Optical Spectroscopy of an Atomic Nucleus: Progress toward Direct Observation of the 229Th Isomer Transition

Abstract The nucleus of the thorium-229 isotope possesses a first excited nuclear state ( 229m Th) at an exceptionally low energy of 7.8±0.5 eV above the nuclear ground state ( 229g Th), as determined by earlier indirect measurements. This is the only nuclear excited state known that is within the range of optical spectroscopy. This paper reports progress toward detecting the 229m Th state directly by luminescence spectroscopy in the vacuum ultraviolet spectral region. The estimated natural linewidth of the 229g Th↔ 229m Th isomer transition of 2 π ×0.1 to 2 π ×10 mHz is expected to broaden to ∼10 kHz for 229 Th 4+ doped into a suitable crystal. The factors governing the choice of crystal system and the substantial challenges in acquiring a sufficiently large quantity of 229 Th are discussed. We show that the 229g Th↔ 229m Th transition energy can be identified to within 0.1 nm by luminescence excitation and luminescence spectroscopy using the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory. This would open the door for subsequent laser-based measurements of the isomer transition and future applications of 229 Th in nuclear clocks. We also show that 233 U-doped materials should produce an intrinsic, continuous, and sufficiently high rate of 229m Th→ 229g Th luminescence and could be a useful aid in the initial direct search of the isomer transition.