Lifetime of the singly charged ²²⁹Th nuclear isomer

Nature Physics (2026)Cite this article The nucleus of thorium-229 (229Th) has an exceptionally low-energy isomeric state (229mTh). Because its excitation energy is close to the energy levels of valence electrons, the lifetime of 229mTh varies substantially depending on its electronic state. Although internal conversion and radiative decay were recently observed, the electronic bridge decay of 229mTh, a higher-order decay process through an electronic transition, has not yet been confirmed. A promising candidate to search for this decay channel is singly charged 229mTh+ in the electronic ground state. Here we produce 229mTh+ by a charge-exchange reaction in an ion trap and detect the isomers through measuring the electrons emitted from internal conversion processes. We determined the half-life of 229mTh+ to be 0.46(8) s. Our result differs by several orders of magnitude from the half-lives of internal conversion and radiative decay, indirectly suggesting the existence of the electronic bridge decay of 229mTh. This will enable the direct observation and further investigation of the electronic bridge process, which will contribute to understanding nuclear–electron interactions and accelerating nuclear deexcitation in the operation of a thorium-based nuclear clock.This is a preview of subscription content, access via your institution Access Nature and 54 other Nature Portfolio journals Get Nature+, our best-value online-access subscription $32.99 / 30 days cancel any timeSubscribe to this journal Receive 12 print issues and online access $259.00 per yearonly $21.58 per issueBuy this articleUSD 39.95Prices may be subject to local taxes which are calculated during checkoutThe data supporting the findings of this study are available from the corresponding author upon request.Emery, G. T. 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A 109, 11224–11235 (2005).Article Google Scholar Download referencesWe are grateful to F. F. Karpeshin for fruitful theoretical discussions. 233U used in this study was provided by the 233U cooperation project between the Japan Atomic Energy Agency and the Inter-University Cooperative Research Program of the Institute for Materials Research, Tohoku University (proposal nos. 17K0204, 17F0011 and 18F0014). This work was supported by JST (CREST grant no. JPMJCR24I6 to Y.S. and A.Y.) and JSPS (KAKENHI grant nos. JP17H01081 to H.H., JP19K23445 to Y.S., JP20K14500 to Y.S., JP21H04473 to Y.S., JP23K25904 to Y.S., JP23H00094 to A.Y. and Y.S., JP24H00469 to H.H. and Y.S., and JP25H00397 to Y.S.).Nishina Center for Accelerator-Based Science, RIKEN, Wako, JapanY. Shigekawa, N. Sato & H. HabaInstitute of Pure and Applied Sciences, University of Tsukuba, Tsukuba, JapanY. ShigekawaSpace-Time Engineering Research Team, RIKEN Center for Advanced Photonics, RIKEN, Wako, JapanA. YamaguchiPRESTO, Japan Science and Technology Agency, Kawaguchi-shi, JapanA. YamaguchiGraduate School of Science, The University of Osaka, Toyonaka, JapanK. Tokoi & Y. KasamatsuResearch Center for Accelerator and Radioisotope Science, Tohoku University, Sendai, JapanH. KikunagaInstitute for Materials Research, Tohoku University, Sendai, JapanK. ShirasakiInstitute for Radiation Science, The University of Osaka, Toyonaka, JapanK. ShirasakiWako Nuclear Science Center, Institute of Particle and Nuclear Studies, High Energy Accelerator Research Organization, Wako, JapanM. WadaAdvanced Energy Science and Technology GuangDong Laboratory, Huizhou, ChinaM. WadaSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarSearch author on:PubMed Google ScholarY.S., A.Y., K.T. and N.S developed the experimental apparatus. Y.S. and A.Y. performed the measurements and analysed the data. Y.S., A.Y., H.K., K.S. and H.H. performed the chemical purification of 233U and developed the 233U source. M.W. developed the ion extracting system including the RF carpet and the quadrupole ion guide. All work was supervised by H.H, Y.K. and M.W. All authors discussed the results and contributed to the preparation of the manuscript.Correspondence to Y. Shigekawa.The authors declare no competing interests.Nature Physics thanks David Leibrandt and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.Black circles show MCP counts as a function of time (bin width 655 ns) at an MCP surface voltage of −35 V for 229m,gTh3+ (a), 229m,gTh+ (b), and 232Th+ (c), while the blue triangles show the time traces obtained at an MCP surface voltage of −2000 V in arbitrary units. The blue solid curves are guides to the eye. The error bars in (a)–(c) represent 1 s.d. statistical uncertainties. The measurements for 229m,gTh3+ and 229m,gTh+ were performed in Campaign C, while those for 232Th+ were performed in Campaign D. The 229m,gTh+ ions in (b) were produced in the RF-carpet gas cell containing 1 kPa of He gas, not in the ion trap.MCP counts at an MCP surface voltage of −35 V are divided by the total number of ions extracted on the MCP detector and shown as a function of time for the extraction of 229m,gTh3+, 229m,gTh2+, 229m,gTh+, and 232Th+ ions produced in the RF-carpet gas cell containing 1 kPa of He gas (red circle, purple square, blue open triangle, and black inverted triangle, respectively), as well as for 229m,gTh+ ions produced by the charge-exchange reaction in the ion trap containing He, Ar, and NO gases at a total pressure of 0.40 Pa (green triangle). The 0 μs in this graph corresponds to the arrival time of the ion bunches at the MCP detector, measured at an MCP surface voltage of −2000 V. The solid curves are guides to the eye. The error bars represent 1 s.d. statistical uncertainties.a, Ion-impact counts (MCP counts at −2000 V) per ion bunch (black circle) as a function of the trapping time and the function (red curve) obtained by fitting Eq. (2) to the data. Here, the ion-impact counts of 229m,gTh+ were regarded as those of 229gTh+ due to the negligibly small fraction of 229mTh. b, IC-electron counts per ion bunch (blue square) as a function of the trapping time, and the function (red curve) obtained by fitting Eq. (4) to the data. IC-electron counts were obtained by subtracting the ion-impact counts (black curve) from the MCP counts at an MCP surface voltage of −35 V. The error bars in (a)–(b) include 1 s.d. statistical uncertainties and fluctuations in the number of 229Th ions supplied by our RF carpet gas cell (5%). The gas pressures in the ion trap were kept at ptotal = 0.50 Pa, pAr ≈ 0.38 Pa, and pNO = 4.6(6)×10−7 Pa.Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.Reprints and permissionsShigekawa, Y., Yamaguchi, A., Tokoi, K. et al. Lifetime of the singly charged 229Th nuclear isomer. Nat. Phys. (2026). https://doi.org/10.1038/s41567-026-03251-1Download citationReceived: 19 February 2025Accepted: 06 March 2026Published: 14 April 2026Version of record: 14 April 2026DOI: https://doi.org/10.1038/s41567-026-03251-1Anyone you share the following link with will be able to read this content:Sorry, a shareable link is not currently available for this article. 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