Measurement of the thermal neutron wave dispersion relations in BeO.

Abstract

The amplitudes and phase shifts of thermal neutron waves were measured in a block of BeO 60.96 x 60.96 x 58.42 cm3 of average density 2.87 g cm-3 and having nominal transverse buckling of 0.0048 cm-2. The attenuation constants (α) and phase shift constants (ζ) were derived from such measurements at 18 different frequencies covering the range 52.4 to 506.1 Hz. The parameters of α2 - ζ2 and 2αζ were evaluated, normalised to a reference density of 2.96 g cm-3, and corrected for the density normalised transverse buckling of 0.00513 cm-2. The normalised and corrected parameters α2 - ζ2 and 2αζ were fitted to polynomials in the angular frequency ω, and the resulting thermal neutron diffusion parameters were compared with those derived from a λ(B2) experiment carried out on the same block of BeO. The parameters λα and L determined by the two methods are in good agreement, but the diffusion coefficient and diffusion cooling constant derived from the sine wave experiment are significantly higher. The high diffusion coefficient (by ~ 9 per cent) could be due to the effect of the sub-Bragg continuum although the values of the parameters α and ζ appeared to be independent of the distance from the source. Agreement of the parameters α2 - ζ 2 and 2αζ with the theoretical values is not good and reasons for the discrepancy are advanced.