The decay channels and the half-life of the proposed anomalously low-energy nuclear isomer ${}^{229}{\mathrm{Th}}^m{(3/2\mathrm{}}^{+},3.5\mathrm{}\pm 1.0\hspace{0.5em}\mathrm{eV})$ in a dielectric and in metals are discussed. The preferred decay channel in wide energy-gap dielectrics is via nuclear $\gamma$ emission in the optical range. The isomer’s half-life lies in the range 10 min–1 h in ${}^{229}{\mathrm{ThO}}_2$ for the spectral range ${\omega }_N=4.5\mathrm{}–2.5\mathrm{eV}.$ 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 ${}^{229}{\mathrm{ThO}}_2.$ The second step is the excitation of a large number of the isomeric nuclei ${}^{229}{\mathrm{Th}}^m{(3/2\mathrm{}}^{+},3.5\mathrm{}\pm 1.0\hspace{0.5em}\mathrm{eV})$ from the ground state by laser radiation in thorium dioxide, and investigation of the $\alpha$ decay of the isomeric level.

• Received 13 January 2000

DOI:https://doi.org/10.1103/PhysRevC.61.064308