17,448 research outputs found
The antiproton component of the primary cosmic ray flux
Solution of Fokker-Planck diffusion equation in intensity prediction of antiproton component of primary cosmic ray flu
Use of quantitative ultrasound scans of the calcaneus to diagnose osteoporosis in patients with rheumatoid arthritis
Background: Patients with rheumatoid arthritis are recognized as being at risk for osteoporosis as a result of the disease process as well as the medication used to treat it. This study was conducted to consider the use of calcaneal scanning with quantitative ultrasoundâcontact ultrasound bone analysis (CUBA)âto diagnose osteoporosis in patients with rheumatoid arthritis.Methods: Forty-six patients (11 men and 35 women) with established rheumatoid arthritis underwent dual-energy x-ray absorptiometry (DEXA) of the nondominant wrist andCUBA of the nondominant heel. Sensitivity, specificity, and positive and negative predictive values were used to determine the correlation between osteoporosis as diagnosedby the CUBA heel scan compared with the DEXA wrist scan given that DEXA is widely seen as the gold standard for the diagnosis of osteoporosis.Results: The CUBA heel scan revealed a sensitivity of 90% and a specificity of 44% for a diagnosis of osteoporosis compared with DEXA. The positive predictive value of theCUBA scan was 31%, and the negative predictive value was 94%. Therefore, if normal bone density is found using CUBA, there is 94% certainty this is correct. However, if osteoporosis is diagnosed using CUBA, there is only 31% certainty this is correct. In such instances a secondary scan using a different method (eg, DEXA) would be required. Future work should consider the effect of minor alterations to the equipment or scanning protocol, because this may improve diagnosis.Conclusions: The CUBA unit could be used as a primary screening device. Given the cost and accessibility issues associated with DEXA, quantitative ultrasound may have arole in screening for osteoporosis in the primary-care setting to determine the most appropriate routes of referral for patients requiring further investigations. <br/
The Relation between Approximation in Distribution and Shadowing in Molecular Dynamics
Molecular dynamics refers to the computer simulation of a material at the
atomic level. An open problem in numerical analysis is to explain the apparent
reliability of molecular dynamics simulations. The difficulty is that
individual trajectories computed in molecular dynamics are accurate for only
short time intervals, whereas apparently reliable information can be extracted
from very long-time simulations. It has been conjectured that long molecular
dynamics trajectories have low-dimensional statistical features that accurately
approximate those of the original system. Another conjecture is that numerical
trajectories satisfy the shadowing property: that they are close over long time
intervals to exact trajectories but with different initial conditions. We prove
that these two views are actually equivalent to each other, after we suitably
modify the concept of shadowing. A key ingredient of our result is a general
theorem that allows us to take random elements of a metric space that are close
in distribution and embed them in the same probability space so that they are
close in a strong sense. This result is similar to the Strassen-Dudley Theorem
except that a mapping is provided between the two random elements. Our results
on shadowing are motivated by molecular dynamics but apply to the approximation
of any dynamical system when initial conditions are selected according to a
probability measure.Comment: 21 pages, final version accepted in SIAM Dyn Sy
Ion yields and erosion rates for Si1âxGex(0x1) ultralow energy O2+ secondary ion mass spectrometry in the energy range of 0.25â1 keV
We report the SIMS parameters required for the quantitative analysis of Si1âxGex across the range of 0 †x †1 when using low energy O2+ primary ions at normal incidence. These include the silicon and germanium secondary ion yield [i.e., the measured ion signal (ions/s)] and erosion rate [i.e., the speed at which the material sputters (nm/min)] as a function of x. We show that the ratio Rx of erosion rates, Si1âxGex/Si, at a given x is almost independent of beam energy, implying that the properties of the altered layer are dominated by the interaction of oxygen with silicon. Rx shows an exponential dependence on x. Unsurprisingly, the silicon and germanium secondary ion yields are found to depart somewhat from proportionality to (1âx) and x, respectively, although an approximate linear relationship could be used for quantification across around 30% of the range of x (i.e., a reference material containing Ge fraction x would give reasonably accurate quantification across the range of ±0.15x). Direct comparison of the useful (ion) yields [i.e., the ratio of ion yield to the total number of atoms sputtered for a particular species (ions/atom)] and the sputter yields [i.e., the total number of atoms sputtered per incident primary ion (atoms/ions)] reveals a moderate matrix effect where the former decrease monotonically with increasing x except at the lowest beam energy investigated (250 eV). Here, the useful yield of Ge is found to be invariant with x. At 250 eV, the germanium ion and sputter yields are proportional to x for all x
Teleportation of continuous variable polarisation states
This paper discusses methods for the optical teleportation of continuous
variable polarisation states. We show that using two pairs of entangled beams,
generated using four squeezed beams, perfect teleportation of optical
polarisation states can be performed. Restricting ourselves to 3 squeezed
beams, we demonstrate that polarisation state teleportation can still exceed
the classical limit. The 3-squeezer schemes involve either the use of quantum
non-demolition measurement or biased entanglement generated from a single
squeezed beam. We analyse the efficacies of these schemes in terms of fidelity,
signal transfer coefficients and quantum correlations
Extraordinarily high leaf selenium to sulfur ratios define âse-accumulatorâ plants
Background and Aims: Selenium (Se) and sulfur (S) exhibit similar chemical properties. In flowering plants (angiosperms) selenate and sulfate are acquired and assimilated by common transport and metabolic pathways. It is hypothesized that most angiosperm species show little or no discrimination in the accumulation of Se and S in leaves when their roots are supplied a mixture of selenate and sulfate, but some, termed Se-accumulator plants, selectively accumulate Se in preference to S under these conditions.
Methods: This paper surveys Se and S accumulation in leaves of 39 angiosperm species, chosen to represent the range of plant Se accumulation phenotypes, grown hydroponically under identical conditions.
Results: The data show that, when supplied a mixture of selenate and sulfate: (1) plant species differ in both their leaf Se ([Se]leaf) and leaf S ([S]leaf) concentrations; (2) most angiosperms show little discrimination for the accumulation of Se and S in their leaves and, in non-accumulator plants, [Se]leaf and [S]leaf are highly correlated; (3) [Se]leaf in Se-accumulator plants is significantly greater than in other angiosperms, but [S]leaf, although high, is within the range expected for angiosperms in general; and (4) the Se/S quotient in leaves of Se-accumulator plants is significantly higher than in leaves of other angiosperms.
Conclusion: The traits of extraordinarily high [Se]leaf and leaf Se/S quotients define the distinct elemental composition of Se-accumulator plants
Stable resonances and signal propagation in a chaotic network of coupled units
We apply the linear response theory developed in \cite{Ruelle} to analyze how
a periodic signal of weak amplitude, superimposed upon a chaotic background, is
transmitted in a network of non linearly interacting units. We numerically
compute the complex susceptibility and show the existence of specific poles
(stable resonances) corresponding to the response to perturbations transverse
to the attractor. Contrary to the poles of correlation functions they depend on
the pair emitting/receiving units. This dynamic differentiation, induced by non
linearities, exhibits the different ability that units have to transmit a
signal in this network.Comment: 10 pages, 3 figures, to appear in Phys. rev.
Ideal isotropic auxetic networks from random networks
Auxetic materials are characterized by a negative Poisson's ratio,
. As the Poisson's ratio becomes negative and approaches the
lower isotropic mechanical limit of , materials show
enhanced resistance to impact and shear, making them suitable for applications
ranging from robotics to impact mitigation. Past experimental efforts aimed at
reaching the limit have resulted in highly anisotropic
materials, which show a negative Poisson's ratio only when subjected to
deformations along specific directions. Isotropic designs have only attained
moderately auxetic behavior, or have led to structures that cannot be
manufactured in 3D. Here, we present a design strategy to create isotropic
structures from disordered networks that leads to Poisson's ratios as low as
. The materials conceived through this approach are
successfully fabricated in the laboratory and behave as predicted. The
Poisson's ratio is found to depend on network structure and bond
strengths; this sheds light on the structural motifs that lead to auxetic
behavior. The ideas introduced here can be generalized to 3D, a wide range of
materials, and a spectrum of length scales, thereby providing a general
platform that could impact technology.Comment: 16 pages, 6 figure
Distribution of periodic points of polynomial diffeomorphisms of C^2
This paper deals with the dynamics of a simple family of holomorphic
diffeomorphisms of \C^2: the polynomial automorphisms. This family of maps
has been studied by a number of authors. We refer to [BLS] for a general
introduction to this class of dynamical systems. An interesting object from the
point of view of potential theory is the equilibrium measure of the set
of points with bounded orbits. In [BLS] is also characterized
dynamically as the unique measure of maximal entropy. Thus is also an
equilibrium measure from the point of view of the thermodynamical formalism. In
the present paper we give another dynamical interpretation of as the
limit distribution of the periodic points of
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Electrospray synthesis of PLGA TIPS microspheres
We successfully demonstrate the synthesis of polymer microspheres using a single electrospray source, and show their physical characterisation. Electrospray has proven to be a versatile method to manufacture particles, giving tight control over size with quasi-monodisperse size distributions. It is a liquid atomisation technique that generates a monodisperse population of highly charged liquid droplets over a broad size range (nanometres to tens of microns). The droplets contain liquid precursors for the in-flight synthesis of particles, and control over the trajectory of these droplets can be precisely manipulated with the use of electric fields to drive them to a grounded substrate. This study reports a method to synthesize poly(lactic-co-glycolic) acid (PLGA) microspheres using the electrospray and thermally induced phase separation (TIPS) techniques, followed by subsequent freeze-drying, for particle production. These microspheres are of interest as vehicles for controlled drug release systems
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