Scattering effects in x-ray imaging systems

Abstract

The optical performance achieved in many x-ray imaging applications is severely degraded by surface scattering effects. Residual surface roughness over a broad range of spatial frequencies must be specified and controlled for many applications of interest. Specifying the traditional surface `figure' and `finish' is inadequate for assuring the desired performance of high resolution imaging systems at these very short wavelengths. `Mid' spatial frequency surface errors that span the gap between the traditional `figure' and `finish' errors are often the dominant error source for both grazing incidence applications and those utilizing enhanced reflectance multilayers at normal incidence. Wide-angle scatter from surface microroughness severely reduces the reflectance of these multilayers whereas the small-angle scatter from the mid spatial frequency surface irregularities will degrade the resolution or image quality. A linear systems formulation of surface scattering theory results in a surface transfer function which characterizes the image degradation effects of residual surface irregularities over the entire range of relevant spatial frequencies. Image quality predictions obtained with this linear systems model agree favorably with experimental data for several different x-ray imaging systems. Parametric performance predictions can thus be used to determine realistic optical fabrication tolerances for x-ray astronomy, microscopy, and synchrotron source applications.