7,997 research outputs found
Influence of crystallographic orientation of biogenic calcite on <i>in situ</i> Mg XANES analyses
Micro X-ray absorption near-edge spectroscopy at the Mg <i>K</i>-edge is a useful technique for acquiring information about the environment of Mg<sup>2+</sup> in biogenic calcite. These analyses can be applied to shell powders or intact shell structures. The advantage of the latter is that the XANES analyses can be applied to specific areas, at high (e.g. micrometre) spatial resolution, to determine the environment of Mg<sup>2+</sup> in a biomineral context. Such in situ synchrotron analysis has to take into account the potential effect of crystallographic orientation given the anisotropy of calcite crystals and the polarized nature of X-rays. Brachiopod shells of species with different crystallographic orientations are used to assess this crystallographic effect on <i>in situ</i> synchrotron measurements at the Mg <i>K</i>-edge. Results show that, owing to the anisotropy of calcite, <i>in situ</i> X-ray absorption spectra (XAS) are influenced by the crystallographic orientation of calcite crystals with a subsequent potentially erroneous interpretation of Mg<sup>2+</sup> data. Thus, this study demonstrates the importance of crystallography for XAS analyses and, therefore, the necessity to obtain crystallographic information at high spatial resolution prior to spectroscopic analysis
Scalar Top Quark Studies with Various Visible Energies
The precision determination of scalar top quark properties will play an
important role at a future International Linear Collider (ILC). Recent and
ongoing studies are discussed for different experimental topologies in the
detector. First results are presented for small mass differences between the
scalar top and neutralino masses. This corresponds to a small expected visible
energy in the detector. An ILC will be a unique accelerator to explore this
scenario. In addition to finding the existence of light stop quarks, the
precise measurement of their properties is crucial for testing their impact on
the dark matter relic abundance and the mechanism of electroweak baryogenesis.
Significant sensitivity for mass differences down to 5 GeV are obtained. The
simulation is based on a fast and realistic detector simulation. A vertex
detector concept of the Linear Collider Flavor Identification
(LCFI)collaboration, which studies pixel detectors for heavy quark flavour
identification, is implemented in the simulations for c-quark tagging. The
study extends simulations for large mass differences (large visible energy) for
which aspects of different detector simulations, the vertex detector design,
and different methods for the determination of the scalar top mass are
discussed. Based on the detailed simulations we study the uncertainties for the
dark matter density predictions and their estimated uncertainties from various
sources. In the region of parameters where stop-neutralino co-annihilation
leads to a value of the relic density consistent with experimental results, as
precisely determined by the Wilkinson Microwave Anisotropy Probe (WMAP), the
stop-neutralino mass difference is small and the ILC will be able to explore
this region efficiently.Comment: 11 pages, 11 figures, presented at SUSY'0
A series solution and a fast algorithm for the inversion of the spherical mean Radon transform
An explicit series solution is proposed for the inversion of the spherical
mean Radon transform. Such an inversion is required in problems of thermo- and
photo- acoustic tomography. Closed-form inversion formulae are currently known
only for the case when the centers of the integration spheres lie on a sphere
surrounding the support of the unknown function, or on certain unbounded
surfaces. Our approach results in an explicit series solution for any closed
measuring surface surrounding a region for which the eigenfunctions of the
Dirichlet Laplacian are explicitly known - such as, for example, cube, finite
cylinder, half-sphere etc. In addition, we present a fast reconstruction
algorithm applicable in the case when the detectors (the centers of the
integration spheres) lie on a surface of a cube. This algorithm reconsrtucts
3-D images thousands times faster than backprojection-type methods
Reconstruction of deglacial sea surface temperatures in the tropical Pacific from selective analysis of a fossil coral
The Sr/Ca of coral skeletons demonstrates potential as an indicator of sea surface temperatures (SSTs). However, the glacial-interglacial SST ranges predicted from Sr/Ca of fossil corals are usually higher than from other marine proxies. We observed infilling of secondary aragonite, characterised by high Sr/Ca ratios, along intraskeletal pores of a fossil coral from Papua New Guinea that grew during the penultimate deglaciation (130 +/- 2 ka). Selective microanalysis of unaltered areas of the fossil coral indicates that SSTs at similar to 130 ka were <= 1 degrees C cooler than at present in contrast with bulk measurements ( combining infilled and unaltered areas) which indicate a difference of 6-7 degrees C. The analysis of unaltered areas of fossil skeletons by microprobe techniques may offer a route to more accurate reconstruction of past SSTs.</p
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Effects of solar wind magnetosphere coupling recorded at different geomagnetic latitudes: Separation of directly-driven and storage/release systems
The effect on geomagnetic activity of solar wind speed, compared with that of the strength of the interplanetary magnetic field, differs with geomagnetic latitude. In this study we construct a new index based on monthly standard deviations in the H-component of the geomagnetic field for all geomagnetic latitudes. We demonstrate that for this index the response at auroral regions correlates best with interplanetary coupling functions which include the solar wind speed while mid- and low-latitude regions respond to variations in the interplanetary magnetic field strength. These results are used to isolate the responsible geomagnetic current systems
The spectrum of large powers of the Laplacian in bounded domains
We present exact results for the spectrum of the Nth power of the Laplacian
in a bounded domain. We begin with the one dimensional case and show that the
whole spectrum can be obtained in the limit of large N. We also show that it is
a useful numerical approach valid for any N. Finally, we discuss implications
of this work and present its possible extensions for non integer N and for 3D
Laplacian problems.Comment: 13 pages, 2 figure
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