Fast Neutron Spectrum for a Subcritical Section of a Homogeneous ²³⁵U-Polyethylene Critical Assembly

The neutron spectrum for a subcritical section of the thermionic critical experiment, Assembly No. 1, was measured by the pulsed source, time-of-flight method. The measurement was performed in a calculable slab configuration so that the results would be directly applicable to the critical assembly. The measured spectrum was compared with multigroup transport calculations based upon various cross-section data sets and cross-section treatments. The results demonstrate the need for consideration of unresolved resonance self-shielding in ²³⁵U, and suggest that the ²³⁵U absorption cross-section data may be slightly high at low energies (100 eV < E < 2 keV).