Seminarium Optyczne

The thorium isotope 229 possesses a unique, low-energy nuclear isomeric state (denoted 229mTh) at8.28(17) eV [1]. This fact has stimulated the development of novel ideas in the borderland between atomicand nuclear physics. For example, the isomer could be used as an optical nuclear clock, using a -transitionas a reference - instead of a transition in the electron shell - which offers many advantages [2, 3]. Anotherapplications of this system can be: a sensitive probe of temporal variations of fundamental constants, a 3Dgravity sensor in relativistic geodesy, a sensor for detection of topological dark matter.Many experimental attempts to induce and detect an optical excitation of this isomer have failed. Thenuclear moments of the isomer 229mTh have been estimated from nuclear structure models. Apart from thespectroscopic determination of the nuclear spin and indirect measurements of the excitation energy [4, 5],no experimental data on the nuclear properties of the isomer have been available until recently. Using recoilions from the decay of 233U as a source of 229mTh, electrons emitted from the internal-conversion decay ofthe isomer in neutral thorium were detected [6], the half-life for this process was measured [7] and the mostprecise value of isomer energy 8.28(17) eV was determine [1].The availability of the isomer through recoil ions provides a way to measure the unknown nuclear propertiesof 229mTh via laser spectroscopy of electronic transitions. In this presentation we report the opticaldetection of ions in the 229mTh isomeric state and of the resolved hyperfine structure (HFS), which arisesfrom the interaction of the isomer nucleus with the valence electrons.This measurement yielded the first experimental determination of the nuclear moments and the meansquare charge radius of the isomer [8]. Our results constitute a key issue in the ongoing experimental searchfor the direct optical excitation of the nuclear transition, as well as the future nuclear clock operation.This work was supported by European Union’s Horizon 2020 Research and Innovation Programme underGrant Agreement number 664732 (nuClock), from DFG through CRC 1227 (DQ-mat, project B04) andTH956-3-2 and from the LMU Department of Medical Physics via the Maier-Leibnitz Laboratory.References[1] B. Seiferle, L. von der Wense, P. V. Bilous, I. Amersdorffer, Ch. Lemell, F. Libisch, S. Stellmer,T. Schumm, Ch. E. D¨ullmann, A. P´alffy, P. G. Thirolf, Nature, 2019, 573, pp 243-246[2] E. Peik, Chr. Tamm, Europhysics Letters, 2003, 61, pp 181-186.[3] E. Peik, M. Okhapkin, Comptes Rendus Physique 2015, 16(5), pp 516-523.[4] B. R Beck, J. A. Becker, P. Beiersdorfer, G.V. Brown, K. J. Moody, J. B. Wilhelmy, F. S. Porter,C. A. Kilbourne, R. L. Kelley, Physical Review Letters, 2007, 98, pp 142501-4.1