Intermediate structures in sub-barrier fission

The formal theory of intermediate structure in nuclear reactions is applied to the problem of subthreshold fission processes induced by a neutron beam. Explicit expressions are obtained for the energy-averaged cross section, related to gross structure experiments, for the distribution of fission widths in the resolved-fine-structure analysis and for the neutron width distribution in the resonance region. Some useful sum rules are also easily found. From a comparison with experimental data it is possible to extract the positions of resonances as well as the widths Γ^↑ and Γ^↓ which are closely related to the height of the two fission barriers. Quantitative results are shown for the ²³⁷Np, ²⁴⁰Pu, and ²⁴²Pu cases. In ²³⁷Np we get the first barrier higher than the second one, E_A>E_B. The opposite result is obtained for the ²⁴²Pu case while the ²⁴⁰Pu, with E_A~=E_B, seems to be a transitional nucleus between the two situations. NUCLEAR REACTIONS, FISSION ²³⁷Np(n, f), ²⁴⁰Pu(n, f), ²⁴²Pu(n, f) calculated σ(E) and <Γ_fD_f>. Deduced the widths Γ^↑ and Γ^↓ for the two fission barriers.