Measurements of Calculations of Spatial Distribution of Fast Neutron Spectra in Iron and Iron-Uranium Assemblies from a 14-Mev Source

Abstract

The spatial distribution of fast neutron spectra in two assemblies of iron and the combination of iron and natural uranium of about equal ratio by volume have been measured with five threshold reactions: - 63Cu(n, 2n), 27Al(n, a), 56Fe (n, p), 27Al(n, p) and 31p(n, p). 14 Mev source neutrons were produced with a 150 KV SAMES accelerator. The assemblies were of cylindrical annular geometry - the inner radius about 10 cm, and the outer radius slightly larger than 30 cm. The source was centrally located. The absolute source strength of the neutrons was determined by the associated alpha-particle counting technique. The induced activities in the foils were measured by counting gamma-rays with a Nal(Tl) scintillator coupled to a multichannel analyser and the beta particles with a plastic scintillator. The counting systems were calibrated for each type of foil and the absolute activity was determined from that. Measurements were also made with the 115In(n, y) reaction for both bare and cadmium covered foils to assess the intensities of the lower energy neutrons. Multigroup diffusion and removal diffusion calculations were made by solving the equations numerically. The diffusion coefficients in the calculations were obtained with higher order transport corrections than commonly used. The group parameters were obtained from the more recent 20 group Yiftah-Sieger cross-section set which considers the presence of a 14 Mev source and divides the Mev energy region into seven groups. Calculations were also made with a “hybrid” set mainly based on the Russian ABBN set, but the top two groups contained data supplimented from the Yiftah-Sieger set. 5 Results are compared for experimentally obtained normalised absolute activities with those predicted by the calculations. Satisfactory agreement is obtained between them and results show some interesting features.