原子核で時を刻む(Keeping time with an atomic nucleus)

2023-05-25 ミュンヘン大学（LMU）

＜関連情報＞

• https://www.lmu.de/en/newsroom/news-overview/news/keeping-time-with-an-atomic-nucleus.html
• https://www.nature.com/articles/s41586-023-05894-z

229Th核時計異性体の放射性崩壊の観測に成功 Observation of the radiative decay of the 229Th nuclear clock isomer

Sandro Kraemer,Janni Moens,Michail Athanasakis-Kaklamanakis,Silvia Bara,Kjeld Beeks,Premaditya Chhetri,Katerina Chrysalidis,Arno Claessens,Thomas E. Cocolios,João G. M. Correia,Hilde De Witte,Rafael Ferrer,Sarina Geldhof,Reinhard Heinke,Niyusha Hosseini,Mark Huyse,Ulli Köster,Yuri Kudryavtsev,Mustapha Laatiaoui,Razvan Lica,Goele Magchiels,Vladimir Manea,Clement Merckling,Lino M. C. Pereira,Sebastian Raeder,Thorsten Schumm,Simon Sels,Peter G. Thirolf,Shandirai Malven Tunhuma,Paul Van Den Bergh,Piet Van Duppen,André Vantomme,Matthias Verlinde,Renan Villarreal & Ulrich Wahl
Nature Published:24 May 2023
DOI:https://doi.org/10.1038/s41586-023-05894-z

Abstract

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 clocks^1,2,3. This nuclear clock will be a unique tool for precise tests of fundamental physics^4,5,6,7,8,9. 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 measured^{12,13,14,15,16}. 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 measurements^14,15,16 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.