Results of a Direct Search Using Synchrotron Radiation for the Low-Energy<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi>Th</mml:mi></mml:mrow><mml:mprescripts/><mml:none/><mml:mrow><mml:mn>229</mml:mn></mml:mrow></mml:mmultiscripts></mml:mrow></mml:math>Nuclear Isomeric Transition

We report the results of a direct search for the $^{229}\mathrm{Th}$ (${I}^{\ensuremath{\pi}}=3/{2}^{+}\ensuremath{\leftarrow}5/{2}^{+}$) nuclear isomeric transition, performed by exposing $^{229}\mathrm{Th}$-doped ${\mathrm{LiSrAlF}}_{6}$ 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) $\mathrm{s}\ensuremath{\lesssim}\ensuremath{\tau}\ensuremath{\lesssim}(2000--5600)\text{ }\text{ }\mathrm{s}$. This measurement excludes roughly half of the favored transition search area and can be used to direct future searches.