Spectral Characterization of the Epithermal Neutron Beam at the Brookhaven Medical Research Reactor

Neutron spectrum measurements are performed on the aluminum-oxide-filtered neutron beam at the Brookhaven Medical Research Reactor (BMRR). Two independent measurement techniques are used in the spectrum characterization: (a) foil activation spectrometry and (b) proton-recoil spectrometry. Activation foil assemblies are irradiated at the exit port of the beam facility. Dominant resonances in selected activation reactions are used to measure the epithermal neutron spectrum. The intermediate and fast energy ranges of the neutron spectrum are measured by threshold reactions and the ¹⁰B-filtered ²³⁵U fission reaction. Neutron spectral data are derived from the activation data by two approaches: (a) an analysis that yields neutron flux values at the energies of the dominant or primary resonances in the epithermal activation reactions and (b) an analysis that utilizes all the activation data simultaneously in a spectrum unfolding process using the FERRET data adjustment code. Hydrogen-filled proton-recoil proportional chambers are used at the beam port exit to acquire data of a higher energy resolution than that obtainable through foil activation techniques. These measurements are made to determine if structure in the aluminum scattering cross section would produce significant structure in the filtered spectrum in the fast neutron region.