866 research outputs found
The Next Round of Hadronic Generator Tuning Heavily Based on Identified Particle Data
Event shape and charged particle inclusive distributions determined from 750
000 hadronic Z events measured with the DELPHI detector at LEP are presented.
The statistical and systematic precision of this data allows for a decisive
confrontation with Monte Carlo models of the hadronization process and a better
understanding of the structure of the Z hadronic final state.
Improved tunings of the JETSET, ARIADNE and HERWIG parton shower models and
the JETSET matrix element model are obtained by fitting the models to
identified particle distributions from all LEP experiments and the DELPHI data
presented. The description of the data distributions by the models is
critically reviewed with special importance attributed to identified particles.Comment: 73+2 pages, latex, 39 figures appended as uuencoded fil
Anpassung und Test von Fragmentierungsmodellen mit präzisen Ereignisform- und Einteilchenverteilungen unter besonderer Berücksichtigung von identifizierten Teilchenspektren
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
When combat prevents PTSD symptoms—results from a survey with former child soldiers in Northern Uganda
Weierstall R, Schalinski I, Crombach A, Hecker T, Elbert T. When combat prevents PTSD symptoms—results from a survey with former child soldiers in Northern Uganda. BMC Psychiatry. 2012;12(1): 41
Coherent X-ray Diffractive Imaging; applications and limitations
The inversion of a diffraction pattern offers aberration-free
diffraction-limited 3D images without the resolution and depth-of-field
limitations of lens-based tomographic systems, the only limitation being
radiation damage. We review our experimental results, discuss the fundamental
limits of this technique and future plans.Comment: 7 pages, 8 figure
SPEDEN: Reconstructing single particles from their diffraction patterns
Speden is a computer program that reconstructs the electron density of single
particles from their x-ray diffraction patterns, using a single-particle
adaptation of the Holographic Method in crystallography. (Szoke, A., Szoke, H.,
and Somoza, J.R., 1997. Acta Cryst. A53, 291-313.) The method, like its parent,
is unique that it does not rely on ``back'' transformation from the diffraction
pattern into real space and on interpolation within measured data. It is
designed to deal successfully with sparse, irregular, incomplete and noisy
data. It is also designed to use prior information for ensuring sensible
results and for reliable convergence. This article describes the theoretical
basis for the reconstruction algorithm, its implementation and quantitative
results of tests on synthetic and experimentally obtained data. The program
could be used for determining the structure of radiation tolerant samples and,
eventually, of large biological molecular structures without the need for
crystallization.Comment: 12 pages, 10 figure
Dose, exposure time, and resolution in Serial X-ray Crystallography
The resolution of X-ray diffraction microscopy is limited by the maximum dose
that can be delivered prior to sample damage. In the proposed Serial
Crystallography method, the damage problem is addressed by distributing the
total dose over many identical hydrated macromolecules running continuously in
a single-file train across a continuous X-ray beam, and resolution is then
limited only by the available molecular and X-ray fluxes and molecular
alignment. Orientation of the diffracting molecules is achieved by laser
alignment. We evaluate the incident X-ray fluence (energy/area) required to
obtain a given resolution from (1) an analytical model, giving the count rate
at the maximum scattering angle for a model protein, (2) explicit simulation of
diffraction patterns for a GroEL-GroES protein complex, and (3) the frequency
cut off of the transfer function following iterative solution of the phase
problem, and reconstruction of an electron density map in the projection
approximation. These calculations include counting shot noise and multiple
starts of the phasing algorithm. The results indicate counting time and the
number of proteins needed within the beam at any instant for a given resolution
and X-ray flux. We confirm an inverse fourth power dependence of exposure time
on resolution, with important implications for all coherent X-ray imaging. We
find that multiple single-file protein beams will be needed for sub-nanometer
resolution on current third generation synchrotrons, but not on fourth
generation designs, where reconstruction of secondary protein structure at a
resolution of 0.7 nm should be possible with short exposures.Comment: 19 pages, 7 figures, 1 tabl
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
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