Civita, M; (2017) Measurements of phase changes in crystals using ptychographic x-ray imaging. Doctoral thesis , UCL (University College London).

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Abstract

In a typical X-ray diffraction experiment we are only able to directly retrieve part of the information which characterizes the propagating wave transmitted through the sample: while its intensity can be recorded with the use of appropriate detectors, the phase is lost. Because the phase term which is accumulated when an X-ray beam is transmitted through a slab of material is due to refraction [1, 2], and hence it contains relevant information about the structure of the sample, finding a solution to the “phase problem” has been a central theme over the years. Many authors successfully developed a number of techniques which were able to solve the problem in the past [3, 4, 5, 6], but the interest around this subject also continues nowadays [7, 8]. With this Thesis work, we aim to give a valid contribution to the phase problem solution by illustrating the first application of the ptychographic imaging technique [9, 10, 11, 12, 13, 14] to measure the effect of Bragg diffraction on the transmitted phase, collected in the forward direction. In particular, we will discuss the experimental methodology which allowed to detect the small phase variations in the transmitted wave when changing the X-ray’s incidence angle around the Bragg condition. Furthermore, we will provide an overview of the theoretical frameworks which can allow to interpret the experimental results obtained. More specifically, we will also discuss a new quasi-kinematic approximation which was recently developed by Gorobtsov and Vartanyants [2] in order to highlight the potential for future applications of the methodology described in this work. In particular, this new theory, used in conjunction with the experimental technique here presented, will permit to investigate further the effects related to the phase of the transmitted beam, thus allowing to study the structure of strained crystals as well as to fully determine the phase of the structure factor.

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