Instrumental resolution effects in small-angle scattering

Experimentally measured scattering data differ from theoretical curves because of departures from point geometry in a real instrument. In a small-angle scattering instrument there are three contributions to the smearing of an ideal curve: (1) the angular divergence of the beam; (2) the finite resolution of the detector; and (3) the polychromatic nature of the beam. In general, instrumental-resolution (smearing) effects are smaller for small-angle neutron scattering (SANS) than for small-angle X-ray scattering (SAXS). This is because most SANS experiments are performed in point geometry (with pinhole collimation) whereas a significant proportion of X-ray experiments have used long-slit sources where smearing effects are much larger, particularly at small angles. Hence, corrections to the measured curve are often applied in SAXS experiments to compensate for angular smearing. Less attention has been given to the effects of wavelength smearing in SAXS which is often assumed to be negligible. Similarly, SANS data are routinely analyzed without reference to instrumental smearing effects. The circumstances under which the above assumptions break down are illustrated by a range of experiments comparing data taken from the same samples, run on different types of SAXS and SANS facilities. These experiments indicate some circumstances under which the above effects cannot be neglected.