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Thorium-229 is a radioactive isotope of Thorium that decays by alpha emission with a half life of 7340 years. 229Th is produced by the decay of uranium-233, and its principal use is for the production of the medical isotopes actinium-225 and bismuth-213.[1]

Gamma ray spectroscopy has indicated that 229Th has a nuclear isomer with a remarkably low excitation energy. Recent measurements of higher-energy gamma rays give 7.6 eV as the energy of the 3/2+ state, with an uncertainty of 0.5 eV.[2] This would make it the lowest-energy nuclear isomer known, and it might be possible to excite this nuclear state using lasers with wavelengths in the vacuum ultraviolet.[3] The half life of this excited state is not known, though it is estimated at 5 hours.


  1. Report to Congress on the extraction of medical isotopes from U-233. U.S. Department of Energy. March 2001
  2. B. R. Beck; et al. (2007-04-06). "Energy splitting in the ground state doublet in the nucleus 229Th"". Physical Review Letters. 98: 142501.
  3. E. Peik and C. Tamm (2003-01-15). "Nuclear laser spesctroscopy of the 3.5 eV transition in 229Th". Europhysics Letters. 61: 181–186.