Measurement of the Frequency Response and Thermal Neutron Wave Dispersion Parameters in Water and Water-Void Assemblies

Abstract

A square wave input of fast neutrons from the D-T reaction was supplied by a S.A.M.E.S. accelarator. The fast neutrons were thermalised by an iron-polypropylene arrangement which designed to provide a plane source of thermal neutrons with minimum pulse broadening. The thermal neutron waves were propagated axially in a water assembly with and without voids. The time profile of the thermal neutron pulse was recorded with lithium glass scintillation counters input to a multi-channel pulse high analyser used in a time sequence storage mode and synchronised with the source pulsing frequency. Form a Fourier analysis of the recorded thermal flux, the frequency response of the system was measured as a function of source frequency, position and void dimensions. This provided information on the space-dependent phase and amplitude of the wave, from which the attenuation factor and the phase shift per unit length were evaluated. The streaming factor of each water-void assembly was found to be a frequency-dependent quantity. The dispersive properties of each assembly were expressed through the dispersion relation which relates the inverse complex relaxation length of the wave to the frequency of excitation and the nuclear properties of the system. Three independent models were developed to calculate the dispersion parameters of the systems. The first model was a one group diffusion theory; the second model was based on a solution of Tlegrapher's equation by P-1 approximation to account for transport effects, and the third model was to account for the thermalisation effects from the presence of a non-Maxwellian neutron energy spectrum. Calculations based on thermalisation theory predicted the experimental observations fairly well at frequencies less than 600 Hz.