1,308 research outputs found
New insights (and new interrogations) in perinatal arterial ischemic stroke
With an incidence of 1/2800 to 1/5000 live-births, perinatal arterial ischemic stroke is the most frequent form of cerebral infarction in children. About 40% of the children do not have specific symptoms in the neonatal period, and are only recognized later with the emergence of motor impairment, developmental delay, specific cognitive deficiency or seizures. In the remaining 60%, children present with early symptoms, mostly recurrent focal seizures in the first 3 days of life. The diagnosis is easily confirmed by cranial ultrasounds and MRI. Early MRI has both a key role in the diagnosis, dating the injury, but also an important prognostic value to predict the motor outcome of the child. Indeed, although the infarct does not recur, the majority of children show subsequent sequels: cerebral palsy, epilepsy, cognitive or behavioural problems. Finding predictors of outcome regarding these latter concerns (and the way to prevent or alleviate them) is of major interest.The main etiological hypothesis for perinatal AIS is a cerebral embolus, originating from the placenta through the foramen ovale. Most of the established risk factors are indeed either determinants or biomarkers of vasculo-placental pathology. Injury to the cervico-cerebral arteries, giving rise to thrombus/embolus during the birthing process is also suggested. Both placento-embolic and traumatic theories are supported by a few, but well-analysed pathological or arteriographic reports. Nevertheless, their relative frequency, the implication of other mechanisms, and their repercussions to evidence-based preventive strategies remain to be determined. Moreover, the mechanism of stroke in the different groups of newborns with stroke (term vs. preterm; symptomatic neonates vs. those with a delayed presentation) is likely to be different, and there is a need for future studies to assess all populations as different entities. Neonatal supportive care remains important for all infants while there is no evidence for preventive anticoagulant use at present. In an effort to reduce neurological dysfunction, and in adjunction with ongoing physical therapy and pharmacological treatment, new rehabilitative interventions, such as constraint-induced movement therapy and mirror therapy, are increasingly being used
Charge transfer electrostatic model of compositional order in perovskite alloys
We introduce an electrostatic model including charge transfer, which is shown
to account for the observed B-site ordering in Pb-based perovskite alloys. The
model allows charge transfer between A-sites and is a generalization of
Bellaiche and Vanderbilt's purely electrostatic model. The large covalency of
Pb^{2+} compared to Ba^{2+} is modeled by an environment dependent effective
A-site charge. Monte Carlo simulations of this model successfully reproduce the
long range compositional order of both Pb-based and Ba-based complex
A(BB^{'}B^{''})O_3 perovskite alloys. The models are also extended to study
systems with A-site and B-site doping, such as
(Na_{1/2}La_{1/2})(Mg_{1/3}Nb_{2/3})O_3,
(Ba_{1-x}La_{x})(Mg_{(1+x)/3}Nb_{(2-x)/3})O_3 and
(Pb_{1-x}La_{x})(Mg_{(1+x)/3}Ta_{(2-x)/3})O_3. General trends are reproduced by
purely electrostatic interactions, and charge transfer effects indicate that
local structural relaxations can tip the balance between different B-site
orderings in Pb based materials.Comment: 15 pages, 6 figure
Polymer Sensors for the Quantification of Waterborne Uranium
Clandestine activities involving the separation, concentration or manipulation of special nuclear material for the express purpose of developing a weapon of mass destruction is likely to result in the contamination of environmental water sources. The capability to conduct isotopic analyses for waterborne special nuclear material, like uranium, would be a powerful nuclear forensics tool. Despite widespread interest, there currently is no on-line or field-able measurement system available for low-level quantification of uranium in aqueous solutions. A recent development in environmental sensing is a portable, flow cell detector that utilizes extractive scintillating (ES) resin. The ES resin serves the dual purpose of (1) concentrating the radionuclide of interest and (2) serving as a radiation transducer. Currently, such resins are produced by physically absorbing organic extractants and fluors into a polymer matrix. Unfortunately, this approach yields resins with poor stability as the active components leach from the resin over time. This contribution describes our work to increase resin stability by synthesizing ES resin in which the active components are bound covalently within the polymer matrix. The extraction and fluorescence properties of the resin were studied separately before the resin was applied in flow cell detector where detection efficiencies of 40% were achieved
Electrostatic model of atomic ordering in complex perovskite alloys
We present a simple ionic model which successfully reproduces the various
types of compositional long-range order observed in a large class of complex
insulating perovskite alloys. The model assumes that the driving mechanism
responsible for the ordering is simply the electrostatic interaction between
the different ionic species. A possible new explanation for the anomalous
long-range order observed in some Pb relaxor alloys, involving the proposed
existence of a small amount of Pb^4+ on the B sublattice, is suggested by an
analysis of the model.Comment: 4 pages, two-column style with 1 postscript figure embedded. Uses
REVTEX and epsf macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/index.html#lb_orde
Geometric analysis of satellite laser ranging data
The analysis of simultaneous laser data is investigated using the method of trilateration. Analysis of data from 1987 to 1992 is presented with selected baseline rates and station positions. The use of simultaneous Etalon data is simulated to demonstrate the additional global coverage these satellites provide. Trilateration has a great potential for regional deformation studies with monthly LAGEOS American solutions between 3-12 millimeters
CVM studies on the atomic ordering in complex perovskite alloys
The atomic ordering in complex perovskite alloys is investigated by the
cluster variation method (CVM). For the 1/3\{111\}-type ordered structure, the
order-disorder phase transition is the first order, and the order parameter of
the 1:2 complex perovskite reaches its maximum near x=0.25. For the
1/2\{111\}-type ordered structure, the ordering transition is the second order.
Phase diagrams for both ordered structures are obtained. The order-disorder
line obeys the linear law.Comment: 10 pages, 6 figure
Measurement of the 3He mass diffusion coefficient in superfluid 4He over the 0.45-0.95 K temperature range
We have measured the mass diffusion coefficient D of 3He in superfluid 4He at
temperatures lower than were previously possible. The experimental technique
utilizes scintillation light produced when neutron react with 3He nuclei, and
allows measurement of the 3He density integrated along the trajectory of a
well-defined neutron beam. By measuring the change in 3He density near a heater
as a function of applied heat current, we are able to infer values of D with
20% accuracy. At temperatures below 0.7 K and for concentrations of order
10^{-4} we find D=(2.0+2.4-1.2)T^-(6.5 -/+ 1.2) cm^2/s, in agreement with a
theoretical approximation.Comment: 8 pages, 5 figures. Submitted to Europhysics Letters and prepared in
that journal's forma
Heterovalent and A-atom effects in A(B'B'')O3 perovskite alloys
Using first-principles supercell calculations, we have investigated
energetic, structural and dielectric properties of three different A(B'B'')O_3
perovskite alloys: Ba(Zn_{1/3}Nb_{2/3})O_3 (BZN), Pb(Zn_{1/3}Nb_{2/3})O_3
(PZN), and Pb(Zr_{1/3}Ti_{2/3})O_3 (PZT). In the homovalent alloy PZT, the
energetics are found to be mainly driven by atomic relaxations. In the
heterovalent alloys BZN and PZN, however, electrostatic interactions among B'
and B'' atoms are found to be very important. These electrostatic interactions
are responsible for the stabilization of the observed compositional long-range
order in BZN. On the other hand, cell relaxations and the formation of short
Pb--O bonds could lead to a destabilization of the same ordered structure in
PZN. Finally, comparing the dielectric properties of homovalent and
heterovalent alloys, the most dramatic difference arises in connection with the
effective charges of the B' atom. We find that the effective charge of Zr in
PZT is anomalous, while in BZN and PZN the effective charge of Zn is close to
its nominal ionic value.Comment: 7 pages, two-column style with 2 postscript figures embedded. Uses
REVTEX and epsf macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/index.html#lb_he
Water in Comet 2/2003 K4 (LINEAR) with Spitzer
We present sensitive 5.5 to 7.6 micron spectra of comet C/2003 K4 (LINEAR)
obtained on 16 July 2004 (r_{h} = 1.760 AU, Delta_{Spitzer} = 1.409 AU, phase
angle 35.4 degrees) with the Spitzer Space Telescope. The nu_{2} vibrational
band of water is detected with a high signal-to-noise ratio (> 50). Model
fitting to the best spectrum yields a water ortho-to-para ratio of 2.47 +/-
0.27, which corresponds to a spin temperature of 28.5^{+6.5}_{-3.5} K. Spectra
acquired at different offset positions show that the rotational temperature
decreases with increasing distance from the nucleus, which is consistent with
evolution from thermal to fluorescence equilibrium. The inferred water
production rate is (2.43 +/- 0.25) \times 10^{29} molec. s^{-1}. The spectra do
not show any evidence for emission from PAHs and carbonate minerals, in
contrast to results reported for comets 9P/Tempel 1 and C/1995 O1 (Hale-Bopp).
However, residual emission is observed near 7.3 micron the origin of which
remains unidentified.Comment: 33 pages, including 11 figures, 2 tables, ApJ 2007 accepte
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