The forward scattering of cold neutrons by mixtures of light and heavy water

The forward scattering of cold neutrons in small-angle scattering experiments on macromolecules dissolved in mixtures of light and heavy water is predominantly determined by the large incoherent cross section of the hydrogen atoms leading to an isotropic background level. The probability per unit solid angle for forward scattering by samples rich in H₂O is approximately equal to the fraction of the primary beam which has not been transmitted through the sample, divided by the full solid angle 4π. When using the camera D11 at the high flux reactor in Grenoble [Ibel (1976). J. Appl. Cryst. 9, 296–309] this simple relation is fulfilled within ±5% by a 1 mm thick sample of H₂O at 281.8 K using neutrons of 1.0 nm wavelength. In D₂O, the coherent cross section is of the same order as the incoherent one, and about one half of all those neutrons which had been removed from the primary beam are scattered into large angles. Our experimental results and the derived graphs allow us to determine the effective forward scattering per unit solid angle for any volume fraction of D₂O in H₂O/D₂O mixtures around room temperature by a simple and accurate transmission experiment in conjunction with the measurement of the scattering of one single sample of H₂O of 1 mm thickness, for neutrons of wavelengths from 0.45 to 1.0 nm.