“The Th-229 Nuclear Clock Isomer: VUV and Laser Spectroscopy Studies”
Prof. Piet van Duppen (KU Leuven, Belgien)

The thorium-229 nucleus contains an isomeric state with a low excitation energy, making it possible to probe using lasers. It is one of the best candidates for the development of a nuclear clock [1,2] which will enable the ability to test fundamental principles in physics (see e.g. [4]). However, to accomplish such a nuclear clock, the nuclear properties of the isomer need to be determined more precisely and two approaches are being followed.

VUV spectroscopy revealed the radiative decay of the thorium-229 isomer in a study at ISOLDE-CERN by populating the isomer via the beta decay of actinium-229, implanting the beam in large bandgap crystals (CaF2 and MgF2). A reduced uncertainty of the isomer’s excitation energy (8.338±0.024 eV) and a first determination of the half-life (670±102 s) in MgF2 was reported [5]. During a follow-up campaign, different crystals were tested, the energy was determined with a better precision and the half-life behaviour of the VUV signal in the different crystals was studied.

Preparatory work to perform laser ionization spectroscopy of the thorium-229 ground and isomeric states, populated in the alpha decay of uranium-233, is performed in an argon gas-jet based system. These studies, aimed to deduce the mean-square charge radii and moments of both ground and isomeric state, are based on singly charged thorium ions and necessitates a search for efficient and effective laser ionization schemes of thorium giving rise to a more precise determination of the first and second ionization potential. Results from these off and on-line studies will be presented and outlook to future work discussed.

[1] E. Peik and C. Tamm, EPL 61, 181 (2003).
[2] C. Campbell et al., Phys. Rev. Lett. 108, 120802 (2012).
[3] L. von der Wense et al. Nature 533 (7601), 47–51 (2016).
[4] E. Peik et al., Quantum Sci. Technol. 6, 034002 (2021).
[5] Kraemer et al., Nature 617, 706–710 (2023).