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 Th-229 (I-pi = 3/2(+) <- 5/2(+)) nuclear isomeric transition, performed by exposing Th-229-doped LiSrAlF6 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 less than or similar to tau less than or similar to (2000-5600) s. This measurement excludes roughly half of the favored transition search area and can be used to direct future searches.