Evaluation of the Weighted Mean X-ray Energy for an Imaging System Via Propagation-Based Phase-Contrast Imaging

Abstract

<jats:p>For imaging events of extremely short duration, like shock waves or explosions, it is necessary to be able to image the object with a single-shot exposure. A suitable setup is given by a laser-induced X-ray source such as the one that can be found at GSI (Helmholtzzentrum für Schwerionenforschung GmbH) in Darmstadt (Society for Heavy Ion Research), Germany. There, it is possible to direct a pulse from the high-energy laser Petawatt High Energy Laser for Heavy Ion eXperiments (PHELIX) on a tungsten wire to generate a picosecond polychromatic X-ray pulse, called backlighter. For grating-based single-shot phase-contrast imaging of shock waves or exploding wires, it is important to know the weighted mean energy of the X-ray spectrum for choosing a suitable setup. In propagation-based phase-contrast imaging the knowledge of the weighted mean energy is necessary to be able to reconstruct quantitative phase images of unknown objects. Hence, we developed a method to evaluate the weighted mean energy of the X-ray backlighter spectrum using propagation-based phase-contrast images. In a first step wave-field simulations are performed to verify the results. Furthermore, our evaluation is cross-checked with monochromatic synchrotron measurements with known energy at Diamond Light Source (DLS, Didcot, UK) for proof of concepts.</jats:p>

Noise Reduction for Single-Shot Grating-Based Phase-Contrast Imaging at an X-ray Backlighter

“…The spectrum of this backlighter setup at PHELIX was analyzed by Borm [ 39 ]. The dominating X-ray energy at PHELIX was determined in [ 40 ] using propagation-based phase-contrast imaging. In [ 26 ], a grating-based phase-contrast imaging setup, optimized for the dominating energy range, was introduced, evaluated, and the dominating energy verified.…”

“…The setup was designed for an X-ray energy of approximately 11 keV, which roughly corresponds to the dominating X-ray energy [ 40 ]. The G1 grating for this energy is approximately -shifting, resulting in a Talbot pattern with twice the frequency of the grating.…”

Radiation measurements and simulations at the PHELIX laser facility, GSI

“…The experiments described in this publication took place in February 2020 during the course of an experiment for the phase-contrast imaging of a micrometer sample using the laser-driven x-ray backlighter source [17,18]. For such experiments, the determination of the particle spectra emitted by the laser plasma has a high relevance [19]. Therefore, the spectrum and dose were measured in parallel to the experiments mentioned above and are described in this publication.…”

Single-shot grating-based phase-contrast imaging of a micrometer sample at a laser-driven x-ray backlighter source

“…Further, the moire visibility and the sensitivity of our imaging detector are energy-dependent (see figure 3 or [35]), so it is difficult to simulate an effective spectrum. Seifert et al [38] determined the weighted mean energy of the imaging system experimentally to lie between 11 keV and 12 keV using the same x-ray source and detector. Instead of gratings the setup had an acrylic window in the beam path with absorption properties comparable to our grating substrates.…”

“…Each calculation of the image signatures is repeated for multiple energies. The energy that best reproduces the measurement data is assumed to be the weighed mean energy of the imaging system (compare [38]). This procedure neglects that spectral contributions at higher or lower energies can distort the signals due to non-linear absorption-or phase-effects.…”