Research On Advanced X-ray Imaging Methods Based On Thomson Scattering X-ray Source

As the development of high brightness electron beam generation and ultrahigh intensity laser technique,a novel x-ray source based on the Thomson scattering of ultrashort laser pulses from relativistic electrons was proposed in 1990 s and has been rapidly developed since then.Thomson scattering x-ray sources have drawn much attention among scientists in various scientific fields,since they can generate x-ray pulses with excellent beam qualities,such as quasi-monochromaticity,continuous tunability of x-ray energy,high spatial coherence,high brightness,straightforward polarization control,and ultrashort pulse length.As an important potential application,x-ray imaging based on this type of x-ray source is in the ascendant around the world,and a series of practical problems about this topic remain to be resolved,such as precise measurement of the whole beam spectrum of Thomson scattering x-ray sources,energy-angle correlation correction,typical photon yield requirement for a single-shot propagation-based phase contrast imaging,etc.This dissertation is devoted to a systematic research on multiple advanced x-ray imaging methods combined with the advantages of Thomson scattering x-ray sources.With regard to x-ray imaging,the spectrum of scattered x-rays is a key parameter that must be precisely described.In this dissertation,we proposed a precise spectrum reconstruction method for Thomson scattering x-ray sources with the diffraction of a Highly Oriented Pyrolytic Graphite(HOPG)crystal and demonstrated it at the Tsinghua Thomson scattering x-ray source(TTX).The pulse pileup problem associated with this type of x-ray source in direct spectrum measurement was avoided and the details of the spectrum,e.g.central energy and bandwidth,were reconstructed simultaneously without rotating the crystal since the mosaicity of HOPG crystals makes it possible to accept x-rays with a finite bandwidth at a given diffraction angle.Taking advantage of the higher integral reflectivity of HOPG crystals and the high flux of x-ray beam,this method is even possible for single-shot measurement.Hence,this method is very applicable and attractive to the spectrum measurement of quasi-monochromatic x-ray sources,e.g.Thomson sources.Based on the quasi-monochromaticity of Thomson scattered x-ray pulses,monochromatic computed tomography(CT)research es were carried out in this dissertation.A peanut sample(Arachis hypogaea L.)was chosen for experiment study and the hull was peeled off the core at the right threshold after quantitative analysis of the reconstructed linear attenuation coefficients.Further studies showed that the intrinsic energy-angle correlation associated with the Thomson source will make it severely compromised to quantitatively evaluate the reconstructed images in monochromatic CT.To resolve this problem,we,inspired by the dual-energy imaging,proposed a dual-spectrum scan method.Not only monochromatic CT can be achieved,the effective atomic number and electron density distributions of an imaging object can also be retrieved simultaneously at the expense of two spectrally different measurements.Hence,the previous small imaging object and/or far source-to-object distance restrictions for monochromatic CT based on Thomson sources have been broken through.Based on the continuous tunability of Thomson scattered x-ray energy,dualenergy digital radiography(DR)imaging and K-edge imaging researches were carried out in this dissertation.The former can retrieve the effective atomic number distribution of an imaging object,while the latter can enhance the absorption contrast of imaging objects.The influence of bandwidth on the retrieval precision of effective atomic number was studied.Relevant experiments were carried out at the TTX using cement sample and Iodine solution respectively,and well agreement was observed between experiment results and theoretical predictions.Based on the high spatial coherence of Thomson scattered x-ray pulses,propagation-based phase contrast imaging(PCI)researches were carried out in this dissertation.Considering that the focal spot characteristic,the spatial distribution of scattered x-ray photons,and the energy-angle correlation for Thomson sources are quite different from other types of x-ray sources,a Monte Carlo simulation tool for propagation-based PCI based on Thomson sources was developed,taking advantage of which,the typical requirement of photon yield for a single-shot propagation-based PCI based on this type of x-ray source was discussed.An experimental platform for propagation-based PCI and phase contrast CT was set up at the TTX,and relevant experiments were carried out at this platform.Clear edge-enhancement effect was observed in the shrimp PCI experiment and quantitative relation between defined Contrast parameter and sample-to-detector distance was analyzed.Meanwhile,the single-step phase retrieval method was adopted at the TTX platform,based on which,a propagation-based phase contrast CT demonstration experiment was carried out using a Teflon sample.