Towards a Th-229 Nuclear Clock: Measurements of the Bandgap of ThF₄ by Electron Spectroscopy Techniques

The existence of an excited nuclear state at about 8.3 eV in Th-229 has stimulated enormous interest, because it opens the possibility to develop solid-state optical nuclear clocks with unparalleled accuracy and stability. In such a device, Th-229 should be embedded in a crystalline matrix with a band gap larger than the isomer energy, in order to suppress decay by internal conversion. The crystal matrix should also be able to host Th-229 atoms in regular lattice positions, in order to minimize color center defects. Here, we present an experimental determination of the bandgap of ThF₄ performed by two different techniques. The first one is based on a combination of x-ray photoemission spectroscopy and bremsstrahlung isochromat spectroscopy. The second measurement exploited the position of the inelastic threshold in reflection electron energy loss spectroscopy. Both measurements gave compatible values of the bandgap, with the average ΔE = 10.2(2) eV [1]. This value is in excellent agreement with theoretical calculations. The measured bandgap is significantly larger than the ^229mTh excitation energy making ThF₄ a possible candidate material for a solid-state nuclear clock based on the vacuum ultraviolet γ decay.
[1] T. Gouder et al., Phys. Rev. Res. 1, 033005 (2019).