Optimisation of coherent X-ray diffraction imaging at ultrabright synchrotron sources

Abstract

Coherent X-ray Diffraction Imaging (CXDI) is a powerful method of imaging single crystalline grains within a powder. Direct Fourier transformation of the oversampled diffraction pattern surrounding a Bragg peak is possible once the phases have been obtained using a 'support' constraint. The image is in general complex with the phase representing a projection of the atomic displacements allowing access to the internal strains inside the crys- tal. CXDI relies crucially on the production of a coherent beam of X-rays, which is one of the technical advances of the latest 3rd generation Synchroton Radiation (SR) sources. It is shown here how the use of a secondary source in the design of the X-ray beamline allows the coherence to be controlled continuously over a finite range without any loss of flux. The conclusions will have significant impact on instruments currently being designed at the Dia- mond Light Source and the new National Synchrotron Light Source (NSLS-2). 1. Imaging of crystals Strain fields inside nanometre-size crystals can be imaged quantitatively using the method of Coherent X-ray Diffraction Imaging (CXDI) (1-5). A coherent X-ray beam is focussed onto a nano-grained powder sample so that relatively few grains are illuminated. One of the diffraction spots of an individual grain can isolated on a Charge-Coupled Device (CCD) detector with sufficient resolution that it becomes oversampled relative to the spacing of the fringes that arise from the finite size of the grain. The angular spacing, ��, of the fringes is given by the spacing formula, d / λ θ =