On the cause of a contrast change in the SR images of micropipes in SiC

Abstract

We propose a theoretical explanation for the experimentally observed feature in phase contrast images of micropipes in silicon carbide in a white synchrotron radiation (SR) beam, which consists in the fact that the contrast of various micropipes or various regions of the same micropipe is of different colors. The contrast is most often white (the intensity is higher than the background) at the center and black at the edges; however, the sign sometimes changes, and the contrast becomes black at the center and white at the edges. We discuss the results of experiments performed at the SR source in Pohang, Republic of Korea. The cause of the contrast change can be a change in the angle between the micropipe axis and the SR beam direction. At not overly small angles, the phase progression in a section of the micropipe is small and the contrast is standard. If the angle becomes very small, the size of the longitudinal section of the micropipe by the beam increases, which leads to wave field oscillations in the region of the section. At large distances in the white beam, these oscillations are averaged, and averaging results in black contrast.