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

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

The 229Th isomer: prospects for a nuclear optical clock

The proposal for the development of a nuclear optical clock has triggered a multitude of experimental and theoretical studies. In particular the prediction of an unprecedented systematic frequency uncertainty of about $10^{-19}$ has rendered a nuclear clock an interesting tool for many applications, potentially even for a re-definition of the second. The focus of the corresponding research is a nuclear transition of the $^{229}$Th nucleus, which possesses a uniquely low nuclear excitation energy of only $8.12\pm0.11$ eV ($152.7\pm2.1$ nm). This energy is sufficiently low to allow for nuclear laser spectroscopy, an inherent requirement for a nuclear clock. Recently, some significant progress toward the development of a nuclear frequency standard has been made and by today there is no doubt that a nuclear clock will become reality, most likely not even in the too far future. Here we present a comprehensive review of the current status of nuclear clock development with the objective of providing a rather complete list of literature related to the topic, which could serve as a reference for future investigations

A laser excitation scheme for 229m^{229\text{m}}Th

Direct laser excitation of the lowest known nuclear excited state in $^{229}$Th has been a longstanding objective. It is generally assumed that reaching this goal would require a considerably reduced uncertainty of the isomer's excitation energy compared to the presently adopted value of $(7.8\pm 0.5)$ eV. Here we present a direct laser excitation scheme for $^{229\text{m}}$Th, which circumvents this requirement. The proposed excitation scheme makes use of already existing laser technology and therefore paves the way for nuclear laser spectroscopy. In this concept, the recently experimentally observed internal-conversion decay channel of the isomeric state is used for probing the isomeric population. A signal-to-background ratio of better than $10^4$ and a total measurement time of less than three days for laser scanning appear to be achievable

Extending Our Knowledge about the Th-229 Nuclear Isomer

The first nuclear excited state in Th-229 possesses the lowest excitation energy of all currently known nuclear levels. The energy difference between the ground- and first-excited (isomeric) state (denoted with Th-229m) amounts only to approximate to 8.2 eV (approximate to 151.2 nm), which results in several interesting consequences: Since the excitation energy is in the same energy range as the binding energy of valence electrons, the lifetime of Th-229m is strongly influenced by the electronic structure of the Th atom or ion. Furthermore, it is possible to potentially excite the isomeric state in Th-229 with laser radiation, which led to the proposal of a nuclear clock that could be used to search for new physics beyond the standard model. In this article, we will focus on recent technical developments in our group that will help to better understand the decay mechanisms of Th-229m, focusing primarily on measuring the radiative lifetime of the isomeric state

The theory of direct laser excitation of nuclear transitions

A comprehensive theoretical study of direct laser excitation of a nuclear state based on the density matrix formalism is presented. The nuclear clock isomer $^{229\text{m}}$Th is discussed in detail, as it could allow for direct laser excitation using existing technology and provides the motivation for this work. The optical Bloch equations are derived for the simplest case of a pure nuclear two-level system and for the more complex cases taking into account the presence of magnetic sub-states, hyperfine-structure and Zeeman splitting in external fields. Nuclear level splitting for free atoms and ions as well as for nuclei in a solid-state environment is discussed individually. Based on the obtained equations, nuclear population transfer in the low-saturation limit is reviewed. Further, nuclear Rabi oscillations, power broadening and nuclear two-photon excitation are considered. Finally, the theory is applied to the special cases of $^{229\text{m}}$Th and $^{235\text{m}}$U, being the nuclear excited states of lowest known excitation energies. The paper aims to be a didactic review with many calculations given explicitly