Ludwig-Maximilians-University Munich, Garching, Germany.
Max-Planck-Institut für Kernphysik, Heidelberg, Germany.
Nature. 2019 Sep;573(7773):243-246. doi: 10.1038/s41586-019-1533-4. Epub 2019 Sep 11.
Owing to its low excitation energy and long radiative lifetime, the first excited isomeric state of thorium-229, Th, can be optically controlled by a laser and is an ideal candidate for the creation of a nuclear optical clock, which is expected to complement and outperform current electronic-shell-based atomic clocks. A nuclear clock will have various applications-such as in relativistic geodesy, dark matter research and the observation of potential temporal variations of fundamental constants-but its development has so far been impeded by the imprecise knowledge of the energy of Th. Here we report a direct measurement of the transition energy of this isomeric state to the ground state with an uncertainty of 0.17 electronvolts (one standard deviation) using spectroscopy of the internal conversion electrons emitted in flight during the decay of neutral Th atoms. The energy of the transition between the ground state and the first excited state corresponds to a wavelength of 149.7 ± 3.1 nanometres, which is accessible by laser spectroscopy through high-harmonic generation. Our method combines nuclear and atomic physics measurements to advance precision metrology, and our findings are expected to facilitate the application of high-resolution laser spectroscopy on nuclei and to enable the development of a nuclear optical clock of unprecedented accuracy.
由于其低激发能量和长辐射寿命，钍-229 的第一激发同质异能态 Th 可以通过激光进行光学控制，是创建核光学钟的理想候选者，预计该光学钟将补充并超越当前基于电子壳层的原子钟。核钟将有各种应用，例如在相对论大地测量学、暗物质研究和观察基本常数潜在的时间变化，但迄今为止，其发展一直受到对 Th 能量的精确知识的阻碍。在这里，我们报告了使用中性 Th 原子衰变过程中飞行中发射的内转换电子的光谱学，以 0.17 电子伏特（一个标准差）的不确定度直接测量该同质异能态到基态的跃迁能量。基态和第一激发态之间的跃迁能量对应于 149.7 ± 3.1 纳米的波长，通过高次谐波产生的激光光谱学可以实现该波长。我们的方法结合了核物理和原子物理测量，以推进精密计量学，我们的发现有望促进高分辨率激光光谱学在原子核上的应用，并能够开发出具有前所未有的精度的核光学钟。
