19,818 research outputs found
Thermal detector model for cryogenic composite detectors for the dark matter experiments CRESST and EURECA
The CRESST (Cryogenic Rare Event Search with Superconducting Thermometers)
and the EURECA (European Underground Rare Event Calorimeter Array) experiments
are direct dark matter search experiments where cryogenic detectors are used to
detect spin-independent, coherent WIMP (Weakly Interacting Massive
Particle)-nucleon scattering events by means of the recoil energy. The
cryogenic detectors use a massive single crystal as absorber which is equipped
with a TES (transition edge sensor) for signal read-out. They are operated at
mK-temperatures. In order to enable a mass production of these detectors, as
needed for the EURECA experiment, a so-called composite detector design (CDD)
that allows decoupling of the TES fabrication from the optimization procedure
of the absorber single-crystal was developed and studied. To further
investigate, understand and optimize the performance of composite detectors a
detailed thermal detector model which takes into account the CDD has been
developed.Comment: To appear in Journal of Physics: Conference Series; Proceedings of
Neutrino 2008, Christchurch, New Zealan
Where is the fuzz? Undetected Lyman alpha nebulae around QSOs at z~2.3
We observed a small sample of 5 radio-quiet QSOs with integral field
spectroscopy to search for possible extended emission in the Ly line.
We subtracted the QSO point sources using a simple PSF self-calibration
technique that takes advantage of the simultaneous availability of spatial and
spectral information. In 4 of the 5 objects we find no significant traces of
extended Ly emission beyond the contribution of the QSO nuclei itself,
while in UM 247 there is evidence for a weak and spatially quite compact excess
in the Ly line at several kpc outside the nucleus. For all objects in
our sample we estimated detection limits for extended, smoothly distributed
Ly emission by adding fake nebulosities into the datacubes and trying
to recover them after PSF subtraction. Our observations are consistent with
other studies showing that giant Ly nebulae such as those found
recently around some quasars are very rare. Ly fuzz around typical
radio-quiet QSOs is fainter, less extended and is therefore much harder to
detect. The faintness of these structures is consistent with the idea that
radio-quiet QSOs typically reside in dark matter haloes of modest masses.Comment: 12 Pages, Accepted for publication in A&
Morphometric approach to many-body correlations in hard spheres
We model the thermodynamics of local structures within the hard sphere liquid
at arbitrary volume fractions through the \textit{morphometric} calculation of
-body correlations. We calculate absolute free energies of local geometric
motifs in excellent quantitative agreement with molecular dynamics simulations
across the liquid and supercooled liquid regimes. We find a bimodality in the
density library of states where five-fold symmetric structures appear lower in
free energy than four-fold symmetric structures, and from a single reaction
path predict a relaxation barrier which scales linearly in the compressibility
factor. The method provides a new route to assess changes in the free energy
landscape at volume fractions dynamically inaccessible to conventional
techniques.Comment: 6+17 pages, 3 figure
A simulation study of scene confusion factors in sensing soil moisture from orbital radar
Simulated C-band radar imagery for a 124-km by 108-km test site in eastern Kansas is used to classify soil moisture. Simulated radar resolutions are 100 m by 100 m, 1 km by 1km, and 3 km by 3 km. Distributions of actual near-surface soil moisture are established daily for a 23-day accounting period using a water budget model. Within the 23-day period, three orbital radar overpasses are simulated roughly corresponding to generally moist, wet, and dry soil moisture conditions. The radar simulations are performed by a target/sensor interaction model dependent upon a terrain model, land-use classification, and near-surface soil moisture distribution. The accuracy of soil-moisture classification is evaluated for each single-date radar observation and also for multi-date detection of relative soil moisture change. In general, the results for single-date moisture detection show that 70% to 90% of cropland can be correctly classified to within +/- 20% of the true percent of field capacity. For a given radar resolution, the expected classification accuracy is shown to be dependent upon both the general soil moisture condition and also the geographical distribution of land-use and topographic relief. An analysis of cropland, urban, pasture/rangeland, and woodland subregions within the test site indicates that multi-temporal detection of relative soil moisture change is least sensitive to classification error resulting from scene complexity and topographic effects
Model-free reconstruction of magnetic correlations in frustrated magnets
Frustrated magnetic systems exhibit extraordinary physical properties but
quantification of their magnetic correlations poses a serious challenge to
experiment and theory. Current insight into frustrated magnetic correlations
relies on modelling techniques such as reverse Monte Carlo methods, which
require knowledge about the exact ordered atomic structure. Here we present a
method for direct reconstruction of magnetic correlations in frustrated magnets
by three-dimensional difference pair distribution function analysis of neutron
total scattering data. The methodology is applied to the disordered frustrated
magnet bixbyite, (Mn1-xFex)2O3, which reveals nearest-neighbor
antiferromagnetic correlations for the metal sites up to a range of
approximately 15 {\AA}. Importantly, this technique allows for magnetic
correlations to be determined directly from the experimental data without any
assumption about the atomic structure
Polymeric forms of carbon in dense lithium carbide
The immense interest in carbon nanomaterials continues to stimulate intense
research activities aimed to realize carbon nanowires, since linear chains of
carbon atoms are expected to display novel and technologically relevant
optical, electrical and mechanical properties. Although various allotropes of
carbon (e.g., diamond, nanotubes, graphene, etc.) are among the best known
materials, it remains challenging to stabilize carbon in the one-dimensional
form because of the difficulty to suitably saturate the dangling bonds of
carbon. Here, we show through first-principles calculations that ordered
polymeric carbon chains can be stabilized in solid LiC under moderate
pressure. This pressure-induced phase (above 5 GPa) consists of parallel arrays
of twofold zigzag carbon chains embedded in lithium cages, which display a
metallic character due to the formation of partially occupied carbon lone-pair
states in \emph{sp}-like hybrids. It is found that this phase remains the
most favorable one in a wide range of pressure. At extreme pressure (larger the
215 GPa) a structural and electronic phase transition towards an insulating
single-bonded threefold-coordinated carbon network is predicted.Comment: 10 pages, 6 figure
Evaluation of the CGIAR Research Program on Climate Change, Agriculture and Food Security (CCAFS)
This evaluation has been commissioned by the Independent Evaluation Arrangement (IEA) of CGIAR. Following consultation with the ISP and CCAFS management, the Evaluation addressed four key evaluation questions.
1) How well is strategic collaboration and integration both within and outside the CGIAR being achieved? 2) To what extent is CCAFS generating unique international public goods for agriculture, food security and climate change? 3) How well do the Flagship Projects link together and combine at output and outcome levels in the regions; and, to what extent are successes toward outcomes transferable from region to region? 4) How robust are the monitoring, evaluation and learning processes of the Program?
As its main approach, the Evaluation selected nine projects for case studies to address the evaluation criteria and questions at three levels: global Program level, Flagship level and Regional Program level. The methodology also included document review, field visits to all regions, interviews of over 150 stakeholders, survey of CCAFS researchers, identification of exemplary cases of CCAFS work with high impact potential using a triple loop learning analysis, bibliometric analysis and H-index analysis of senior researchers. An independent third-party review team contributed to the evaluation of quality of science by assessing a random sample of journal articles published by CCAFS
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