87,310 research outputs found
High-resolution ab initio three-dimensional X-ray diffraction microscopy
Coherent X-ray diffraction microscopy is a method of imaging non-periodic
isolated objects at resolutions only limited, in principle, by the largest
scattering angles recorded. We demonstrate X-ray diffraction imaging with high
resolution in all three dimensions, as determined by a quantitative analysis of
the reconstructed volume images. These images are retrieved from the 3D
diffraction data using no a priori knowledge about the shape or composition of
the object, which has never before been demonstrated on a non-periodic object.
We also construct 2D images of thick objects with infinite depth of focus
(without loss of transverse spatial resolution). These methods can be used to
image biological and materials science samples at high resolution using X-ray
undulator radiation, and establishes the techniques to be used in
atomic-resolution ultrafast imaging at X-ray free-electron laser sources.Comment: 22 pages, 11 figures, submitte
Disentangling the Cosmic Web I: Morphology of Isodensity Contours
We apply Minkowski functionals and various derived measures to decipher the
morphological properties of large-scale structure seen in simulations of
gravitational evolution. Minkowski functionals of isodensity contours serve as
tools to test global properties of the density field. Furthermore, we identify
coherent objects at various threshold levels and calculate their partial
Minkowski functionals. We propose a set of two derived dimensionless
quantities, planarity and filamentarity, which reduce the morphological
information in a simple and intuitive way. Several simulations of the
gravitational evolution of initial power-law spectra provide a framework for
systematic tests of our method.Comment: 26 pages including 12 figures. Accepted for publication in Ap
On the reliability of initial conditions for dissipationless cosmological simulations
We present the study of ten random realizations of a density field
characterized by a cosmological power spectrum P(k) at redshift z=50. The
reliability of such initial conditions for n-body simulations are tested with
respect to their correlation properties. The power spectrum P(k), and the mass
variance sigmaM(r) do not show detectable deviations from the desired behavior
in the intermediate range of scales between the mean interparticle distance and
the simulation volume. The estimator for xi(r) is too noisy to detect any
reliable signal at the initial redshift z=50. The particle distributions are
then evolved forward until z=0. This allows us to explore the cosmic variance
stemming from the random nature of the initial conditions. With cosmic variance
we mean the fact that a simulation represents a single realization of the
stochastic initial conditions whereas the real Universe contains many
realizations of regions of the size of the box; this problem affects most
importantly the scales at about the fundamental mode. We study morphological
descriptors of the matter distribution such as the genus, as well as the
internal properties of the largest object(s) forming in the box. We find that
the scatter is at least comparable to the scatter in the fundamental mode.Comment: 22 pages, 12 figures, replaced with major revision to previous
submission, PASA in pres
Extending Continuum Models for Atom Probe Simulation
This work describes extensions to existing level-set algorithms developed for
application within the field of Atom Probe Tomography (APT). We present a new
simulation tool for the simulation of 3D tomographic volumes, using advanced
level set methods. By combining narrow-band, B-Tree and particle-tracing
approaches from level-set methods, we demonstrate a practical tool for
simulating shape changes to APT samples under applied electrostatic fields, in
three dimensions. This work builds upon our previous studies by allowing for
non-axially symmetric solutions, with minimal loss in computational speed,
whilst retaining numerical accuracy
The Area Distribution of Solar Magnetic Bright Points
Magnetic Bright Points (MBPs) are among the smallest observable objects on
the solar photosphere. A combination of G-band observations and numerical
simulations is used to determine their area distribution. An automatic
detection algorithm, employing 1-dimensional intensity profiling, is utilized
to identify these structures in the observed and simulated datasets. Both
distributions peak at an area of 45000 km, with a sharp decrease
towards smaller areas. The distributions conform with log-normal statistics,
which suggests that flux fragmentation dominates over flux convergence.
Radiative magneto-convection simulations indicate an independence in the MBP
area distribution for differing magnetic flux densities. The most commonly
occurring bright point size corresponds to the typical width of intergranular
lanes.Comment: Astrophysical Journal, accepte
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