The Exotic Isomer Th-229m and the Way to a Nuclear Clock

Long-living, excited nuclear states, so called isomers, have usually transition energies in the range of a few keV to MeV, so that they are not accessible for direct laser-induced excitation. One exception of this rule is the Th-229m isomer, predicted already in 1976, in order to explain measured γ-lines of the decay of the mother nuclide U-233. The excitation energy of this isomer was predicted to be in the range of only a few eV. The first experimental prove of its existence was achieved 2016 at LMU Munich, and in the meanwhile the value of the excitation energy could be pinned down to 8.28(17) eV

Beside the pure physics interest to investigate this exotic isomer, a possible application in the future would be the usage as frequency generator in a so-called nuclear clock. A nuclear clock would be based on the same fundamental principle as atomic clocks, which are nowadays the clocks with the highest precision, but instead of using an electronic transition of an atom, to stabilize a certain frequency, the nuclear transition of Th-229m could be used. The usage of a nuclear transition has the chance to improve the accuracy of such a clock by about one order of magnitude compared to a traditional atomic clock, because the nucleus is shielded by its electrons, making the transition more insensitive with respect to external perturbations, like electric or magnetic fields. One major requirement to build such a nuclear clock, is to have a transition in an energy range that is excitable with an appropriate laser system, which makes Th-229 so unique, because its low energy isomeric state corresponds to a wavelength in the UV region of the electromagnetic spectrum.

Our group participates in a number of international collaborations, e.g. with LMU Munich, PTB Braunschweig, UCLA Los Angeles, JILA Boulder, and the TACTICa collaboration at JGU Mainz, that follow different questions around Th-229m. Our main contribution in those collaborations are to provide the experimentalists with adequate targets. Some experiments aim for the direct excitation of the isomeric state, requesting Th-229 targets. Other approaches, aiming for some fundamental investigations of Th-229m, need U-233 samples, because a 2% branch of the alpha decay of U-233 populates the isomeric state of its daughter, being up to now the only experimental way to access Th-229m.

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