Optical Spectroscopy of an Atomic Nucleus: Progress toward Direct Observation of the 229Th Isomer Transition
CALIFORNIA UNIV LOS ANGELES DEPT OF PHYSICS AND ASTRONOMY
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The nucleus of the thorium-229 isotope possesses a first excited nuclear state 229mTh at an exceptionally low energy of 7.8 or - 0.5 eV above the nuclear ground state 229gTh, 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 229mTh state directly by luminescence spectroscopy in the vacuum ultraviolet spectral region. The estimated natural linewidth of the 229gTh if and only if 229mTh isomer transition of 2 pi x 0.1 to 2pi x 10 mHz is expected to broaden to approximately 10 kHz for 229Th4 doped into a suitable crystal. The factors governing the choice of crystal system and the substantial challenges in acquiring a sufficiently large quantity of 229Th are discussed. We show that the 229gTh if and only if 229mTh 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 229Th in nuclear clocks. We also show that 233U-doped materials should produce an intrinsic, continuous, and sufficiently high rate of 229mTh yields 229gTh luminescence and could be a useful aid in the initial direct search of the isomer transition.
- Nuclear Physics and Elementary Particle Physics
- Solid State Physics