing Ce:LaCl₃-Based Eutectic Scintillator for Neutron Detection, IEEE Transactions on Nuclear Science, 70, 1337-1341(2023).
   DOI: 10.1109/TNS.2023.3271639
   
3. Rei Sasaki, Fabrication and Properties for Thermal Neutron Detection of 6LiCl/Rb2CeCl5 Eutectic Scintillator, Crystals, 14, 154(2024).
   DOI: 10.3390/cryst14020154
   

口頭発表、ポスター発表および、その他の論文 / Oral Presentations etc.

1. Rei Sasaki, Kei Kamada, Akira Yoshikawa et al., Fabrication and evaluation of LiCl/Rb2CeCl5 scintillator　for thermal neutron detection. Directionally Solidified Eutectics Conference – VII(DSEC). Oral. 2023年5月25日～27日
2. Rei SASAKI, Kei KAMADA, Akira Yoshikawa et al., Growth and scintillation properties of LaCl3/6LiCl /SrCl2 ternary eutectic for thermal neutron detection.9th International Symposium on Optical Materials (IS-OM'9).oral.2023年6月26日～30日
3. Rei Sasaki, Kei Kamada, Akira Yoshikawa et al., Investigation of the phase diagram of the CsI-LiBr system and fabrication of the eutectic scintillator for thermal neutron detection.International Conference on Crystal Growth and Epitaxy-(ICCGE).Oral.2023年7月30日～8月4日

特許 / Patents

特許出願件数 / Number of Patent Applications：0件
特許登録件数 / Number of Registered Patents：0件