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

229mTh 异构体基态跃迁的精确能量测定：在核钟的道路上取得进展

• 批准号: 235069165
• 负责人: Privatdozent Dr. Peter G. Thirolf
• 金额: --
• 依托单位: Lehrstuhl für Experimentalphysik - Medizinische Physik
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2013
• 资助国家: 德国
• 起止时间: 2012-12-31 至 2019-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/235069165?language=en
• 关键词: Precise; energy; determination; isomeric; ground

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.

在最近终止并申请续展的DFG项目中，我们课题组在全球范围内研究了40年的研究领域取得了突破性成果：首次直接识别出所有已知原子核中能量最低激发态的衰变（从而确定了其存在），即229-钍的同分异构第一激发态。这种状态因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，需要更好地了解跃迁能量（目前假设为 7.8(5) eV），以便构建用于光学操纵跃迁的优化激光器。我们目前的测量是为了识别跃迁而设计的，而不是为了确定跃迁的能量。这将是本项目更新提案的中心目标。我们对 229mTh 衰变的检测是基于在探测器表面收集双电荷或三电荷 Th 离子，并随后记录衰变产物。这些是来自内部转换（IC）衰变通道的电子，在中性钍的情况下（由于探测器表面上的电荷中和，这里必须预期）是能量允许的，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于低能 IC 电子动能的精确测定。为此，我们将建立一台高分辨率磁瓶电子能谱仪。在这里，IC 电子将首先被有效收集并聚焦在合适的磁场配置中，然后引导至电子探测器。在途中，它们必须经过几个与减速电势相啮合的电势，这将导致电子光谱中 IC 衰变电子动能处出现尖锐的截止边缘。这将允许确定优于 50 meV 的异构跃迁能量，比当前值精确一个数量级。这足以构建激光系统，以光学方式激发核转变为异构体。所需的钍离子中和将在第一个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，同时将使用基于铯原子蒸气的电荷交换电池开发无接触中和。最终的能量确定将以此配置进行。光谱仪的表征和校准将使用氦气放电灯进行。该项目将使我们课题组在229mTh核时钟跃迁的探索中保持甚至扩大目前的领先地位，最终目标是为实现超精确核频率标准奠定基础。

项目成果

Laser spectroscopic characterization of the nuclear-clock isomer 229mTh

• DOI: 10.1038/s41586-018-0011-8
• 发表时间: 2017-09
• 期刊: Nature
• 影响因子: 64.8
• 作者: J. Thielking;M. Okhapkin;P. Głowacki;D. Meier;L. Wense;B. Seiferle;C. Düllmann;P. Thirolf;E. Peik

Improving Our Knowledge on the 229mThorium Isomer: Toward a Test Bench for Time Variations of Fundamental Constants

• DOI: 10.1002/andp.201800381
• 发表时间: 2019-02
• 期刊: Annalen der Physik
• 影响因子: 2.4
• 作者: P. Thirolf;B. Seiferle;Lars der Wense

Energy of the 229Th nuclear clock transition

• DOI: 10.1038/s41586-019-1533-4
• 发表时间: 2019-09-12
• 期刊: NATURE
• 影响因子: 64.8
• 作者: Seiferle, Benedict;von der Wense, Lars;Thirolf, Peter G.
• 通讯作者: Thirolf, Peter G.