Intermediate structure and the shape isomer in 233Th

Abstract With a neutron-capture experiment the s-wave resonances in 233 Th between 50 eV and 4.2 keV were investigated for delayed γ-γ coincidences. At least two intermediate structures are found in the distribution of the relative delayed γ-yield R at about 1 and 2 keV incident neutron energy confirmed by statistical tests. We interpret them as being due to the population of a shape-isomeric state via class-II states admixed to the resonances. From the derived class-II spacing D II and the gross shape of the structures the isomeric ground-state energy E II as well as the coupling width Γ↓ may then be estimated. These results are compatible with the decay of a shape isomer with a half life between 1 and 100 ns. They also would suggest that the height of the inner potential barrier E A is less than or of the order of the neutron binding energy, and that the inner barrier is relatively thin.