Speaker
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Due to its low excitation energy around 8.4 eV, the unique $^{229}$Th isomer is the ideal candidate for developing a nuclear clock [1]. Such a clock would be particularly suited for fundamental physics studies [1]. In the past, measuring the isomer's radiative decay from a large-bandgap crystal doped with $^{229}$Th, has proven difficult: the commonly used population of the isomer via the $^{233}$U $\alpha$-decay has a limited branching ratio towards the isomer and creates a high-radioluminescence background [2, 3]. However, recently, a new approach to populate the isomer through the $\beta$-decay of $^{229}$Ac was proposed [2]. This approach made it possible to observe, for the first time, the radiative decay of the $^{229}$Th isomer with vacuum-ultraviolet (VUV) spectroscopy, which allowed to successfully determine the resulting photon's wavelength at a value of $\lambda = 148.7\pm 0.4$ nm ($E=8.338\pm 0.024$ eV) and the isomer's radiative half-life in a MgF$_2$ crystal at a value of $ t_{1/2}=670\pm102$ s [4, 5]. Based on this work, narrow-band laser excitation of the nuclear isomer was achieved [6] with a frequency comb, determining the energy to $10^{-12}$ precision, boosting the development of a solid-state nuclear clock.
A new measurement campaign in July 2023 took place at ISOLDE, aimed at investigating different large-bandgap crystals and accurately determining the time behaviour of the radiative decay of $^{229\text{m}}$Th, embedded in different crystal materials. This allowed to (1) observe, for the first time, the radiative decay in a LiSrAlF$_6$ crystal, (2) determine the radiative decay fraction of the isomer in different crystals [7], and (3) study the time behaviour of an ensemble of $^{229}$Th isomers. These studies revealed the presence of a crystal-material-dependent quenching mechanism induced by the $\beta$-decay of the precursor isotopes. Results of these experiments will be presented, as well as the results of the most recent ISOLDE campaign which took place in May 2025. This campaign extended the earlier radiative-decay fraction measurements with new crystalline materials, and further investigated the $\alpha$- and $\beta$-decay-induced quenching mechanism in order to link it to laser- and X-ray-induced quenching as reported in [8, 9].
References
[1] E Peik et al. “Nuclear clocks for testing fundamental physics”. In: Quantum Science and Technology 6.3 (Apr. 2021), p. 034002. doi: 10.1088/2058-9565/abe9c2. url: https://dx.doi.org/10.1088/2058-9565/abe9c2.
[2] M. Verlinde et al. “Alternative approach to populate and study the $^{229}$Th nuclear clock isomer”. In:Phys. Rev. C100 (2019), p. 024315.
[3] K. Beeks and T. Schumm. “The Nuclear Excitation of Thorium-229 in the CaF$_2$ Environment”. In: eng. PhD thesis. Wien: TU Wien (2022).
[4] S. Kraemer and J. Moens et al. “Observation of the radiative decay of the $^{229}$Th nuclear clock isomer”. In: Nature 617.7962 (2023), pp. 706–710.
[5] S. Kraemer and P. Van Duppen. “Vacuum-ultraviolet spectroscopy of the radiative decay of the low-energy isomer in $^{229}$Th”. PhD thesis. KU Leuven - Instituut voor Kern- en Stralingsfysica, 2022.
[6] Chuankun Zhang et al. “Frequency ratio of the $^{229m}$Th nuclear isomeric transition and the $^{87}$Sr atomic clock”. In: Nature 633.8028 (2024), pp. 63–70.
[7] S. V. Pineda et al. “Radiative decay of the $^{229m}$Th nuclear clock isomer in different host materials”. In: Phys. Rev. Res. 7 (1 Jan. 2025), p. 013052. doi: 10 . 1103 / PhysRevResearch . 7 . 013052. url: https://link.aps.org/doi/10.1103/PhysRevResearch.7.013052.
[8] F. Schaden et al. “Laser-induced quenching of the Th-229 nuclear clock isomer in calcium fluoride”. In: Phys. Rev. Res. 7 (2 May 2025), p. L022036. doi: 10.1103/PhysRevResearch.7.L022036. url: https://link.aps.org/doi/10.1103/PhysRevResearch.7.L022036.
[9] J. E. S. Terhune et al. “Photoinduced quenching of the $^{229}$Th isomer in a solid-state host”. In: Phys. Rev. Res. 7 (2 June 2025), p. L022062. doi: 10.1103/glzr-thyw. url: https://link.aps.org/doi/10.1103/glzr-thyw.