Results of a direct search for the thorium-229 nuclear isomeric transition

The nucleus of thorium-229 has an exceptionally low-energy isomeric transition in the vacuum-ultraviolet spectrum around $7.8 \pm 0.5$eV [1]. The prospects of a laser-accessible nuclear transition are manifold but require spectroscopically resolving the transition. Our approach is a direct search using thorium-doped crystals as samples and exciting the isomeric state with vacuum-ultraviolet synchrotron radiation [2]. In a recent experiment, we were able to search for the transition at the Advanced Light Source synchrotron, LBNL, between $7.3$eV and $8.8$eV. We found no evidence for the transition within a lifetime range of 1--2s to 2000--5600s [3]. This result excludes large parts of the theoretically expected region. We conclude reporting on our efforts of a search using laser-generated vacuum-ultraviolet light. \\[4pt] [1] B. R. Beck et al.: LLNL-PROC-415170 (2009)\\[0pt] [2] W. G. Rellergert et al.: Phys. Rev. Lett. 104, 200802 (2010)\\[0pt] [3] J. Jeet et al.: arXiv 1502.02189 (2015)