The nuclear excitation of Thorium-229 in the CaF2 environment : Development of a crystalline nuclear clock

Abstract

The 229Th nucleus has the lowest known 1rst nuclear excited state out of all isotopes.The transition energy from ground to1rst excited state was measured to be between 7.88 eV and 8.47 eV (1 sigma), or between 146.4nm and 157.3 nm. The lifetime of this state depends heavily on the chemical environment due to the two competing nuclear decay processes: Radiative photon emission and internal conversion. In internal conversion the energy of the excited nucleus is transferred to a bound electron of the atom which is promoted to the continuum. Due to the nuclear structure and selection rules of 229Th, photon emission is strongly suppressed while internal conversion is preferred if energetically allowed.The low energy of the 1rst nuclear excited state is too low for internal conversion to take place if the charge state of the atom is higher than 1+. Above 1+, this 1rst nuclear excited state is long lived with an estimated lifetime of around 5000 seconds. Metastable nuclear states are commonly referred to as isomers and are denoted as 229mTh.The low energy of 229mTh has sparked many ideas for application over the years. Out of all isotopes, 229Th seems to have the only nuclear excited state accessible by lasers and frequency combs. This allows to probe the nuclear level structure with a precision unprecedented in nuclear physics. The most sought after application for this unique state is currently to build an optical clock. The low energy nuclear excited state even allows to build a solid state optical clock by doping the 229Th in a large bandgap material, such as CaF2. The CaF2 single crystal is transparent up to 122.5 nm, hence transparent to the predicted emission wavelength of the isomer. Doping 229Th into CaF2 thus allows optical excitation and detection of the nucleus. The 229Th will substitutionally replace the Ca in the crystal lattice, which ensures a 4+ oxidation state and a long lifetime. Although the nucleus couples to its environment, it allows for a high precision clock to be built even in a solid state environment due to the high number of nuclei (>1e16) in the doped crystal. Such a clock would be a highly sensitive probe to new physics due to the unique nature of the nuclear excitation.In this thesis, the doping and nuclear excitation of 229Th into CaF2 is investigated. The crystal environment is characterized through crystal growing and vacuum ultraviolet (VUV) optical techniques. The nuclear excitation is investigated through viability calculations using experimental characterizations of three distinctly different nuclear excitation methods: Resonant VUV photon irradiation, nuclear decay and x-ray irradiation. A detailed description is given on how cylindrical 229Th doped CaF2 single crystals were grown (3.2mm diameter, 1 cm length). The size of crystals was reduced by a factor of 200 compared to previous work.