Experimental setup for the rocking curve measurements on α-SiO2 crystals. (a) Dispersive setup using a Ge 220 four-crystal monochromator (4C), (b) α-SiO2 (Y-cut) dispersive rocking curve, (c) high-sensitivity nondispersive setup and (d) α-SiO2 (Y-cut) nondispersive rocking curve. Open circles are the measured data; solid lines are the theoretical calculation based on the dynamical theory of X-ray diffraction. There is an agreement between measured and theoretical data. The major difference is on the tails of the nondispersive rocking curve. This can be attributed to point defects and/or bad surface quality which enhance the ultra-small-angle X-ray scattering.