Ovcharenko A. Detection of changes in optical properties and structural inhomogeneities of materials by X-ray phase contrast methods.

The thesis examines the detection of structural changes and visualization of the internal structure of objects of various nature using X-ray phase contrast methods. Changes in the optical properties of materials in the X-ray range under conditions of deformation, thermal expansion, phase transitions, variations in elemental composition are also investigated. The thesis consists of an introduction, four chapters, conclusions, a list of used sources and one appendix. The volume of the thesis is 158 pages of typewritten text, the main part is 124 pages and contains 42 drawings. The list of used sources includes 151 names. The results of the thesis work are published in 4 scientific works indexed by the Scopus and/or Web of Science databases. The object of the research is three-dimensional structural inhomogeneities of arbitrary geometric shape from micro to microscopic size in samples of living and non-living nature. The purpose of this work is to study the internal structure of complex three-dimensional objects with different values of refraction decrements and absorption coefficients using X-ray phase contrast methods. In this paper, a new approach to modeling phase-contrast X-ray images using the free propagation method based on the Fresnel-Kirchhoff theory was developed, a generator of three-dimensional computer models of optically homogeneous and heterogeneous, single-layer and multilayer, single-component and multi-component samples of arbitrary geometric shape was created, an own algorithm for calculating thicknesses was implemented in practice and phase shifts of X-ray radiation when passing through the studied objects. A method of calculating the refraction decrements and absorption coefficients of metals and their alloys based on known theoretical ratios and the results of molecular dynamic modeling is proposed. Simulation of X-ray diffraction in the free propagation scheme was carried out on samples with linear dimensions of the order of 0.1 mm and optical decrements of refraction of 10^-6 characteristic of biological tissues. As a result of the study, distributions of intensities and phase changes of X-rays on the screen (detector) were calculated, which showed the possibility of obtaining clear images of objects with small values of refraction decrements. The technique of restoring the shape and dimensions of a three-dimensional object using software analysis of diffraction results obtained at different angles has been implemented in practice. On the basis of the created algorithm for generating computer models of multilayer three-dimensional objects of irregular geometric shape with different refraction decrements for each layer, a study of the internal multilayer structure of optically heterogeneous objects was carried out using X-ray phase contrast imaging. The influence of dimensional effects on the interaction of materials with X-ray radiation has been studied. It is shown that the quality of the image on the screen depends on the number of Fresnel zones that fall into the region of the geometric shadow of the studied sample during the propagation of the wave front. The sensitivity of the X-ray phase contrast method in relation to compression-tension mechanical deformations for metal samples based on Fe-1.3%Cu-1%Ni alloy was studied. Phase transitions in pure Zr crystals were identified by X-ray diffraction. The depth of X-ray penetration into Fe-13%Cr-(2,4,8)%Al metal alloys at different temperatures was calculated. The high sensitivity of the X-ray phase contrast method to the processes associated with changes in electron density is shown. The peculiarities of the formation of diffraction patterns of objects when using long-range sources of X-ray radiation are studied, taking into account the degree of their spatial coherence. Using the theoretical provisions of the Van Zittert Zernike theorem, the dimensions of the coherence region for a quasi-monochromatic source were calculated. At the same time, such important parameters as: the physical dimensions of the source, the nature of the radiation intensity distribution and its wavelength were taken into account. On the basis of the obtained data, the optimal distances between the radiation source and the object under study were determined, which ensure the maximum clarity and image quality of the sample under the given experimental conditions. The results of this thesis research can be useful to specialists in the field of materials science, biology and medicine, will help to optimize the processes of materials diagnosis, detect defects and study the structural characteristics of objects of various nature with the aim of increasing the safety and quality of products, solving the main problems of medical diagnostics: reducing the radiation dose, increasing resolution and contrast for imaging soft tissues.