Speaker
Description
Due to its low excitation energy around 8.3 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 with $^{229\text{m}}$Th embedded, 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, the excitation of the nuclear isomer was achieved [6] determining the energy to the $10^{-12}$ precision, boosting the development of a solid-state nuclear clock. A new measurement campaign in July 2023 took place at ISOLDE, aimed at testing different large-bandgap crystals and accurately determining the half-life of $^{229\text{m}}$Th, embedded in different crystals. 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 isomer's time behaviour. Results of these studies will be presented, as well as the plans for future campaigns.
[1] E. Peik et al. Nuclear clocks for testing fundamental physics. Quantum Science and Technology, 6(3):034002, apr 2021.
[2] M. Verlinde et al. Alternative approach to populate and study the $^{229}$Th nuclear clock isomer. Phys. Rev. C100, page 024315, 2019.
[3] K. Beeks and T. Schumm. The nuclear excitation of Thorium-229 in the CaF2 environment. eng. PhD thesis. Wien: TU Wien, 2022.
[4] S. Kraemer et al. Observation of the radiative decay of the $^ {229}$Th nuclear clock isomer. Nature, 617(7962):706–710, 2023.
[5] S. Kraemer. 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] C. Zhang et al. Frequency ratio of the $^{229\mathrm{m}}$Th nuclear
isomeric transition and the $^{87}$Sr atomic clock. Nature, 633(8028):63–70, 2024.
[7] S. V. Pineda, P. Chhetri, S. Bara, Y. Elskens et al. Radiative Decay of the $^{229}$Th Nuclear Clock Isomer in Different Host Materials, 2024. Submitted to Phys. Rev. R.
