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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