Elucidation of the nuclear decay property of Th-229m utilizing Ac-229

2020 Fiscal Year Final Research Report

• Project/Area Number: 19K23445
• Research Category: Grant-in-Aid for Research Activity Start-up
• Allocation Type: Multi-year Fund
• Review Section: 0203:Particle-, nuclear-, astro-physics, and related fields
• Research Institution: Institute of Physical and Chemical Research
• Principal Investigator: Shigekawa Yudai 国立研究開発法人理化学研究所, 仁科加速器科学研究センター, 基礎科学特別研究員 (60845626)
• Project Period (FY): 2019-08-30 – 2021-03-31
• Keywords: トリウム229 / 原子核時計 / アクチニウム / プロトアクチニウム / Th-229m / 核異性体 / 真空紫外光

Outline of Final Research Achievements

We aimed to inhibit the internal conversion process of Th-229m and observe its γ-ray emission process using the new method in which a fluoride crystal is doped with Ac-229 or Pa-229. We developed a method for producing a large amount of Pa-229 using an accelerator and separating it from a target and fission products with high efficiency. We also developed an apparatus for measuring γ rays (VUV photons) originating from the decay of Th-229m. Using a CaF2 crystal on which a Pa-229 solution was heated, we measured background photons originating from the decay of Pa-229. We will be able to clearly observe the γ rays of Th-229m by improving the method for introducing Pa-229 into a CaF2 crystal.

Free Research Field

核化学・放射化学

Academic Significance and Societal Importance of the Research Achievements

本研究課題では、今まで誰も観測に成功していないTh-229mのγ線放出過程の直接観測を目指して、Ac-229やPa-229を用いた手法を新しく開発した。手法をさらに改善することで、Th-229mのγ線を観測し、Th-229原子核時計を開発する上で重要なパラメータであるγ線放出の半減期を決定できると期待される。将来的に既存の原子時計よりも精度の高い原子核時計を実現できれば、基礎物理学や測地学などの発展に大きく貢献できる。