Laser excitation of the thorium-229 nuclear isomeric transition in a solid-state host

LiSrAlF₆ crystals doped with ²²⁹Th are used in a laser-based search for the nuclear isomeric transition. Two spectroscopic features near the nuclear transition energy are observed. The first is a broad excitation feature that produces red-shifted fluorescence that decays with a timescale of a few seconds. The second is a narrow, laser-linewidth-limited spectral feature at 148.38219(4)_stat(20)_sys nm (2020407.3(5)_stat(30)_sys GHz) that decays with a lifetime of 568(13)_stat(20)_sys s. This feature is assigned to the excitation of the ²²⁹Th nuclear isomeric state, whose energy is found to be 8.355733(2)_stat(10)_sys eV in ²²⁹Th:LiSrAlF₆. We will report these experiments and discuss ongoing work to understand and harness the effect of the crystalline host on the isomeric transition. This work was funded by the NSF and ARO.