Directional dark-field implicit x-ray speckle tracking using an anisotropic-diffusion Fokker-Planck equation

TL;DR

This paper introduces a novel anisotropic-diffusion Fokker-Planck model for directional dark-field x-ray speckle tracking, enabling simultaneous recovery of attenuation, refraction, and diffuse scatter components in coherent x-ray imaging.

Contribution

It develops a new theoretical framework using anisotropic diffusion Fokker-Planck equations for modeling bifurcated x-ray energy flow in speckle tracking, validated with experimental data.

Findings

Successfully models attenuation, refraction, and diffuse scatter components.

Demonstrates the approach's effectiveness with experimental x-ray data.

Provides a comprehensive method for directional dark-field imaging.

Abstract

When a macroscopic-sized non-crystalline sample is illuminated using coherent x-ray radiation, a bifurcation of photon energy flow may occur. The coarse-grained complex refractive index of the sample may be considered to attenuate and refract the incident coherent beam, leading to a coherent component of the transmitted beam. Spatially-unresolved sample microstructure, associated with the fine-grained components of the complex refractive index, introduces a diffuse component to the transmitted beam. This diffuse photon-scattering channel may be viewed in terms of position-dependent fans of ultra-small-angle x-ray scatter. These position-dependent fans, at the exit surface of the object, may under certain circumstances be approximated as having a locally-elliptical shape. By using an anisotropic-diffusion Fokker-Planck approach to model this bifurcated x-ray energy flow, we show how all three components (attenuation, refraction and locally-elliptical diffuse scatter) may be recovered. This is done via x-ray speckle tracking, in which the sample is illuminated with spatially-random x-ray fields generated by coherent illumination of a spatially-random membrane. The theory is developed, and then successfully applied to experimental x-ray data.