Prospects for a Nuclear Optical Frequency Standard based on Thorium-229
  The 7.6-eV-isomer of Thorium-229 offers the opportunity to perform high resolution laser spectroscopy of a nuclear transition. We give a brief review of the investigations of this isomer. The nuclear resonance connecting ground state and isomer may be used as the reference of an optical clock of very high accuracy using trapped and laser-cooled thorium ions, or in a compact solid-state optical frequency standard of high stability. 
E. Peik, K. Zimmermann, M. Okhapkin, Chr. Tamm