Sample geometry effects on incoherent small-angle scattering of light water

Light water is frequently used as a standard for calibrating small-angle neutron scattering (SANS) data. The intensities collected for 1 and 2 mm of light water in standard quartz cells may differ by up to about 50% due to the presence of multiple and inelastic scattering [Rennie & Heenan (1993). Proceedings of ISSI Meeting, Dubna, pp. 254-260, Report E3-93-65; Teixeira (1992). Structure and Dynamics of Strongly Interacting Colloids and Supramolecular Aggregates in Solution, edited by S. H. Chen, pp. 625-658. Dordrecht: Kluwer Academic Publishers]. Multiple scattering increases with the thickness of the sample. The use of only elastically scattered neutrons may lead to an absolute intensity of the SANS data of about a factor of 2 higher than that obtained by taking into account all of the neutrons on the detector [Ghosh & Rennie (1990). Inst. Phys. Conf. Ser. 107, 233-244]. However, it is shown here that the scattering intensities collected with different ratios of sample-to-beam dimension do present large differences as a function of sample thickness. In particular, ratios smaller and larger than unity are considered and the results are discussed and compared with Monte Carlo simulations.