Direct search for the thorium-229 nuclear isomeric transition with a pulsed VUV laser

The nucleus of thorium-229 has an exceptionally low-energy isomeric transition in the vacuum-ultraviolet (VUV) spectrum around $7.8 \pm 0.5$eV [1]. While inaccessible to standard nuclear physics techniques, there are various prospects for a laser-accessible nuclear transition. Our direct search for the transition uses thorium-doped crystals as samples. In a previous experiment [2] at the Advanced Light Source (ALS) synchrotron, LBNL, we were able to exclude a large portion of the transition lifetime-vs.-frequency region-of-interest (ROF) [3]. Here, we will report on our ongoing efforts of a search using a pulsed VUV laser system as light source, which allows us to enhance our sensitivity up to $10^4 \times$ over the ALS and extend the accessible frequency range over the entire ROF [3]. An updated exclusion region will be presented. \\[2ex] {[1]} B. R. Beck et al.: LLNL-PROC-415170 (2009)\\ {[2]} J. Jeet et al.: Phys. Rev. Lett. 114, 253001 (2015)\\ {[3]} E. V. Tkalya et al.: Phys. Rev. C 92, 054324 (2015)