Transmission of Fast Neutrons Through an Iron Sphere

Integral experiments have been performed using a homogeneous iron spherical shell to test neutron cross-section data. Neutron leakage spectra from the shell were measured using 252Cf-fission and (deuterium-tritium) D-T-fusion neutron sources and an NE-213 spectrometry system. An associated particle detector was used to monitor the absolute D-T neutron source strength as well as any accompanying deuterium-deuterium neutron contamination. The leakage spectra were calculated using the continuous-energy Monte Carlo code VIM and the discrete ordinates Sn code ANISN employing ENDF/B-IV. For neutron energies between 1 and 5 MeV, the calculations underpredicted the leakage spectrum by factors of 1.4 to 2 for the californium neutron source and of 2 to 3 for the D-T neutron source. The large discrepancies are attributed to inadequate representation of cross-section resonance structure (namely, minima); inadequate representation of the angular and secondary energy distributions for continuum inelastic scattering and (n,2n) reactions also contribute to these discrepancies.