The ²²⁹Th nucleus has an isomeric state at an energy of about 8 eV above the ground state, several orders of magnitude lower than typical nuclear excitation energies. This has inspired the development of a field of low-energy nuclear physics in which nuclear transition rates are influenced by the electron shell. The low energy makes the ²²⁹Th isomer accessible to resonant laser excitation. Observed in laser-cooled trapped thorium ions or with thorium dopant ions in a transparent solid, the nuclear resonance may serve as the reference for an optical clock of very high accuracy. Precision frequency comparisons between such a nuclear clock and conventional atomic clocks will provide sensitivity to the effects of hypothetical new physics beyond the standard model. Although laser excitation of ²²⁹Th remains an unsolved challenge, recent experiments have provided essential information on the transition energy and relevant nuclear properties, advancing the field.

在^229个钍核在约8电子伏特以上的基态的能量具有异构态，几个数量级比典型的核激发能量低。这激发了低能核物理领域的发展，在该领域中，核跃迁速率受电子壳的影响。低能耗使²²⁹该异构体易于共振激光激发。在激光冷却的捕获th离子或透明固体中的do掺杂离子中观察到，核共振可作为非常高精度的光学时钟的参考。这样的核钟与常规原子钟之间的精确频率比较将提供对超出标准模型的假设新物理效应的敏感性。尽管²²⁹ Th的激光激发仍然是一个尚未解决的挑战，但最近的实验已经提供了有关跃迁能和相关核特性的重要信息，从而推动了该领域的发展。