Dynamical diffraction imaging of voids in nearly perfect silicon

X-ray diffraction topographs of extremely pure and perfect silicon single c rystals are made using low-energy undulator radiation from a positron stora ge ring. Typical defect images observed are rather large round images havin g a black-white contrast and a diameter of about 40 mum. Applying the dynam ical theory of x-ray diffraction, the defect contrast is explained by tensi le strain in the lattice around voids close to the exit surface. This disco very of void-like microdefects explains, at least in part, the reduced dens ity of the crystal intended to be used for a redefinition of the unit of ma ss, the kilogram.