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The thorium-229 nucleus contains an isomeric state with a low excitation energy, making it accessible to laser excitation. It is presently the only known candidate for the development of a nuclear clock [1,2,3,4] which will enable testing fundamental principles in physics, such as e.g. potential variations of fundamental constants [5] or the search for ultralight dark matter candidates [6]. Moreover, practical applications like relativistic geodesy are possible [7].
The radiative decay of the thorium-229 isomer was observed at ISOLDE in a previous experiment by populating it via the beta decay of actinium-229, implanting its shorter-lived decay precursors in large bandgap crystals and observing the isomer’s VUV photons in a dedicated spectrometer. A reduced uncertainty of the isomer’s excitation energy (8.338±0.024 eV) and a first determination of the ionic half-life (670±102 s) in MgF$_2$ was reported [8]. During the July 2023 campaign seven crystals (including SiO$_2$, AlN, LiSrAlF$_6$) were tested, the energy was determined with better precision, and the half-life behavior of the VUV signal in the different crystals was studied. Preliminary results and ongoing analysis as well as future prospects will be discussed.
[1] E. Peik and C. Tamm, EPL 61, 181 (2003).
[2] C. Campbell et al., Phys. Rev. Lett. 108, 120802 (2012).
[3] P. Thirolf et. al., J. Phys. B: At. Mol. Opt. Phys. 52, 203001 (2019).
[4] E. Peik et al., Quantum Sci. Technol. 6, 034002 (2021).
[5] V. Flambaum, Phys. Rev. Lett. 97, 092502 (2006).
[6] P. Wcisło et. al., Nature Astronomy 1, 0009 (2016).
[7] M. Bondarescu et. al., Eur. Phys. J. Web Conf. 95, 04009 (2015).
[8] Kraemer et al., Nature 617, 706–710 (2023).
