Hyperfine studies of Th-229 in its nuclear ground and isomeric state

The 229Th nucleus possesses an exceptionally low-lying isomeric state of about 8eV, about four orders of magnitude lower than common nuclear excitation energies. This state is coupled to the ground state by a magnetic dipole transition with an estimated linewidth of less than 1mHz. It has been proposed to use this unique transition as a reference for a new type of optical clock. Such a nuclear clock is expected to be highly immune to perturbations from external electromagnetic elds, as well as a sensitive probe for temporal variations of fundamental constants. A non-destructive optical detection method for the isomeric state is a key feature in the ongoing experimental search for the direct excitation of the nucleus, as well as the future clock operation. The realization of such a detection scheme based on hyperne spectroscopy is the main focus of this thesis. To this end, the hyperne structures of several electronic transitions in 229Th+ and 229Th2+ are investigated. Their splitting constants, as well as their isotope shift with respect to 232Th are measured using laser spectroscopy. Measurements are also performed on thorium ions produced by the α decay of 233U, where 2% of the thorium ions are in the isomeric state. This led to the rst optical detection of the isomer via hyperne spectroscopy and to the measurement of its nuclear magnetic dipole moment µ = −0.37(6)µN, its electric quadrupole moment Qs = 1.74(6)eb, as well as the increase in mean squared charge radius compared to the nuclear ground state δhr2i229m,229 = 0.0107(16)fm2. Based on these values, a rst experimental estimation on the sensitivity of the nuclear transition to temporal variations of the ne structure constant α is given. In a further experiment, the excitation of the isomeric state in 229Th2+ via electronic bridge processes is investigated. Selected electronic states of 229Th2+ are addressed in a two-step excitation scheme to search for a coupling to the nuclear transition in the energy range from 7.14eV to 10.46eV. Since no excitation has been observed so far, limits on the detectable excitation rate are given.