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 cm 3 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 (ξ) were derived from such measurements at 18 different frequencies covering the range 52·4–506·1 Hz. The parameters α 2 - ξ 2 and 2αξ were evaluated, normalized to a reference density of 2·96 g cm −3, and corrected for the density normalized transverse buckling of 0·00513 cm −2. The normalized and corrected parameters α 2 - ξ 2 and 2αξ were fitted to polynomials in the angular frequency ω, and the resulting thermal neutron diffusion parameters were compared with those derived from a λ( B 2) experiment carried out on the same block of BeO. The parameters λ a 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 ξ appeared to be independent of the distance from the source. Agreement of the parameters α 2 - ξ 2 and 2αξ with the theoretical values is not good and reasons are advanced for the discrepancy.