X-ray laminography and SAXS on beryllium grades and lenses and wavefront propagation through imperfect compound refractive lenses

摘要

Hard X-ray free electron lasers provide almost fully transverse coherent X-rays. Though the natural divergence of these X-rays is a few micro-radians, they still need to be collimated or focused while traveling up to 1 km towards the sample. This can be done with beryllium compound refractive lenses (CRLs). Due to the coherence of the beam, it is important that the impurities or granular boundaries in these CRLs do not distort the wavefront of the X-ray beam to a measurable extend. We measured the SAXS signal of various beryllium grades and of 2D parabolic lenses made of IF-1 beryllium. Then, we imaged these samples using X-ray computed laminography at a resolution of around 1 micrometer. Computed laminography is a 3D imaging technique similar to computed tomography, but particularly adapted for flat extended objects. These measurements are used to characterize the voids and granular boundaries in the beryllium samples. Boundaries between the former powder particles are easily seen for beryllium grades produced via powder metallurgy methods. This is not the case for cast ingots. Common to all samples are voids with diameters in the 10 micrometer range as well as smaller sized, denser impurities. Finally, we use wavefront propagation simulations in order to analyze the effect of voids in the CRLs on the wavefront of the XFEL beam. If the distance "lens to focus and sample" is large enough, the diffraction patterns emerging from the voids smoothen out.