University thesis:
Dissertation, Universität Freiburg, 2020
Footnote:
Description:
Abstract: Synchrotron-based X-ray imaging is widely used in various fields of material science and biological research. Mostly used state-of-the-art indirect synchrotron cameras do not offer good efficiency in the detection of X-rays at high energies around 30 keV at simultane- ously high spatial resolutions. This thesis aims to close this gap by developing a complex and elaborate instrumentation of a novel, mobile and stand-alone imaging system with highly efficient X-ray magnifying crystal optics of a so-called in-line Bragg magnifier (BM), which coupled to a high-Z (semiconductor material of high atomic number) direct converting photon counting detector (PCD), enables dose-efficient and simultaneously μm spatial resolution X-ray imaging at synchrotrons. The novel imaging device performs a two dimensional magnification of the X-ray beam cross section based on multiple crystal arrangement by using the principle of asymmetric Bragg diffraction. The BM instrument which uses two pairs of asymmetrically cut Si(220) crystals with a cut of 5.92° and 0.94° has been designed to operate at ~30 keV and this goal is actually a world ́s first. The remarkable reflectivity of all four crystals is at least 92%, which means that almost no flux is lost in the optics and thus provides an excellent contribution to the efficiency of the overall imaging system. The bright and enlarged X-ray image is captured by a large area, 500μm thick Gallium Arsenide (GaAs) PCD, which again dramatically increases efficiency. The imaging system is seven times more efficient than conventional indirect synchrotron cameras even at μm spatial resolution. Due to the in-line capability of the BM, the instrument can be operated in two modes, first the Bragg magnifier microscope (BMM) mode providing dose-efficient imaging of small ~mm sized samples and second the Bragg magnifier conditioner (BMC) mode enabling large field of view (FoV) imag- ing of ~cm sized samples which is not available at modern synchrotrons. The imaging system has been successfully operated at energies ranging from 29 keV to 31 keV with cor- responding magnification factors of 35 × 35 up to 200 × 200 producing two dimensional, magnified radiographic projections enabling efficient, μm spatial resolution tomography. On the other hand, the system allows for large FoV imaging with a beam cross sections up to 55 × 55 mm^2 even with small synchrotron beams