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M. Schreitl, G. Winkler, G. Steinhauser, G. Kazakov, Thorsten Schumm:
"Towards a nuclear clock with ²²⁹Thorium";
Poster: 487. WE-Heraeus-Seminar: Astrophysics, Clocks and Fundamental Constants (ACFC 2011), Bad Honnef; 2011-07-18 - 2011-07-21.

The current time standard uses a hyperfine transition of the electronic ground state of Cesium. An improvement of the clock´s Q-factor by some orders of magnitude could be achieved by exploiting transitions with higher energy differences, e.g. in the optical range. However, challenges like Doppler-broadening due to the thermal movement of the atoms and finding a sufficiently long-living state which provides a narrow line width have to be mastered. Promising approaches are Sr lattice clocks [1] and Hg+ ion clocks [2].
Nuclear transitions are much more stable against external perturbations due to the shielding of the electrons and offer a huge range of possible lifetimes. Usually the energy of nuclear states are in the range of keV or even MeV which reflects in entirely different tools and methods compared to atomic physics: particle accelerators instead of lasers for state manipulation.
The isotope Thorium-229 however is predicted to provide a unique low-energy excited state which is separated by only 7.6±0.5 eV [3] from the ground state and thereby in the range of UV lasers which would make coherent manipulation of this nuclei possible. An expected lifetime of several hours [3] makes this transition an excellent candidate for a new time standard, with a potential to outperform existing clocks by orders of magnitudes.
Our experimental approach consists in embedding 229Th4+ in the UV transparent crystal structure of Calcium fluoride (CaF2). This provides the advantages of having room-temperature solid-state sample with a great number of nuclei (crystal with up to 1018 nuclei/cm3 of the chemically identical 232Th are already available). Furthermore this method does not require a complex experimental setup like in atom or ion traps and the small crystal which hosts the nuclei of interest can easily be combined with high flux excitation sources like UV lamps and synchrotrons. Since the Thorium4+-ions in the crystal lattice are expected to be confined in the Lamb-Dicke regime, the nuclei experience no sensitivity to recoil or first-order Doppler effects.
The next steps in our experiment include the characterization of the crystals doped with 232Th in order to ensure the transparency in the relevant wavelength region, the reliable substitution of the Thorium ions in the crystal lattice and the role of defects. A broadband UV-lamp will subsequently be used to try to excite the predicted nuclear transition and detect fluorescence in a crystal doped with 229Th. A frequency comb is currently set up which will be transferred to the 160nm region by a high-harmonic generation build-up cavity and will be used as a precision measurement tool for comparison of the nuclear transition to other frequency standards.
Current theories that attempt to unify gravity with the other fundamental forces can lead to spatial and temporal variation of fundamental constants [4]. Nuclear energy states are mainly affected by the strong interaction and Coulomb repulsion. A precise measurement of the uniquely low nuclear transition frequency in 229Th will therefore allow to measure possible variations of the fine-structure constant with increased precision [5, 6] since the variations in a nucleus are enhanced compared to atomic transitions.
References:
[1] A. D. Ludlow et al, Science 319, 1805 (2008)
[2] T. Rosenband et al., Science 319, 1808 (2008)
[3] B. R. Beck et al., Phys. Rev. Lett. 98, 142501 (2007)
[4] K. Olive and Y. Qian, Phys. Today 57, 10, 40 (2004)
[5] V. V. Flambaum, Phys. Rev. Lett. 97, 092502 (2006)
[6] X.-T. He, Z.-Z. Ren, Nucl. Phys. A 806, 117 (2008)

time standard, Thorium, nuclear clock, Calcium fluoride, variation of fine-structure constant