Decay of the low-energy nuclear isomer ²²⁹Th^m(3/2⁺,3.5±1.0 eV) in solids (dielectrics and metals): A new scheme of experimental research

The decay channels and the half-life of the proposed anomalously low-energy nuclear isomer ²²⁹Th^m(3/2⁺,3.5±1.0 eV) in a dielectric and in metals are discussed. The preferred decay channel in wide energy-gap dielectrics is via nuclear γ emission in the optical range. The isomer’s half-life lies in the range 10 min–1 h in ²²⁹ThO₂ for the spectral range ω_N=4.5–2.5eV. 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 ²²⁹ThO₂. The second step is the excitation of a large number of the isomeric nuclei ²²⁹Th^m(3/2⁺,3.5±1.0 eV) from the ground state by laser radiation in thorium dioxide, and investigation of the α decay of the isomeric level.