LiSrAlF$_6$ crystals doped with $^{ 229}$Th are used in a laser-based search for the nuclear isomeric transition. Two spectroscopic features near the nuclear transition energy are observed. The first is a broad excitation feature that produces red-shifted fluorescence that decays with a timescale of a few seconds. The second is a narrow, laser-linewidth-limited spectral feature at $148.38219(4)_{ \textrm{ stat}}(20)_{ \textrm{ sys}}$ nm ($2020407.3(5)_{ \textrm{ stat}}(30)_{ \textrm{ sys}}$ GHz) that decays with a lifetime of $568(13)_{ \textrm{ stat}}(20)_{ \textrm{ sys}}$ s. This feature is assigned to the excitation of the $^{ 229}$Th nuclear isomeric state, whose energy is found to be $8.355733(2)_{ \textrm{ stat}}(10)_{ \textrm{ sys}}$ eV in $^{ 229}$Th:\thor:LiSrAlF$_6$.
Categories: physics.atom-ph nucl-ex