Characterization of the Th-229 nuclear clock transition

Of all presently known about 180,000 excited nuclear states, the first isomeric excited state of Th-229 (called Th-229m or 'thorium isomer') has the lowest excitation energy of less than 10 eV. Its energy is accessible with today's laser technology and therefore allows for direct laser excitation of a nucleus. This opens up a new field of precision nuclear spectroscopy leading to a multitude of applications, such as the realization of a nuclear clock, which, besides utilization in relativistic geodesy or satellite-based navigation, could be employed in dark-matter research or the investigation of a potential temporal variation of fundamental constants. A long-standing problem, however, was that not much was known about the properties of the excited state. Therefore the goal of the thesis was to provide measurements of the energy and lifetime of the first excited state in Th-229. The lifetime of the Th-229 nuclear isomer depends strongly on its charge state, as different charge states make it possible to energetically open or close decay channels. There exist two main decay channels, gamma-decay and internal conversion (IC), where the latter proceeds several orders of magnitude faster than the former. In this decay channel the nucleus interacts with the electronic shell of the atom and transfers the energy of the excited nuclear state to a shell electron, thereby ionizing the atom. The kinetic energy of the expelled electron allows to conclude on the isomer's energy. For Th-229m this is only possible in a neutral atom, as only in this case the nuclear excitation energy exceeds the electron binding energy. Within the scope of this thesis, the IC decay channel was used to investigate the properties of the isomer. First measurements of the lifetime of Th-229m in neutral, surface-bound atoms were realized. Additionally, energy measurements using the emitted internal conversion electrons were performed. The results will allow to develop a laser that can be used for a direct optical excitation of the first excited nuclear state in Th-229 and thus for the realization of nuclear clock. The experimental setup that is employed in this thesis consists of a buffer-gas stopping cell that is used to thermalize and extract Th-229(m) ions from a U-233 alpha-recoil source. In order to measure the lifetime of Th-229m following the decay by internal conversion and to increase the signal-to-noise ratio for energy measurements, phase-space cooled Th-229(m) ion bunches are generated in a radio-frequency quadrupole (RFQ) ion buncher. For lifetime measurements, the ions are collected directly on the surface of an MCP detector, where they neutralize and subsequently decay via internal conversion. A half-life of 7±1 µs was measured. Moreover, a strong dependence of the lifetime on the electronic environment of the nucleus could be shown. In order to measure the excitation energy of the isomeric state, Th-229(m) ions are sent through a bi-layer of graphene for neutralization. They continue their flight as neutral atoms through a magnetic-bottle type retardation electron spectrometer. The isomeric state decays in-flight and the kinetic energy of the emitted internal conversion electrons can be measured with the spectrometer. By a careful analysis and comparison with theoretical spectra it is possible to measure the isomeric energy to 8.28±0.17 eV. Thus, the present reference value of 7.8±0.5 eV, measured indirectly more than 10 years ago, could be replaced with threefold improved precision. The thesis is structured as follows: The first chapter provides an introduction to the isomeric first excited state in Th-229, gives a short overview on previously performed energy measurements, summarizes current experimental approaches and outlines possible applications. The second chapter summarizes the theoretical background. The experimental setup is detailed in the third chapter and its characterization in preparatory measurements is described in chapter 4. Lifetime and energy measurements are presented in chapter 5 and 6, respectively. The last chapter provides a conclusion and an outlook