Final Report Summary - NAC (Nuclear Atomic Clock) The aim of the „Nuclear Atomic Clock - NAC“ project was to realize a novel frequency standard based on a low-energy nuclear transition in Thorium-229. This nuclear transition has never been directly observed, it’s energy is predicted from indirect measurements only with a large uncertainty. The predictions places it at 7.8 eV, corresponding to a wavelength of 160 nm un the ultraviolet (UV) regime.Our approach was to implement a „solid-state clock“ using crystals which are transparent in the UV range and dope those with the Th-229, to provide a “frozen” ensemble of nuclei for a variety of spectroscopy methods. Ultimately, we aim to frequency-stabilize a laser to this transition to provide a high-stability optical frequency standard.In the project we have successfully (for the first time) produced Thorium-doped Calcium fluoride single crystals in an specifically developed in-house growing system. We could show, that the crystal indeed accepts the doping (limited only be the accessible amount of Th-229) and shows a largely homogeneous distribution of the material. Furthermore, we could show theoretically and experimentally, that the doping does not change the favourable optical properties (UV transmission), which was not clear from the beginning. This technology is now established and we provide these samples to a variety of collaboration groups.In a spectroscopy experiment, and especially in a nuclear spectroscopy with extremely low light-matter interaction cross section, signal-to-noise is a crucial point. In particular, a crystal doped with radioactive material “glows” all by itself, due to radioluminescence. Furthermore, exposure to high-energy spectroscopy light might induce additional luminescence. We could show that the luminescence spectrum of doped crystals is not depending on the doping but depends on the host bulk properties alone. In Calcium Fluoride, the luminescence in the UV range is determined by a self-trapped exciton lime around 230 nm, there is no light emitted below that. We have hence a window between about 200 nm down to 140 nm (transmission window of Calcium Fluoride) where spectroscopy can be performed with high signal-noise-ratio.We have established two collaborations with synchrotron groups (MLS Berlin and Spring-8 Japan), measurement setups are installed at the specific beam lines and the spectroscopy search has begun. At this stage, we can not report a measurement of the Thorium-229 isomer state energy, we are confident that the search will be successful soon.To lay the foundations for a high-resolution spectroscopy and the “clockwork” for a nuclear clock, we have established a frequency comb system in the ultraviolet regime. This system uses high-harmonic generation (5th harmonic of a 800 nm TiSa femtosecond laser = 160 nm) in a Xenaon gas jet inside a passive enhancement cavity. We have developed a novel 3D resonator design, that overcomes some problems of astigmatism and which increases the accessible enhancement. 5th harmonic light at 160 nm has been generated successfully. To further enhance the yield, we are currently investigating using electronically pre-excited noble gases in the jet.The efforts during the NAC ERC project have produced numerous collaborations and established a European community on Thorium research. This has culminated into a successful H2020 FET-Open project “nuClock” (coordinated by ERC PI Schumm) with 8 European Partners, that has started end 2015. The ERC Starting grant has hence indeed developed the intended incubator effect.
