Nuclear Physics with a Laser: 229Thorium

The aim of the START 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, its energy is predicted 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 establish a high-stability optical frequency standard.In the project we have successfully produced Thorium-doped Calcium Fluoride single crystals, a material that didnt exist before. We could show, that the crystal indeed accepts the doping and shows a largely homogeneous dopant distribution. 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 around the world.In the planned spectroscopy and laser stabilization experiments, the signal-to-noise ratio will be the critical point, due to the extremely low light-matter interaction cross section of nuclei. Background is produced by the inherent radioactivity of the material (radio luminescence) and the materials response to intense UV illumination (induced luminescence).We could show that the luminescence spectrum of doped crystals is not depending on the doping but depends on the host crystal properties alone. In Calcium Fluoride, the luminescence in the UV range is determined by a self-trapped exciton line 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 good signal-noise-ratio.Spectroscopy experiments are currently performed in a variety of collaborations, most notably two synchrotron facilities (MLS Berlin and Spring-8 Japan). We are confident that these activities will yield a precise energy measurement of the low-energy isomer state in Thorium-229.The START project has lead to numerous collaborations and established a European community on Thorium research. This has culminated into a successful H2020 FET-Open project nuClock (coordinated by PI Schumm) with 8 European Partners, that has started end 2015. The START grant has hence indeed developed the intended incubator effect.