Photofission of ²³⁵U and ²³⁷Np with Bremsstrahlung Photons Below 20 MeV: Measurements of Delayed Neutron Yields and Time Spectra

Recently, new applications based on detection of delayed neutrons (DNs) or delayed photons produced during photofission have shown the need for new basic nuclear data. Indeed, available data for DNs are scarce, incomplete, and sometimes contradictory. An experimental campaign has been dedicated to DN measurements for some important actinides with a bremsstrahlung end-point energy spectrum up to 20 MeV. The experiments were carried out at the electron accelerator facility ELSA installed at the Commissariat à l'Energie Atomique (CEA) center of Bruyères-le-Châtel, France. Delayed neutrons were measured by 12 ³He counters embedded in an annular polyethylene cylinder. Delayed neutron absolute yields were determined. Different irradiation-decay time combinations allowed the extraction of the averaged six-group parameters of DNs. Similarly to the nuclear reactor physics, these groups are formed by lumping DN precursors according to their half-lives. The DN time dependence is then expressed by the sum of group contributions.

In this paper, DN absolute yields and six-group parameters for ²³⁵U at 15 and 18 MeV and ²³⁷Np at 15 MeV are presented. The ²³⁵U and ²³⁷Np absolute DN yields are found slightly lower than those published in the literature. For ²³⁵U, the six-group parameters show no dependence on incident electron energy from 10 to 18 MeV. In addition, the group parameters are very close to those of ²³⁷Np. This work helped to resolve the discrepancies of the photofission DN parameters for ²³⁵U while the six-group parameters for ²³⁷Np are reported for the first time.