Th-Doped CaF2: Towards an Ion-Implanted Nuclear Clock

The extremely low-energy 229Th isomeric state (8.28 ± 0.17 eV [1]) can be used to construct a nuclear clock [2], expected to outperform current electronic-shell-based atomic clocks. One of the key issues towards a clock is obtaining a detailed understanding of (and have a grip on!) the competition between radiative and internal conversion (IC) decay channels. In the solid state approach, the latter is largely determined by the Th incorporation in a suitable lattice.

We experimentally assessed the local configuration of Th dopants in a CaF2 lattice using radioactive ion implantation (at ISOLDE, CERN). We used the emission channeling technique to study the lattice location of both 231Th (mimicking the end of the decay chain to 229Th) and of 229Ac (which decays to 229Th). In parallel experiments on these implanted crystals, the 229Th radiative decay has been observed for the first time, which constitutes a major milestone towards a solid state nuclear clock [3].

[1] T. Sikorsky et al., Phys. Rev. Lett. 125 (2020) 142503

[2] E. Peik and C. Tamm, EPL 61 (2003) 181

[3] S. Kraemer et al., Nature 617 (2023) 706-710