Small-angle X-ray scattering due to sample shape

It is usually assumed that Fraunhofer conditions prevail for sample-shape scattering (SSS) in small-angle X-ray scattering (SAXS) and that SSS is negligibly small at all observable angles. By contrast, calculation showed that Fresnel diffraction conditions exist and that SSS may be large at observable angles if the beam has very flat edges perpendicular to the observed h and these edges intersect the sample. SSS intensity was calculated for sheet-like samples (both flat and slightly curved) approximately perpendicular to the beam. It was very large at observable angles [h≲0.006 Å⁻¹ where h =4π(sin θ)/λ] especially for thinner samples, but may be much reduced for a beam with rough edges. Predicted intensities seem to be far higher than measured values of total scattering. Possible reasons are discussed. The shape of the wavefront at the sample may require much more detailed consideration than is usual in SAXS theory.