Fig. 1: Fluorescence x-ray tomography setup at the Hard X-ray Nanoprobe at NSLS-II.

a a monochromatic x-ray beam is focused using a Fresnel zoneplate, together with a central beam stop (not shown) and an order-sorting aperture. The transmitted scattering pattern and fluorescence signals are collected simultaneously. Raster scanning is performed to produce a 2D XRF projection image, and tomography measurement is carried out by collecting a series of 2D XRF images as the sample is rotated about the vertical axis, perpendicular to the x-ray beam. b Fluorescence x-rays emitted from an arbitrary point along the incidence x-ray beam, represented by a green voxel. Only a fraction of fluorescence x-rays is captured by a detector with a given solid angle (represented by a pink shape), encoding the absorption through a 3D local region of the sample defined by the detector solid angle (i.e., traversed region). A gray cube represents a voxel within this traversed region, where the absorption is corrected. The traversed region changes with the orientation and position of the sample.