X-ray microscopy and phase imaging towards time-resolved applications in laboratory astrophysics

Authors: Andreas Wolf (Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU))
Co-authored by industrial partner: No

Type: Thesis

State: Published (Approved)
Published: August 2022

Abstract: In this work, a characterization and optimization of phase-sensitive X-ray imaging techniques with a focus on the field of laboratory astrophysics is given. Here, the advent of hard X-ray free electron lasers offers novel opportunities as single-pulse imaging with sub-picosecond temporal resolution becomes possible. The use of phase-sensitive techniques is often mandatory as micro- and nanoscopic samples show little or no attenuation contrast. In order to fully benefit from the short pulse lengths at X-ray free electron lasers, these methods should be reconcilable with single-exposure acquisition schemes. This task is complicated by zeroes in the respective transfer functions of the imaging systems. Therefore, direct inversions are typically not possible and sophisticated algorithms are required for the image reconstruction. Overall, this thesis mainly focuses upon the grating-based X-ray imaging technique, also known as Talbot interferometry. For comparison, propagation-based phase contrast imaging will also be considered. The investigations are divided into analytical considerations, numerical simulations, and experimental implementations of the respective imaging techniques. An analytical examination of the image formation within a Talbot interferometer is presented. This process can become complicated, especially for applications in X-ray microscopes. Here, transverse shifts of the interference pattern in general depend nonlinearly on the phase differences across the X-ray wave field. Existing reconstruction methods on the basis of deconvolutions then rely on idealized conditions, thus limiting the experimental applicability of the method. In addition, the achievable spatial resolution of Talbot interferometry in single-exposure applications is typically limited to the demagnified fringe period of the interference pattern. In order to resolve the limitations regarding the applicability, three novel reconstruction methods for Talbot interferometry are conceptualized and implemented: the design of a beam-splitting diffraction grating featuring only two diffraction orders, a two-stage deconvolution approach, and a statistical image reconstruction method based on an analytical forward model of the imaging process and a regularized maximum likelihood approach. The three schemes are validated on the basis of simulated data. They all prove advantageous when the premises for standard deconvolution-based reconstructions are not met. The statistical image reconstruction technique seems most promising as it achieves the best reconstruction quality at low photon numbers and also circumvents the abovementioned limitations regarding the spatial resolution. Building up on the simulative studies, two experimental realizations of Talbot interferometry at synchrotron light sources are presented. In the first experiment, the single-exposure phase imaging capabilities of Talbot interferometry in conjunction with the statistical image reconstruction method are investigated and characterized on the basis of simple test samples. The broadened experimental applicability is demonstrated through the retrieval of Fresnel diffraction images in an X-ray projection microscope. While a comparative implementation of propagation-based phase contrast imaging at the same instrument still yields a superior spatial resolution, the mitigation of limitations due to the fringe period is also verified experimentally. In the second experiment, single-exposure phase imaging with both the grating-based and the propagation-based approach is employed in order to monitor the moistening process of wood on the level of single wood cells...

Journal Keywords: X-ray imaging; X-ray microscopy; grating interferometry; laboratory astrophysics; near-field holography; phase retrieval

Subject Areas: Physics

Instruments: I13-1-Coherence

Added On: 14/08/2022 11:15

Discipline Tags:

Physics Technique Development - Physics Astronomy & Astrophysics

Technical Tags:

Imaging Tomography