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

The nucleus of $^{229}$Th 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 (ROI) [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 ROI [3]. An updated exclusion region will be presented. \newline \newline [1] B. R. Beck et al.: LLNL-PROC-415170 (2009) \newline[2] J. Jeet et al.: Phys. Rev. Lett. 114, 253001 (2015) \newline[3] E. V. Tkalya et al.: Phys. Rev. C 92, 054324 (2015)