A possibility of the amplification of the 7.6 eV γ radiation by the stimulated γ emission of the ensemble of the (229m)Th isomeric nuclei in a host dielectric crystal is proved theoretically. This amplification is a result of (1) the excitation of a large number of (229m)Th isomers by laser radiation, (2) the creation of the inverse population of nuclear levels in a cooled sample owing to the interaction of thorium nuclei with the crystal electric field or with an external magnetic field, (3) the emission or absorption of the optical photons by thorium nuclei in the crystal without recoil, and (4) the nuclear spin relaxation through the conduction electrons of the metallic covering.

We report the results of a direct search for the 229 Th (I p = 3/2 + ← 5/2 + ) nuclear isomeric transition, performed by exposing 229 Th-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 τ (2000-5600) s. This measurement excludes roughly half of the favored transition search area and can be used to direct future searches.

The decay channels and the half-life of the proposed anomalously low-energy nuclear isomer 229 Th m (3/2 ϩ ,3.5Ϯ1.0 eV) in a dielectric and in metals are discussed. The preferred decay channel in wide energy-gap dielectrics is via nuclear ␥ emission in the optical range. The isomer's half-life lies in the range 10 min-1 h in 229 ThO 2 for the spectral range N ϭ4.5-2.5 eV. Nonradiative decay channels dominate in metals. A two-step experimental scheme is developed. The first step is the excitation of the low-energy isomeric level by synchrotron radiation via the 29-and 72-keV levels, and precise measurement of the wave length of the low-energy nuclear transition in 229 ThO 2 . The second step is the excitation of a large number of the isomeric nuclei 229 Th m (3/2 ϩ ,3.5Ϯ1.0 eV) from the ground state by laser radiation in thorium dioxide, and investigation of the ␣ decay of the isomeric level.

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