Back-face bragg diffraction from a perfect and ultralightly deformed thick crystal

The intensity of neutrons Bragg diffracted from the back face of a 9 mm thick slab-shaped Si(111) analyzer crystal has been measured experimentally at the ORNL double crystal diffractometer and calculated theoretically. The back-face rocking curve of a strain-free perfect crystal contains two symmetrical peaks which become asymmetrical under an ultrasmall static deformation strain (bending with a radius of tens of km). The asymmetry is shown to be a sensitive measure of both the magnitude and direction of bending.