Thorium-229

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.

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. 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. The half life of this excited state is not known, though it is estimated at 5 hours.