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Observation of the radiative decay of the 229Th nuclear clock isomer


The radionuclide thorium-229 features an isomer with an exceptionally low excitation energy that enables direct laser manipulation of nuclear states. It constitutes one of the leading candidates for use in next-generation optical clocks1,2,3. This nuclear clock will be a unique tool for precise tests of fundamental physics4,5,6,7,8,9. Whereas indirect experimental evidence for the existence of such an extraordinary nuclear state is substantially older10, the proof of existence has been delivered only recently by observing the isomer’s electron conversion decay11. The isomer’s excitation energy, nuclear spin and electromagnetic moments, the electron conversion lifetime and a refined energy of the isomer have been measured12,13,14,15,16. In spite of recent progress, the isomer’s radiative decay, a key ingredient for the development of a nuclear clock, remained unobserved. Here, we report the detection of the radiative decay of this low-energy isomer in thorium-229 (229mTh). By performing vacuum-ultraviolet spectroscopy of 229mTh incorporated into large-bandgap CaF2 and MgF2 crystals at the ISOLDE facility at CERN, photons of 8.338(24) eV are measured, in agreement with recent measurements14,15,16 and the uncertainty is decreased by a factor of seven. The half-life of 229mTh embedded in MgF2 is determined to be 670(102) s. The observation of the radiative decay in a large-bandgap crystal has important consequences for the design of a future nuclear clock and the improved uncertainty of the energy eases the search for direct laser excitation of the atomic nucleus.

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Fig. 1: Low-resolution wavelength spectra of different crystals.
Fig. 2: Time behaviour of the signal.
Fig. 3: Typical high-resolution wavelength spectrum.
Fig. 4: Wavelength of the radiative decay and energy of the isomer.
Fig. 5: Emission channelling patterns of electrons.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request. Source data are provided with this paper.


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We thank the ISOLDE collaboration and technical group at CERN for their extensive support and assistance. This work has received funding from Research Foundation Flanders (FWO, Belgium), from grant no. GOA/2015/010 (BOF KU Leuven) and from FWO and F.R.S.-FNRS under the Excellence of Science (EOS) programme (grant no. 40007501), the Portuguese Foundation for Science and Technology (FCT, project no. CERN/FIS-TEC/0003/2019), the Austrian Science Fund (FWF) project no. I5971 (REThorIC), the European Union’s Horizon 2020 research and innovation programme under the ENSAR2 grant agreement no. 654002, under the Marie Skłodowska-Curie grant agreement no. 101026762 and the European Research Council (ERC) under the Thorium Nuclear Clock agreement no. 856415 and under the LRC agreement no. 819957.

Author information

Authors and Affiliations



P.V.D., M.H., L.M.C.P., Y.K., S.R., M.V., S.K., J.M. and A.V. conceived and planned the experiments. S.K. developed the VUV setup with help from H.D.W. and P.V.D.B. S.K. and P.C. prepared the VUV-spectroscopy experiments. J.M., U.W. and L.M.C.P. prepared the emission channelling experiments. C.M. grew the CaF2 thin films. S.K., J.M., M.A.-K., S.B., K.B., P.C., K.C., A.C., T.E.C., J.M.C., R.F., S.G., R.H., N.H., M.L., R.L., G.M., S.R., S.S., P.G.T., P.V.D., S.M.T., U.K., R.V. and U.W. performed the measurements. S.K., S.B., P.C. and S.S. analysed the VUV-spectroscopy data. J.M. analysed the emission channelling data. S.K., J.M., L.M.C.P., S.S. and P.V.D. prepared the manuscript. All authors provided critical feedback and helped shape the research, analysis and manuscript. This article results from the PhD thesis work of S.K. and J.M.

Corresponding author

Correspondence to Sandro Kraemer.

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The authors declare no competing interests.

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Nature thanks Iain Moore and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

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Extended data figures and tables

Extended Data Fig. 1 High-purity germanium detector spectra.

Single γ spectra recorded with the HPGe detector for a typical implantation at A = 229 (a) and A = 230 (b). The main peaks in the spectrum are marked with the β-decaying isotope and the energy. In this study, new γ lines in the decay of 229Ra were identified, these tentative assignments are indicated with an asterisk. The inset shows the high-energy part of the spectrum with smaller peaks.

Source data

Extended Data Fig. 2 Characteristics of the CaF2 thin-film crystal surface.

An atomic force microscopy (AFM) image of the surface of the CaF2 thin film (a) and a reflection high-energy electron diffraction (RHEED) pattern along [11-2] azimuthal direction (b) are shown.

Extended Data Fig. 3 Vacuum-ultraviolet spectroscopy setup.

The implantation beam (1), target wheel with large-bandgap crystals (2), entrance slit (3), parabolic collimation mirror (4), diffraction grating (5), parabolic camera mirror (6), detector slit (7) photomultiplier detector (8) and plasma VUV-photon source used for calibration (9) are shown. The setup includes additionally γ-radiation detectors placed close to the implantation position of the crystal.

Extended Data Table 1 Characteristics of the isobaric β-decay chains
Extended Data Table 2 List of large-bandgap crystals

Supplementary information

Source data

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Kraemer, S., Moens, J., Athanasakis-Kaklamanakis, M. et al. Observation of the radiative decay of the 229Th nuclear clock isomer. Nature 617, 706–710 (2023).

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