近日，日本RIKEN量子计量实验室的Atsushi Yamaguchi及其研究团队取得一项新进展。经过不懈努力，他们对核时钟中三电荷²²⁹Th异构体的激光光谱进行研究。相关研究成果已于2024年4月17日在国际权威学术期刊《自然》上发表。

本文报道了成功利用233U源连续捕获^229mTh³⁺的突破，并通过精密的核态选择激光光谱技术，测定了分离得到的^229mTh³⁺的核衰变半衰期为1400±600-300秒。此外，通过精确测定^229mTh³⁺的超精细常数，研究人员将229号核钟对精细结构常数变化的灵敏度的不确定性降低了四倍。这些重要结果为²²⁹Th³⁺核时钟的研制，及其在探索新物理领域的应用提供了不可或缺的关键参数。

据悉，钍-229（²²⁹Th）具有基态与低洼异构体（^229mTh）之间的光核跃迁特性。这种核跃迁频率的核钟，由于不受周围磁场干扰，有望超越现有的原子钟技术。在众多电荷态中，三电荷态的²²⁹Th³⁺因其闭合的电子跃迁特性而备受青睐，它能够实现激光冷却、激光诱导荧光检测以及离子的状态制备，从而满足高精度核时钟的需求。尽管已有研究对^229mTh³⁺在核基态下的激光光谱进行了探索，但关于^229mTh³⁺的核衰变寿命等关键性质，目前仍属未知，而这些性质对于确定核时钟跃迁的本征线宽至关重要。

附：英文原文

Title: Laser spectroscopy of triply charged ²²⁹Th isomer for a nuclear clock

Author: Yamaguchi, Atsushi, Shigekawa, Yudai, Haba, Hiromitsu, Kikunaga, Hidetoshi, Shirasaki, Kenji, Wada, Michiharu, Katori, Hidetoshi

Issue&Volume: 2024-04-17

Abstract: Thorium-229 (²²⁹Th) possesses an optical nuclear transition between the ground state (^229gTh) and low-lying isomer (^229mTh). A nuclear clock based on this nuclear-transition frequency is expected to surpass existing atomic clocks owing to its insusceptibility to surrounding fields. In contrast to other charge states, triply charged ²²⁹Th (²²⁹Th³⁺) is the most suitable for highly accurate nuclear clocks because it has closed electronic transitions that enable laser cooling, laser-induced fluorescence detection and state preparation of ions. Although laser spectroscopic studies of ²²⁹Th³⁺ in the nuclear ground state have been performed, properties of ^229mTh³⁺, including its nuclear decay lifetime that is essential to specify the intrinsic linewidth of the nuclear-clock transition, remain unknown. Here we report the trapping of ^229mTh³⁺ continuously supplied by a ²³³U source and the determination of nuclear decay half-life of the isolated ^229mTh³⁺ to be 1400⁺⁶⁰⁰_-300s through nuclear-state-selective laser spectroscopy. Furthermore, by determining the hyperfine constants of ^229mTh³⁺, we reduced the uncertainty of the sensitivity of the ²²⁹Th nuclear clock to variations in the fine-structure constant by a factor of four. These results offer key parameters for the ²²⁹Th³⁺ nuclear clock and its applications in the search for new physics.

DOI: 10.1038/s41586-024-07296-1

Source: https://www.nature.com/articles/s41586-024-07296-1