Measurement of the Uranium-238 Subthreshold Fission Cross Section for Incident Neutron Energies Between 0.6 and 100 keV

We have measured the neutron-induced ²³⁸U subthreshold fission cross section in the neutron energy range between 0.6 and 100 keV. A total of 28 fission clusters were identified. The well-known clusters at 721 and 1210 eV appeared resolved into their Class I components. Average ²³⁸U subthreshold fission cross sections were determined and compared with available results in the literature. We interpreted the measurement in terms of fission doorway (Class II levels) arising from the assumption of the existence of a double-humped fission barrier for the (²³⁸U + n) compound nucleus at large deformations. On the basis of this model, several fission barrier parameters were determined.