Precise energy determination of the isomeric ground-state transition in 229mTh: Advancing on the route towards a nuclear clock

在最近终止的DFG项目中，我们的团队在一个40年来在世界范围内进行了深入研究的研究领域取得了突破性成果：我们首次直接识别了所有已知原子核中能量最低激发态的衰变（从而存在），即229-钍中的同分异构第一激发态。这种状态由于其作为高精度核频率标准（核钟）的潜力而引起广泛关注。然而，为了实现核时钟，需要改进跃迁能量的知识（目前假定为7.8（5）eV），以允许构造用于跃迁的光学操纵的优化激光器。我们目前的测量是为了识别跃迁而设计的，而不是为了确定它的能量。这将是本项目更新提案的中心目标。我们的229 mTh衰变的检测是基于收集的双或三重带电的Th离子的检测器表面上的衰变产物的后续登记。这些是来自内转换（IC）衰变通道的电子，在中性钍的情况下（由于检测器表面上的电荷中和，在此必须预期中性钍），其在能量上是允许的并且必须预期占主导地位。因此，跃迁能的精确测量必须集中在低能IC电子动能的精确测定上。为此，我们将建立一个高分辨率磁瓶电子谱仪。在这里，IC电子将首先被有效地收集并聚焦在合适的磁场配置中，然后被引导到电子检测器。在它们的途中，它们将不得不通过几个与阻滞电位相啮合的电位，这将导致在IC衰变电子的动能处的电子谱中的尖锐截止边缘。这将允许确定的同质异能跃迁能量优于50毫电子伏，一个数量级更精确的比目前的价值。这足以使激光系统的建设，将光学激发核跃迁到异构体。所需的钍离子中和将在第一项目阶段在金属收集表面上进行。然而，为了排除任何潜在的扭曲表面效应，平行的无接触中和将开发，使用铯原子蒸气为基础的电荷交换电池。最终的能量测定将在此配置下进行。将使用氦气放电灯对光谱仪进行表征和校准。该项目将使我们的团队在寻求229 mTh核钟跃迁方面保持甚至扩大目前的领先地位，最终目标是为实现超精密核频率标准奠定基础。

In the recently terminated DFG project, which is hereby applied for renewal, our group achieved a breakthrough result in a research field which is intensely studied worldwide since 40 years: we achieved the first direct identification of the decay (and thus the existence) of the energetically lowest excited states amongst all known atomic nuclei, namely the isomeric first excited state in 229-thorium. This state attracts widespread attention due to its potential to serve as a highly precise nuclear frequency standard (nuclear clock). However, in order to realize a nuclear clock it needs an improved knowledge of the transition energy (presently assumed to be 7.8(5) eV) to allow for the construction of an optimized laser for an optical manipulation of the transition. Our present measurements were designed for the identification of the transition, not for its energy determination. This will be the central objective of the present project renewal proposal. Our detection of the decay of 229mTh was based on the collection of doubly or triply charged Th ions on a detector surface with subsequent registration of the decay products. These were electrons from the internal conversion (IC) decay channel, which in case of neutral thorium (which has to be expected here due to the charge neutralization on the detector surface) is energetically allowed and has to be expected to dominate. Thus a precise measurement of the transition energy will have to focus on a precise determination of the kinetic energy of low-energy IC electrons. For this purpose we will set up a high-resolution magnetic bottle electron spectrometer. Here the IC electrons will first be efficiently collected and focused in a suitable magnetic field configuration and then guided to an electron detector. On their way they will have to pass several potential meshed with retarding potentials, which will lead to a sharp cut-off edge in the electron spectrum at the kinetic energy of the IC decay electrons. This will allow to determine the isomeric transition energy to better than 50 meV, one order of magnitude more precise than the present value. This is sufficient to enable the construction of a laser system that will optically excite the nuclear transition to the isomer. The required neutralization of the thorium ions will be induced in a first project phase on a metallic collection surface. However, in order to exclude any potentially distorting surface effects, in parallel a contact-free neutralization will be developed, using a Cs atomic-vapor based charge exchange cell. The final energy determination will be conducted with this configuration. Characterization and calibration of the spectrometer will be performed with a helium gas discharge lamp. This project will allow our group to keep or even extend the present lead in the quest for the 229mTh nuclear clock transition, with the final goal to lay the foundations for the realization of an ultra-precise nuclear frequency standard.