A pulsed VUV laser for the search for the thorium-229 nuclear isomeric transition

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]. We will present the technical aspects of our ongoing efforts at UCLA, including a newly developed pulsed VUV laser system with wide tunability and VUV pulse energies up to $40 \mu J$/pulse, the absolute measurement of these pulse energies, and the characterization of the frequency spectrum of the pulsed laser light. A preliminary, updated exclusion region obtained with the new experimental setup will be depicted. \\[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)