5,907 research outputs found
Ab initio compressive phase retrieval
Any object on earth has two fundamental properties: it is finite, and it is
made of atoms. Structural information about an object can be obtained from
diffraction amplitude measurements that account for either one of these traits.
Nyquist-sampling of the Fourier amplitudes is sufficient to image single
particles of finite size at any resolution. Atomic resolution data is routinely
used to image molecules replicated in a crystal structure. Here we report an
algorithm that requires neither information, but uses the fact that an image of
a natural object is compressible. Intended applications include tomographic
diffractive imaging, crystallography, powder diffraction, small angle x-ray
scattering and random Fourier amplitude measurements.Comment: 7 pages, 4 figures, presented at the XXI IUCr Congress, Aug. 2008,
Osaka Japa
Spatial Coherence of Synchrotron Radiation
Theory and measurement of spatial coherence of synchrotron radiation beams
are briefly reviewed. Emphasis is given to simple relationships between
electron beam characteristics and far field properties of the light beam.Comment: 8 pages, 3 figure
Constrained MC for QCD evolution with rapidity ordering and minimum kT
With the imminent start of LHC experiments, development of phenomenological
tools, and in particular the Monte Carlo programs and algorithms, becomes
urgent. A new algorithm for the generation of a parton shower initiated by the
single initial hadron beam is presented. The new algorithm is of the class of
the so called ``constrained MC'' type algorithm (an alternative to the backward
evolution MC algorithm), in which the energy and the type of the parton at the
end of the parton shower are constrained (predefined). The complete kinematics
configurations with explicitly constructed four momenta are generated and
tested. Evolution time is identical with rapidity and minimum transverse
momentum is used as an infrared cut-off. All terms of the leading-logarithmic
approximation in the DGLAP evolution are properly accounted for. In addition,
the essential improvements towards the so-called CCFM/BFKL models are also
properly implemented. The resulting parton distributions are cross-checked up
to the 0.1% precision level with the help of a multitude of comparisons with
other MC and non-MC programs. We regard these tests as an important asset to be
exploited at the time when the presented MC will enter as a building block in a
larger MC program for W/Z production process at LHC.Comment: Submitted to Computer Physics Communication
Experimental determination of the b quark mass in DELPHI
The running mass of the b quark as defined in the MS-bar renormalization
scheme, m_b, was measured at the M_Z scale using 2.8 million hadronic Z^0
decays collected by the DELPHI experiment at LEP. The result is m_b(M_Z) = 2.67
+- 0.25 (stat.) +- 0.34 (frag.) +- 0.27(theo.) GeV/c^2 which differs from that
obtained at the Upsilon scale, by m_b(M_\Upsilon/2)-m_b(M_Z) = 1.49 +- 0.52
GeV/c^2. This measurement, performed far from the production
threshold, provides the first experimental observation of the running of the
quark masses.Comment: Talk given at the QCD 97 conference held in Montpellier, July 1997.
Also available here http://hep.ph.liv.ac.uk/~martis
Coherent Diffraction Imaging of Single 95nm Nanowires
Photonic or electronic confinement effects in nanostructures become
significant when one of their dimension is in the 5-300 nm range. Improving
their development requires the ability to study their structure - shape, strain
field, interdiffusion maps - using novel techniques. We have used coherent
diffraction imaging to record the 3-dimensionnal scattered intensity of single
silicon nanowires with a lateral size smaller than 100 nm. We show that this
intensity can be used to recover the hexagonal shape of the nanowire with a
28nm resolution. The article also discusses limits of the method in terms of
radiation damage.Comment: 5 pages, 5 figure
The size-star formation relation of massive galaxies at 1.5<z<2.5
We study the relation between size and star formation activity in a complete
sample of 225 massive (M > 5 x 10^10 Msun) galaxies at 1.5<z<2.5, selected from
the FIREWORKS UV-IR catalog of the CDFS. Based on stellar population synthesis
model fits to the observed restframe UV-NIR SEDs, and independent MIPS 24
micron observations, 65% of galaxies are actively forming stars, while 35% are
quiescent. Using sizes derived from 2D surface brightness profile fits to high
resolution (FWHM_{PSF}~0.45 arcsec) groundbased ISAAC data, we confirm and
improve the significance of the relation between star formation activity and
compactness found in previous studies, using a large, complete mass-limited
sample. At z~2, massive quiescent galaxies are significantly smaller than
massive star forming galaxies, and a median factor of 0.34+/-0.02 smaller than
galaxies of similar mass in the local universe. 13% of the quiescent galaxies
are unresolved in the ISAAC data, corresponding to sizes <1 kpc, more than 5
times smaller than galaxies of similar mass locally. The quiescent galaxies
span a Kormendy relation which, compared to the relation for local early types,
is shifted to smaller sizes and brighter surface brightnesses and is
incompatible with passive evolution. The progenitors of the quiescent galaxies,
were likely dominated by highly concentrated, intense nuclear star bursts at
z~3-4, in contrast to star forming galaxies at z~2 which are extended and
dominated by distributed star formation.Comment: 6 pages, 4 figures, accepted for publication in Ap
A unified evaluation of iterative projection algorithms for phase retrieval
Iterative projection algorithms are successfully being used as a substitute
of lenses to recombine, numerically rather than optically, light scattered by
illuminated objects. Images obtained computationally allow aberration-free
diffraction-limited imaging and the possibility of using radiation for which no
lenses exist. The challenge of this imaging technique is transfered from the
lenses to the algorithms. We evaluate these new computational ``instruments''
developed for the phase retrieval problem, and discuss acceleration strategies.Comment: 12 pages, 9 figures, revte
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
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