13,565 research outputs found
Uranium and Associated Heavy Metals in Ovis aries in a Mining Impacted Area in Northwestern New Mexico.
The objective of this study was to determine uranium (U) and other heavy metal (HM) concentrations (As, Cd, Pb, Mo, and Se) in tissue samples collected from sheep (Ovis aries), the primary meat staple on the Navajo reservation in northwestern New Mexico. The study setting was a prime target of U mining, where more than 1100 unreclaimed abandoned U mines and structures remain. The forage and water sources for the sheep in this study were located within 3.2 km of abandoned U mines and structures. Tissue samples from sheep (n = 3), their local forage grasses (n = 24), soil (n = 24), and drinking water (n = 14) sources were collected. The samples were analyzed using Inductively Coupled Plasma-Mass Spectrometry. Results: In general, HMs concentrated more in the roots of forage compared to the above ground parts. The sheep forage samples fell below the National Research Council maximum tolerable concentration (5 mg/kg). The bioaccumulation factor ratio was >1 in several forage samples, ranging from 1.12 to 16.86 for Mo, Cd, and Se. The study findings showed that the concentrations of HMs were greatest in the liver and kidneys. Of the calculated human intake, Se Reference Dietary Intake and Mo Recommended Dietary Allowance were exceeded, but the tolerable upper limits for both were not exceeded. Food intake recommendations informed by research are needed for individuals especially those that may be more sensitive to HMs. Further study with larger sample sizes is needed to explore other impacted communities across the reservation
Modelling electron distributions within ESA's Gaia satellite CCD pixels to mitigate radiation damage
The Gaia satellite is a high-precision astrometry, photometry and
spectroscopic ESA cornerstone mission, currently scheduled for launch in 2012.
Its primary science drivers are the composition, formation and evolution of the
Galaxy. Gaia will achieve its unprecedented positional accuracy requirements
with detailed calibration and correction for radiation damage. At L2, protons
cause displacement damage in the silicon of CCDs. The resulting traps capture
and emit electrons from passing charge packets in the CCD pixel, distorting the
image PSF and biasing its centroid. Microscopic models of Gaia's CCDs are being
developed to simulate this effect. The key to calculating the probability of an
electron being captured by a trap is the 3D electron density within each CCD
pixel. However, this has not been physically modelled for the Gaia CCD pixels.
In Seabroke, Holland & Cropper (2008), the first paper of this series, we
motivated the need for such specialised 3D device modelling and outlined how
its future results will fit into Gaia's overall radiation calibration strategy.
In this paper, the second of the series, we present our first results using
Silvaco's physics-based, engineering software: the ATLAS device simulation
framework. Inputting a doping profile, pixel geometry and materials into ATLAS
and comparing the results to other simulations reveals that ATLAS has a free
parameter, fixed oxide charge, that needs to be calibrated. ATLAS is
successfully benchmarked against other simulations and measurements of a test
device, identifying how to use it to model Gaia pixels and highlighting the
effect of different doping approximations.Comment: 12 pages, 6 figures, appearing in Proc. of SPIE Optics and Photonics
Conference (Focal Plane Arrays for Space telescopes IV), 2-6 August 2009, San
Diego, US
Locked and Unlocked Polygonal Chains in 3D
In this paper, we study movements of simple polygonal chains in 3D. We say
that an open, simple polygonal chain can be straightened if it can be
continuously reconfigured to a straight sequence of segments in such a manner
that both the length of each link and the simplicity of the chain are
maintained throughout the movement. The analogous concept for closed chains is
convexification: reconfiguration to a planar convex polygon. Chains that cannot
be straightened or convexified are called locked. While there are open chains
in 3D that are locked, we show that if an open chain has a simple orthogonal
projection onto some plane, it can be straightened. For closed chains, we show
that there are unknotted but locked closed chains, and we provide an algorithm
for convexifying a planar simple polygon in 3D with a polynomial number of
moves.Comment: To appear in Proc. 10th ACM-SIAM Sympos. Discrete Algorithms, Jan.
199
Dynamical density functional theory for the dewetting of evaporating thin films of nanoparticle suspensions exhibiting pattern formation
Recent experiments have shown that the striking structure formation in
dewetting films of evaporating colloidal nanoparticle suspensions occurs in an
ultrathin `postcursor' layer that is left behind by a mesoscopic dewetting
front. Various phase change and transport processes occur in the postcursor
layer, that may lead to nanoparticle deposits in the form of labyrinthine,
network or strongly branched `finger' structures. We develop a versatile
dynamical density functional theory to model this system which captures all
these structures and may be employed to investigate the influence of
evaporation/condensation, nanoparticle transport and solute transport in a
differentiated way. We highlight, in particular, the influence of the subtle
interplay of decomposition in the layer and contact line motion on the observed
particle-induced transverse instability of the dewetting front.Comment: 5 pages, 5 figure
Topology and Bistability in liquid crystal devices
We study nematic liquid crystal configurations in a prototype bistable device
- the Post Aligned Bistable Nematic (PABN) cell. Working within the Oseen-Frank
continuum model, we describe the liquid crystal configuration by a unit-vector
field, in a model version of the PABN cell. Firstly, we identify four distinct
topologies in this geometry. We explicitly construct trial configurations with
these topologies which are used as initial conditions for a numerical solver,
based on the finite-element method. The morphologies and energetics of the
corresponding numerical solutions qualitatively agree with experimental
observations and suggest a topological mechanism for bistability in the PABN
cell geometry
Maslov Indices and Monodromy
We prove that for a Hamiltonian system on a cotangent bundle that is
Liouville-integrable and has monodromy the vector of Maslov indices is an
eigenvector of the monodromy matrix with eigenvalue 1. As a corollary the
resulting restrictions on the monodromy matrix are derived.Comment: 6 page
Complex Periodic Orbits and Tunnelling in Chaotic Potentials
We derive a trace formula for the splitting-weighted density of states
suitable for chaotic potentials with isolated symmetric wells. This formula is
based on complex orbits which tunnel through classically forbidden barriers.
The theory is applicable whenever the tunnelling is dominated by isolated
orbits, a situation which applies to chaotic systems but also to certain
near-integrable ones. It is used to analyse a specific two-dimensional
potential with chaotic dynamics. Mean behaviour of the splittings is predicted
by an orbit with imaginary action. Oscillations around this mean are obtained
from a collection of related orbits whose actions have nonzero real part
Toward the End of Time
The null-brane space-time provides a simple model of a big crunch/big bang
singularity. A non-perturbative definition of M-theory on this space-time was
recently provided using matrix theory. We derive the fermion couplings for this
matrix model and study the leading quantum effects. These effects include
particle production and a time-dependent potential. Our results suggest that as
the null-brane develops a big crunch singularity, the usual notion of
space-time is replaced by an interacting gluon phase. This gluon phase appears
to constitute the end of our conventional picture of space and time.Comment: 31 pages, reference adde
- …