Analysis of neutron flux distributions generated by sources of fast neutrons in a range of bituminous coals

Monte Carlo calculations of fast neutron flux distributions have been made for ²⁵²Cf, ²⁴¹Am/Be and 14 MeV monoenergetic neutron sources in a range of bituminous coals. The configurations examined were sources at the centre of a sphere, at the midpoint of the plane surface of a hemisphere and at or near the surface of a parallel-sided slab. The neutron fluxes were analysed in terms of the resultant yield of γ-rays from inelastic scattering on silicon, carbon and oxygen, which are the key reactions for determining the oxygen content of coal by processes yielding prompt γ-rays. The fast neutron spectra were found to undergo significant changes in shape as a function of distance from the source, in contrast with the assumptions made in the model developed by Wormald and Clayton. ⁽¹⁾ However, the ratios of the response functions for inelastic scattering by the critical elements were found to be very insensitive to the type of coal, indicating that oxygen analyses could be performed for a given configuration and neutron source by using calibration factors derived from a single calculation or benchmark experiment on a coal of intermediate composition. The calculations described also provided information on the yield and spatial distribution of thermal neutrons from the selected sources.