Optimum intensity in small-angle neutron scattering. An experimental comparison between symmetric and asymmetric geometries

Results of an experimental comparison of the performance of symmetric (with the flight path between source and sample, L₁, equal to the flight path between sample and detector, L₂) and asymmetric (L₁ ≠ L₂) geometries of a small-angle neutron scattering instrument are reported. The comparison was performed under the assumption that a specific range of scattering vectors is required to be measured with a certain resolution. The measurements were performed at the spectrometer installed at the Risø National Laboratory, using as test samples a crystalline powder, a single-crystal and a sample that produces a smooth scattering curve at small angles. The results obtained show that, when a specific scattering-vector range has to be measured with a specific resolution, the optimized asymmetric spectrometer performs better than the symmetric instrument. A simple graphical approach for the determination of the best geometry to perform a given measurement is introduced.