标题： X射线泵浦钍-229核钟同质异能素 显示英文标题

标题： X-ray pumping of the Th-229 nuclear clock isomer

摘要： 钍-229是核物理中的一个独特案例：它表现出一个亚稳态的第一激发态钍-229m，仅比核基态高几电子伏特。这种所谓的同质异能素可以通过真空紫外激光器进行激发，这使得原子激光光谱的惊人精度可以应用于核物理。能够随意操控钍-229核状态，将开辟众多前景，从物理学中基本相互作用的研究到作为紧凑且坚固的核钟的应用。然而，尚未观察到该同质异能素的直接光学激发或其辐射衰变回基态，以及诸如钍-229低能级的确切能量和半衰期等一系列关键核结构参数仍未知。在此，我们展示了首次对钍-229m的主动光学泵浦。我们的方案采用窄带29 keV同步辐射来共振激发第二激发态，该态随后主要衰变为同质异能态。我们以0.07 eV的精度确定了共振能量，测得半衰期为82.2皮秒，激发线宽为1.70纳电子伏特，并分别提取了第二激发态进入基态和同质异能态的分支比。这些测量使我们能够重新评估过去40多年收集的伽马光谱数据。 显示英文摘要

摘要： Thorium-229 is a unique case in nuclear physics: it presents a metastable first excited state Th-229m, just a few electronvolts above the nuclear ground state. This so-called isomer is accessible by VUV lasers, which allows transferring the amazing precision of atomic laser spectroscopy to nuclear physics. Being able to manipulate the Th-229 nuclear states at will opens up a multitude of prospects, from studies of the fundamental interactions in physics to applications as a compact and robust nuclear clock. However, direct optical excitation of the isomer or its radiative decay back to the ground state has not yet been observed, and a series of key nuclear structure parameters such as the exact energies and half-lives of the low-lying nuclear levels of Th-229 are yet unknown. Here we present the first active optical pumping into Th-229m. Our scheme employs narrow-band 29 keV synchrotron radiation to resonantly excite the second excited state, which then predominantly decays into the isomer. We determine the resonance energy with 0.07 eV accuracy, measure a half-life of 82.2 ps, an excitation linewidth of 1.70 neV, and extract the branching ratio of the second excited state into the ground and isomeric state respectively. These measurements allow us to re-evaluate gamma spectroscopy data that have been collected over 40~years.