Title: Hyperfine studies of Th-229 in its nuclear ground and isomeric state
Authors: Thielking, Johannes, Physikalisch-Technische Bundesanstalt (PTB), Fachbereich 4.4 Zeit und Frequenz, ORCID: 0000-0002-4511-2407
Contributors: HostingInstitution: Physikalisch-Technische Bundesanstalt (PTB), ISNI: 0000 0001 2186 1887
Resource Type: Text / Dissertation
Publisher: Physikalisch-Technische Bundesanstalt (PTB)
Rights: Download for personal/private use only, if your national copyright law allows this kind of use.
Relationships: IsPartOf: ISSN 0341-6712
IsIdenticalTo: ISBN 978-3-95606-527-9
Dates: Available: 2020-08-31
Created: 2020-06
File: Download File (application/pdf) 33.31 MB (34928165 Bytes)
MD5 Checksum: 91b0c5e77b4f96fb238413fcc9d7cdbf
SHA256 Checksum: 0adc5c1542bdd8fc453dcab4374fc11b43061b0f182edb0d801a72c3a2f8ffef
Keywords: Thorium ; Th-229 ; nuclear clock ; hyperfine spectroscopy ; electronic bridge
Abstract: The Th-229 nucleus possesses an exceptionally low-lying isomeric state of about 8 eV, 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 1 mHz. 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 fields, 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 hyperfine spectroscopy is the main focus of this thesis. To this end, the hyperfine structures of several electronic transitions in singly and doubly charged Th-229 are investigated. Their splitting constants, as well as their isotope shift with respect to Th-232 are measured using laser spectroscopy. Measurements are also performed on thorium ions produced by the alpha decay of U-233, where 2% of the thorium ions are in the isomeric state. This led to the first optical detection of the isomer via hyperfine spectroscopy and to the measurement of its nuclear magnetic dipole moment, its electric quadrupole moment, as well as the increase in mean squared charge radius compared to the nuclear ground state. Based on these values, a first experimental estimation on the sensitivity of the nuclear transition to temporal variations of the fine-structure constant is given. In a further experiment, the excitation of the isomeric state in doubly charged Th-229 via electronic bridge processes is investigated. Selected electronic states are addressed in a two-step excitation scheme to search for a coupling to the nuclear transition in the energy range from 7.14 eV to 10.46 eV. Since no excitation has been observed so far, limits on the detectable excitation rate are given.
Citation: Thielking, Johannes. Hyperfine studies of Th-229 in its nuclear ground and isomeric state, 2020. Physikalisch-Technische Bundesanstalt (PTB). DOI: https://doi.org/10.7795/110.20200831