Quasi-Elastic Scattering of Slow Neutrons by Water and an Aqueous Solution of Sodium Chloride

Using chopper time-of-flight techniques and a beryllium-filtered incident spectrum, we have examined the quasi-elastic scattering of neutrons by water and by a $4N$ solution of NaCl at a scattering angle of 90°. After substracting an inelastic component attributed to scattering by a gas of mass-18 particles, we completed our analysis in terms of the continuous-diffusion model of neutron scattering by fitting a Lorenztian-broadened incident spectrum to our corrected data. In this way we determined the self-diffusion coefficient $D$ of water in each sample. The two values obtained are 2.2×${10}^{-5\mathrm{}}$ ${\mathrm{cm}}^2$/sec and 1.7×${10}^{-5\mathrm{}}$ ${\mathrm{cm}}^2$/sec for pure water and for the salt solution, respectively. These values are in good agreement with NMR and radiotracer measurements. When considered in the light of current theories of water structure and hydration, they have led us to the conclusion that the quasi-elastic peak in the neutron spectrum of water arises primarily from the diffusive motion of individual water molecules.