Demystifying the 229-Thorium Isomer: Towards the Nuclear ClockP.G. ThirolfLudwig-Maximilians-Universität München, Munich, Germany*

Nowaday’s gold standard for most precise time and frequency measurements are optical atomic clocks. However, they could potentially be outperformed by a nuclear clock, which employs a nuclear transition instead of an atomic shell transition. Presently only the isomeric first excited state of ²²⁹Th qualifies to serve as a nuclear clock. For more than 4 decades nuclear physicists have targeted the identification and characterization of this elusive nuclear state (^229mTh), representing the lowest nuclear excitation in the whole landscape of known isotopes. In recent years considerable progress was achieved on unveiling the properties of ^229mTh: in 2016, the first direct detection of this nuclear state could be realized via its internal conversion decay branch, laying the foundation for precise studies of its decay parameters. Subsequently, a measurement of the half-life of the neutral isomer was achieved, confirming the expected reduction of 9 orders of magnitude compared to the one of charged ^229mTh. Collinear laser spectroscopy was applied to resolve the hyperfine structure of the thorium isomer, providing information on nuclear moments and the nuclear charge radius. Recently, also the cornerstone on the road towards the nuclear clock, which is a precise and direct determination of the excitation energy of the isomer, could be achieved in two independent experiments, resulting in a weighted mean of the isomeric excitation energy as 8.19(12) eV. These important findings open the door towards an optical, laser-based control of ^229mTh and thus the development of an ultra-precise nuclear frequency standard. Such a nuclear clock promises intriguing applications in applied as well as fundamental physics, ranging from geodesy and seismology to the investigation of possible variations of fundamental constants and the search for Dark Matter. The collaborative project ‘ThoriumNuclearClock’, funded by the European Union, recently embarked to consolidate and improve the present knowledge on the thorium isomer, realize first prototypes of a Nuclear Clock and apply them to fundamental physics studies.