Results of a direct search using synchrotron radiation for the low-energy ²²⁹Th nuclear isomeric transition

We report the results of a direct search for the $^{229}$Th ($I^{p} = 3/2^+\leftarrow 5/2^+$) nuclear isomeric transition, performed by exposing $^{229}$Th-doped LiSrAlF$_6$ crystals to tunable vacuum-ultraviolet synchrotron radiation and observing any resulting fluorescence. We also use existing nuclear physics data to establish a range of possible transition strengths for the isomeric transition. We find no evidence for the thorium nuclear transition between $7.3 {eV}$ and $8.8 {eV}$ with transition lifetime $(1-2){s} \lesssim \tau \lesssim (2000-5600){s}$. This measurement excludes roughly half of the favored transition search area and can be used to direct future searches.