Measurement of Space Dependent Angular Thermal Neutron Spectra in Natural Uranium-Light Water Slab

Space dependent 0° angular thermal neutron spectra were measured by time-of-flight method at several positions in a spectrum source, which is a single natural uranium plate sandwiched between two layers of pure water. Theoretical analysis was performed by means of multigroup discrete S8 method, using Haywood's model for the scattering of thermal neutrons. Fairly good agreement was observed between experiment and theory. The experiment proved that; (1) The Haywoods model is valid for describing the thermal neutron spectra in heterogeneous multiplying media like the present spectrum source, under the condition that anisotropics present in the densities of either neutron scattering or the slowingdown of source neutrons to the thermal neutrons is taken into consideration. The discrepancy between the measured and calculated spectra is very small, and the error induced therefrom in the neutron absorption of 235U is below 3%. (2) The spatial distribution of the source neutrons requires to be taken into consideration, particularly in the case of thermal neutron spectra in the light water region surrounding the natural uranium plate.