Double Bragg scattering observed in small-angle X-ray scattering from highly oriented pyrolytic graphite

In small-angle X-ray scattering from highly oriented pyrolytic graphite, radial streak patterns are observed. The streaks change their direction with sample rotation and appear and disappear in pairs. This streak pattern can be explained by double Bragg scattering. The directions of the streaks calculated as functions of the rotation angle are in good agreement with experiment. In addition, asymmetry in the intensity of the streaks and a small deviation from the radial pattern are observed, and they can be explained by the finite sample-size effect. The necessity of taking double Bragg scattering into account in small-angle X-ray scattering studies of crystalline materials is emphasized.