Direct measure of the phase shift of an x-ray beam

Although it is widely known that an x-ray beam suffers from absorption on passing through matter, little attention has been given to the change in the phase of the transmitted x-ray beam. Using a recently developed method for differential phase-contrast imaging with hard x rays, we investigate the phase shift induced in an x-ray beam when it passes through a nonabsorbing plastic film, a pure-phase object. The film is bent into a hemicylinder so that it presents a thickness to the x rays that changes with position. This leads to phase changes that vary up to several multiples of 2π, which are observed in the x-ray images as changes in the contrast. We discuss the method of hard x-ray phase-contrast imaging and describe the experiments that enable us to measure the phase change induced in a beam of x rays of wavelength 0.154 nm.