18 research outputs found
Detector tilt considerations in high-energy Bragg coherent diffraction imaging: a simulation study
This paper addresses three-dimensional signal distortion and image
reconstruction issues in x-ray Bragg coherent diffraction imaging (BCDI) in the
event of a general non-orthogonal orientation of the area detector with respect
to the diffracted beam. Growing interest in novel BCDI adaptations at
fourth-generation synchrotron light sources has necessitated improvisations in
the experimental configuration and the subsequent data analysis. One such
possibly unavoidable improvisation that is envisioned in this paper is a
photon-counting area detector whose face is tilted away from the perpendicular
to the Bragg-diffracted beam during acquisition of the coherent diffraction
signal. We describe a likely circumstance in which one would require such a
detector configuration, along with experimental precedent at third generation
synchrotrons. Using physically accurate diffraction simulations from synthetic
scatterers in the presence of such tilted detectors, we analyze the general
nature of the observed signal distortion qualitatively and quantitatively, and
provide a prescription to correct for it during image reconstruction. Our
simulations and reconstructions are based on an adaptation of the known theory
of BCDI sampling geometry as well as recently developed projection-based
methods of wavefield propagation. Such configurational modifications and their
numerical remedies are potentially valuable in realizing unconventional
coherent diffraction measurement geometries and eventually paving the way for
the integration of BCDI into new materials characterization experiments at
next-generation light sources.Comment: 13 pages, 5 figure
Using Automatic Differentiation as a General Framework for Ptychographic Reconstruction
Coherent diffraction imaging methods enable imaging beyond lens-imposed
resolution limits. In these methods, the object can be recovered by minimizing
an error metric that quantifies the difference between diffraction patterns as
observed, and those calculated from a present guess of the object. Efficient
minimization methods require analytical calculation of the derivatives of the
error metric, which is not always straightforward. This limits our ability to
explore variations of basic imaging approaches. In this paper, we propose to
substitute analytical derivative expressions with the automatic differentiation
method, whereby we can achieve object reconstruction by specifying only the
physics-based experimental forward model. We demonstrate the generality of the
proposed method through straightforward object reconstruction for a variety of
complex ptychographic experimental models.Comment: 23 pages (including references and supplemental material), 19
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Gas-induced segregation in Pt-Rh alloy nanoparticles observed by in-situ Bragg coherent diffraction imaging
Bimetallic catalysts can undergo segregation or redistribution of the metals
driven by oxidizing and reducing environments. Bragg coherent diffraction
imaging (BCDI) was used to relate displacement fields to compositional
distributions in crystalline Pt-Rh alloy nanoparticles. 3D images of internal
composition showed that the radial distribution of compositions reverses
partially between the surface shell and the core when gas flow changes between
O2 and H2. Our observation suggests that the elemental segregation of
nanoparticle catalysts should be highly active during heterogeneous catalysis
and can be a controlling factor in synthesis of electrocatalysts. In addition,
our study exemplifies applications of BCDI for in situ 3D imaging of internal
equilibrium compositions in other bimetallic alloy nanoparticles