Neutron Spectrum Perturbations Due to Scattering Materials and Their Effect on the Average Neutron Energy, The Spectral Index, and the Hardness Parameter

Measurements have been performed on the perturbing effect of a number of scattering materials by the ´free-field´ neutron leakage spectrum from a Godiva Type Critical Assembly (White Sands Missile Range Fast Burst Reactor). The results of these measurements are interpreted in relation to some of the general parameters characterizing a neutron environment, namely, the average neutron energy >10 KeV, the spectral index and the hardness parameter. Three neutron spectrum measurements have been performed, each under different experimental configurations of scattering materials. Results from these measurements show the following with relation to the spectral index: (1) The neutron environment on the core surface and at 12-inches from the core surface (free-field) yield a spectral index of 6.8, (2) The neutron environment behind a 4.75-inch Plexiglas plate yield 4.6 for the spectral index and (3) The neutron environment behind a 2-inch aluminum plate yield 6.7 for the spectral index. With relation to the hardness parameter of a neutron environment, the respective values are 1.11, 1.22 and 1.12. Other spectral parameters are 1.54 MeV, 1.89 MeV and 1.55 MeV respectively, for the average neutron energy >10 KeV. These measurements are compared to the ´free-field´ neutron leakage spectrum (measured at 20-inches from the core surface) in which the average neutron energy is 1.53 MeV, the spectral index is 6.7 and the hardness parameter is 1.12.