Growth and characterization of (doped) calcium fluoride crystals for the nuclear spectroscopy of Th-229

Abstract

Thorium-229 provides the unique feature of a nuclear transition with an exceptionally low energy. The isomer, i.e. the excited state of the transition, is predicted to be only a few eV above the ground state, making typical atomic physics tools like lasers applicable for nuclear spectroscopy. Possible applications are a nuclear optical clock with unprecedented stability, an optical gamma laser or a sensitive system for probing variations of fundamental constants. Our experimental approach consists in embedding thorium into the ionic crystal lattice of calcium fluoride. CaF2 provides a simple crystal structure and still transparent in the UV region down to 125 nm. It allows doping with a large numbers of thorium atoms, thereby facilitating a spectroscopic search for the nuclear transition. In this thesis, a furnace for the growth of small-volume and highly-doped calcium fluoride crystals was developed. The vertical gradient freeze method was applied for crystal growth. High doping concentrations were demonstrated for Th:CaF2. The absolute concentration and the distribution of the dopant in the crystals have been determined by neutron activation analysis and gamma spectroscopy. It was shown by UV spectroscopy that the thorium-doped CaF2 crystals remain sufficiently transparent in the UV region, making nuclear excitation by lasers or synchrotrons possible, as well as the detection of gamma photons emitted during deexcitation of the Th-229 isomer. These photons have to be distinguished from the radio- and photoluminescence background of the doped crystals, which was also characterized in this thesis.