Pulsed-Neutron Diffusion Parameters in Mixtures of Heavy and Light Water and Ice

The diffusion parameters for mixtures of 0, 20, 50, 80, and 100% D₂O in H₂O have been measured by the pulsed-neutron method at temperatures near the freezing point and in ice at -20°C. A 250-keV Cockcroft-Walton accelerator was used to produce neutron bursts in cylindrical samples by the ²K(d,n)³Re reaction. The waiting time method was used for establishment of the asymptotic spectrum in each sample. The infinite medium decay constants for D₂O were evaluated from known density and nuclear cross-section data; those for H₂O, H₂O ice, and (H₂O + D₂O) mixtures were determined by a three parameter least-squares fit of the experimental data to the equation λ = λ₀ + D_oB² - CB⁴. An iterative procedure was used to make the value of the extrapolated distance compatible with the diffusion coefficient D₀ derived from the least-squares analysis. The results are compared with those of similar measurements by other workers for H₂O and D₂O at various temperatures. The effect of the liquid-solid phase transition on the diffusion coefficient and diffusion cooling coefficient in H₂O and D₂O is discussed. The expression D₀ = 1 / αi/D₀,i, where αi and D₀,i are the fractional volume and diffusion coefficient of the i’th component of the mixture, respectively, gave lower values than the experimental results for the mixtures.