X-ray speckle contrast variation across absorption edges

We measured static x-ray speckle contrast variation with the incident photon energy across a sample-specific absorption edge. In this paper, we present a theoretical description of this energy dependency consistent with our data. We found that the contrast depends mainly on the imaginary part of the complex index of refraction in the sample, as well as on the instrumental resolution. The speckle contrast decreases as the absorption cross section in the sample increases at the absorption edge. This result is not predicted by commonly used theory.