6,184 research outputs found
Compressive Phase Contrast Tomography
When x-rays penetrate soft matter, their phase changes more rapidly than
their amplitude. In- terference effects visible with high brightness sources
creates higher contrast, edge enhanced images. When the object is piecewise
smooth (made of big blocks of a few components), such higher con- trast
datasets have a sparse solution. We apply basis pursuit solvers to improve SNR,
remove ring artifacts, reduce the number of views and radiation dose from phase
contrast datasets collected at the Hard X-Ray Micro Tomography Beamline at the
Advanced Light Source. We report a GPU code for the most computationally
intensive task, the gridding and inverse gridding algorithm (non uniform
sampled Fourier transform).Comment: 5 pages, "Image Reconstruction from Incomplete Data VI" conference
7800, SPIE Optical Engineering + Applications 1-5 August 2010 San Diego, CA
United State
Theoretical Aspects of Particle Production
These lectures describe some of the latest data on particle production in
high-energy collisions and compare them with theoretical calculations and
models based on QCD. The main topics covered are: fragmentation functions and
factorization, small-x fragmentation, hadronization models, differences between
quark and gluon fragmentation, current and target fragmentation in deep
inelastic scattering, and heavy quark fragmentation.Comment: 26 pages, 27 figures. Lectures at International Summer School on
Particle Production Spanning MeV and TeV Energies, Nijmegen, The Netherlands,
August 199
Tunable porous boron nitride: Investigating its formation and its application for gas adsorption
Boron nitride (BN) has applications in a number of areas: it can be used as lubricant, as insulating thermoconductive filler or UV-light emitter. BN can also capture large amounts of hydrocarbons and gaseous molecules, provided that it exhibits a porous structure. This porous structure also enables its application as a drug-delivery nanocarrier. Little if anything is known on controlling the porosity of BN, even though it has tremendous implications in terms of adsorption performance and drug delivery properties. To address this aspect, we provide for the first time an in-depth investigation of the effects of the synthesis conditions on the formation of porous BN. The material was also tested for CO2 capture. We found that the intermediate preparation is of paramount importance and can in fact be used to tune the porosity of BN. Owing to a combination of spectroscopic and thermal analyses, we attributed this phenomenon to the variation of the thermal decomposition pattern of the intermediates. The most microporous BN produced was able to capture CO2 while not retaining N2. Overall, this study opens the route for the design of well-controlled porous BN structures to be applied as adsorbents and drug-delivery carriers
Holographic analysis of diffraction structure factors
We combine the theory of inside-source/inside-detector x-ray fluorescence
holography and Kossel lines/x ray standing waves in kinematic approximation to
directly obtain the phases of the diffraction structure factors. The influence
of Kossel lines and standing waves on holography is also discussed. We obtain
partial phase determination from experimental data obtaining the sign of the
real part of the structure factor for several reciprocal lattice vectors of a
vanadium crystal.Comment: 4 pages, 3 figures, submitte
Direct Observation of High-Temperature Polaronic Behavior In Colossal Magnetoresistive Manganites
The temperature dependence of the electronic and atomic structure of the
colossal magnetoresistive oxides (x = 0.3, 0.4) has
been studied using core and valence level photoemission, x-ray absorption and
emission, and extended x-ray absorption fine structure spectroscopy. A dramatic
and reversible change of the electronic structure is observed on crossing the
Curie temperature, including charge localization and spin moment increase of
Mn, together with Jahn-Teller distortions, both signatures of polaron
formation. Our data are also consistent with a phase-separation scenario.Comment: 5 pages, 4 figures, revte
Phasing diffuse scattering. Application of the SIR2002 algorithm to the non-crystallographic phase problem
A new phasing algorithm has been used to determine the phases of diffuse
elastic X-ray scattering from a non-periodic array of gold balls of 50 nm
diameter. Two-dimensional real-space images, showing the charge-density
distribution of the balls, have been reconstructed at 50 nm resolution from
transmission diffraction patterns recorded at 550 eV energy. The reconstructed
image fits well with scanning electron microscope (SEM) image of the same
sample. The algorithm, which uses only the density modification portion of the
SIR2002 program, is compared with the results obtained via the
Gerchberg-Saxton-Fienup HIO algorithm. In this way the relationship between
density modification in crystallography and the HiO algorithm used in signal
and image processing is elucidated.Comment: 7 pages, 12 figure
Use of extended and prepared reference objects in experimental Fourier transform X-ray holography
The use of one or more gold nanoballs as reference objects for Fourier
Transform holography (FTH) is analysed using experimental soft X-ray
diffraction from objects consisting of separated clusters of these balls. The
holograms are deconvoluted against ball reference objects to invert to images,
in combination with a Wiener filter to control noise. A resolution of ~30nm,
smaller than one ball, is obtained even if a large cluster of balls is used as
the reference, giving the best resolution yet obtained by X-ray FTH. Methods of
dealing with missing data due to a beamstop are discussed. Practical prepared
objects which satisfy the FTH condition are suggested, and methods of forming
them described.Comment: 7 pages, 2 figures, submitted to Applied Physics Letter
Iterative Algorithms for Ptychographic Phase Retrieval
Ptychography promises diffraction limited resolution without the need for
high resolution lenses. To achieve high resolution one has to solve the phase
problem for many partially overlapping frames. Here we review some of the
existing methods for solving ptychographic phase retrieval problem from a
numerical analysis point of view, and propose alternative methods based on
numerical optimization.Comment: 32 pages, 15 figure
Fully Unintegrated Parton Correlation Functions and Factorization in Lowest Order Hard Scattering
Motivated by the need to correct the potentially large kinematic errors in
approximations used in the standard formulation of perturbative QCD, we
reformulate deeply inelastic lepton-proton scattering in terms of gauge
invariant, universal parton correlation functions which depend on all
components of parton four-momentum. Currently, different hard QCD processes are
described by very different perturbative formalisms, each relying on its own
set of kinematical approximations. In this paper we show how to set up
formalism that avoids approximations on final-state momenta, and thus has a
very general domain of applicability. The use of exact kinematics introduces a
number of significant conceptual shifts already at leading order, and tightly
constrains the formalism. We show how to define parton correlation functions
that generalize the concepts of parton density, fragmentation function, and
soft factor. After setting up a general subtraction formalism, we obtain a
factorization theorem. To avoid complications with Ward identities the full
derivation is restricted to abelian gauge theories; even so the resulting
structure is highly suggestive of a similar treatment for non-abelian gauge
theories.Comment: 44 pages, 69 figures typos fixed, clarifications and second appendix
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