(a) Part of the detector image of the X-ray beam reflected from the silicon single-crystal sample. The x-axis is parallel to the horizontal direction across the X-ray beam direction, while the y-axis is parallel to the vertical direction. Units on the x- and y-axes are pixel numbers of the detector and the size of a pixel is 172 µm × 172 µm. The dark parts are shadows of a slit downstream of the sample. The inclined broad and slightly bright band represents X-ray intensity coming through the slit downstream of the sample and this consists of coplanar and non-coplanar diffuse scattering. A much brighter inclined line in the middle of the broad band is the specularly reflected beam. (b) Intensity profiles along white lines parallel to x (blue) and y (red) axes in (a) including a specular reflection at Q = 0.4 Å−1.