Search Efforts for the Thorium-229 Nuclear Isomeric Transition

Unique among all known nuclei, ²²⁹Th has an exceptionally low-energy isomeric transition in the vacuum-ultraviolet (VUV) spectrum around 8 eV [1,2]. The prospect of a laser accessible nuclear system has inspired proposals for a "nuclear clock" based on the $^{229}$Th nuclear transition [3]. However, these applications can only be realized if the transition energy is known more precisely.

One of our efforts to determine the energy of the isomeric transition has been a sustained  campaign to directly excite the transition by firing a VUV laser into a LiSAF crystal doped with ²²⁹Th. This doped-crystal scheme is an attempt to directly observe the VUV radiative decay of the isomer.

To supplement this effort a laser experiment is being developed to leverage the shorter-lived internal conversion (IC) decay channel of the isomer. A ²²⁹Th doped metal target will be illuminated with our VUV laser system. The excited nuclei should then impart their energy to electrons in the metal. By scanning the frequency of our VUV laser, the excitation of the nucleus can then be detected a microsecond time-scale electron signal emerging from the metal [4].

We will also report on the progress of a lower fidelity measurement of the isomeric decay using several superconducting nano-wires. A ²³³U source has been mounted near several SNSPI pixels inside of a cryogenic apparatus. As ²³³U undergoes α-decay, a fraction of the resulting ²²⁹Th ends up in the isomeric state. The initial impact of the ²²⁹Th destroys the superconducting state in a local region of the nano-wire, generating a detectable "click". It is expected that within a few microseconds, a subsequent "click" is observed at the same location in the nanowire. By comparing our sensitivity to these secondary clicks with VUV photon measurements in the same nanowires, the isomeric transition energy can be bounded. This measurement will then aid in setting a tighter bound on where to focus our efforts with our VUV laser system.

[1] Seiferle, B. et al. Energy of the 229Th nuclear clock transition. Nature (2019).

[2] B. R. Beck et al. LLNL-PROC-415170 (2009).

[3] Campbell, C.J. et al. Single-Ion Nuclear Clock for Metrology at the 19th Decimal Place. PRL (2012).

[4] von der Wense, L.C. et al. Hyperfine Interact (2019).