Photofission and photoneutron cross sections for U-238 and Th-232

A specific objective of the recent IAEA Coordinated Research Project on Photonuclear Data and Photon Strength Functions (Code F41032; Duration 2016-2019) has been to measure photonuclear cross-section data where needed, for unexplored nuclei and cases of discrepant existing data. A dedicated experimental campaign has been conducted at the laser Compton-scattering gamma-ray source of the NewSUBARU synchrotron radiation facility of SPring8, Japan, where photoneutron reactions for 11 nuclei from Be-9 to Bi-209 have been investigated in the Giant Dipole Resonance energy region. The measurements followed the development of a flat-efficiency moderated neutron detection array and the associated neutron-multiplicity sorting techniques. The IAEA CRP campaign has been followed at NewSUBARU by new measurements of photofission and photoneutron reactions on U-238 and Th-232 in the energy range of 5.87 MeV 20.14 MeV. The neutron -multiplicity sorting of high-multiplicity fission neutron coincidence events has been performed using a dedicated energy dependent, multiple firing statistical treatment. The photoneutron (gamma, xn) and photofission (gamma, fxn) reactions have been discriminated by considering a Gaussian distribution of prompt-fission-neutrons (PFN) multiplicities predicted by the evaporation theory. We here provide preliminary experimental (gamma, n), (gamma, 2n) and (gamma, F) cross sections, average energies of PFN and of photoneutrons emitted in (gamma, n) and (gamma, 2n) reactions, as well as the mean number of PFN per fission act. The new U-238 cross sections are compared with recent statistical -model calculations performed with the EMPIRE code on existing data.