Measurements and Calculations of the Neutron Emission Spectra from Materials Used in Fusion-Fission Reactors

Spherical assemblies of copper with radii (R) equal to 1.0, 3.0, and 5.0 mean-free-paths (mfp) for 14-MeV neutrons, niobium (R = 0.9 and 3.2 mfp), ²³²Th (R = 1.0 mfp), and ²³⁸U (R = 1.0 and 3.1 mfp) have been bombarded with a centered nominal 14-MeV neutron source. The neutron leakage spectra have been measured from 0.8 to 14 MeV using a stilbene scintillator, time-of-flight techniques, pulse-shape discrimination and ∼10-m flight paths. The measured spectra are compared with calculations carried out with TARTNP, a coupled neutron-photon Monte Carlo transport code, using the ENDF/B-IV and -V neutron libraries. For copper and ²³⁸U, the reevaluated cross sections at ∼14 MeV in ENDF/B-V are somewhat less satisfactory in fitting the experimental results than those in ENDF/B-IV. For ²³²Th, ENDF/B-V shows significant improvement. The niobium cross sections, which were not reevaluated between ENDF/B-IV and -V, provide poor fits to the measurements.