Thirolf Peter G, Kraemer Sandro, Moritz Daniel, Scharl Kevin
Faculty of Physics, Ludwig-Maximilians-Universität München, Am Coulombwall 1, Garching, 85748 Germany.
Eur Phys J Spec Top. 2024;233(5):1113-1131. doi: 10.1140/epjs/s11734-024-01098-2. Epub 2024 Jan 31.
Today's most precise timekeeping is based on optical atomic clocks. However, those could potentially be outperformed by a nuclear clock, based on a nuclear transition instead of an atomic shell transition. Such a nuclear clock promises intriguing applications in applied as well as fundamental physics, ranging from geodesy and seismology to the investigation of possible time variations of fundamental constants and the search for dark matter. Only one nuclear state is known so far that could drive a nuclear clock: the "Thorium Isomer" Th, i.e., the isomeric first excited state of Th, representing the lowest nuclear excitation so far reported in the landscape of nuclear isotopes. Indirectly conjectured to exist already in 1976, decades of experimental efforts were dedicated to unambiguously identify this elusive nuclear state and to characterize its properties. However, for 40 years, these efforts remained inconclusive. The turning point was marked by the first direct detection of Th via its internal conversion decay branch in 2016. Since then, remarkable progress could be achieved in characterizing the properties and decay parameters. The half-life of the neutral isomer was determined, the hyperfine structure was measured via collinear laser spectroscopy, providing information on nuclear moments and the nuclear charge radius and also the excitation energy of the isomer could be directly determined with different techniques. In a recent experiment at CERN's ISOLDE facility, the long-sought radiative decay of the Thorium isomer could be observed for the first time via implantation of ( -decaying) Ac into a vacuum-ultraviolet (VUV) transparent crystal and subsequent fluorescence detection in a VUV spectrometer. Thus, the excitation energy of Th could be determined with unprecedented precision to 8.338(24) eV, corresponding to a wavelength of 148.71(42) nm. These achievements, together with ongoing laser developments for the required VUV wavelength, open the door toward a laser-driven control of the isomeric transition and thus to the development of an ultra-precise nuclear frequency standard.
当今最精确的计时是基于光原子钟。然而，基于核跃迁而非原子壳层跃迁的核钟可能会超越它们。这样的核钟在应用物理和基础物理领域都有着引人入胜的应用前景，从大地测量学和地震学到对基本常数可能的时间变化的研究以及暗物质的搜寻。到目前为止，已知只有一种核态可以驱动核钟：“钍异构体”Th，即Th的同质异能第一激发态，它代表了核同位素领域中迄今为止报道的最低核激发态。1976年就有人间接推测它的存在，几十年来，人们进行了大量实验，致力于明确识别这种难以捉摸的核态并表征其性质。然而，40年来，这些努力都没有定论。转折点是2016年通过其内转换衰变分支首次直接探测到Th。从那时起，在表征其性质和衰变参数方面取得了显著进展。确定了中性异构体的半衰期，通过共线激光光谱测量了超精细结构，提供了有关核矩、核电荷半径的信息，并且还可以用不同技术直接确定异构体的激发能。在欧洲核子研究中心（CERN）的ISOLDE设施最近进行的一项实验中，通过将（衰变的）Ac注入真空紫外（VUV）透明晶体并随后在VUV光谱仪中进行荧光检测，首次观测到了长期以来寻找的钍异构体的辐射衰变。因此，Th的激发能以前所未有的精度确定为8.338(24) eV，对应波长为148.71(42) nm。这些成就，连同针对所需VUV波长正在进行的激光技术发展，为激光驱动的同质异能跃迁控制以及超精确核频率标准的开发打开了大门。
