535 research outputs found

    Potential bottom-up control of blue crab distribution at various spatial scales

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    Top-down (i.e., predation), bottom-up (i.e., food availability), and physical factors may influence blue crab (Callinectes sapidus) distribution. To assess the role of bottom-up and physical process in blue crab distributions, we concurrently measured density of the blue crab (Callinectes sapidus), density of its principal prey, the Baltic clam (Macoma balthica), and physical characteristics in mud and sand habitats spanning various spatial scales (1-10 km and 10-50 km) in the York River, Chesapeake Bay. Clam and crab densities were intermediate in mud, low in downriver sand, and high in upriver sand. Clam and crab densities were not correlated in mud. whereas in sand, they were significantly and positively correlated at both the local scale (I km) and at the broadest spatial scale (10-50 km). Crab density also correlated with salinity at the broad spatial scale. After removing the effect of salinity, crab density remained significantly correlated with clam density. Using a hydrodynamic model for the York River, potential transport of clams from downriver coves was primarily to upriver habitats, but transport of crabs was mainly to downriver seagrass habitats. At the local scale, upriver zones where crab and clams were abundant, crab density was highly correlated (r(2) = 0.93) with clam density, but not with salinity, suggesting that the distribution of blue crabs was driven mostly by their primary food item-clams. The collective findings are consistent with the hypotheses that crab density is driven by both food availability and salinity at broad spatial scales (10-50 km), whereas food availability is a primary control at smaller spatial scales (1-10 km). Bottom-up control of upper trophic levels may be distinctive; thus far, in marine and freshwater systems, bottom-up control has not been demonstrated to filter from basal to upper trophic levels. Furthermore, unvegetated habitats where food, such as clams, is abundant may be important in the population dynamics of the blue crab, even in systems where seagrass beds are common

    ROFRAQ: A statistics-based empirical method for assessing accident risk from rockfalls in quarries. International Journal of Rock Mechanics and Mining Sciences.

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    This paper describes an empirical method, called Rockfall Risk Assessment for Quarries (ROFRAQ), which assesses the risk associated with rock falls in quarries. The method is based on aprobabilistic approach that assumes that an accident occurs as a consequence of as equence of events. This method has been applied to slopes in a number of quarries, and has proved useful indetecting trouble some slopes on the basis of empirical evidence. Thus far, it has been applied to around 100 slopes from various quarries of different rocks. These results show satisfactory agreement with results for empirical methods applied in the civil engineering field to highways and roads. The authors describe a case study of a granite aggregate quarry that highlights a number of issues in relation to practical application of the method

    Intra-pulse transition between ion acceleration mechanisms in intense laser-foil interactions

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    Multiple ion acceleration mechanisms can occur when an ultrathin foil is irradiated with an intense laser pulse, with the dominant mechanism changing over the course of the interaction. Measurement of the spatial-intensity distribution of the beam of energetic protons is used to investigate the transition from radiation pressure acceleration to transparency-driven processes. It is shown numerically that radiation pressure drives an increased expansion of the target ions within the spatial extent of the laser focal spot, which induces a radial deflection of relatively low energy sheath-accelerated protons to form an annular distribution. Through variation of the target foil thickness, the opening angle of the ring is shown to be correlated to the point in time transparency occurs during the interaction and is maximized when it occurs at the peak of the laser intensity profile. Corresponding experimental measurements of the ring size variation with target thickness exhibit the same trends and provide insight into the intra-pulse laser-plasma evolution

    Ion acceleration and plasma jet formation in ultra-thin foils undergoing expansion and relativistic transparency

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    At sufficiently high laser intensities, the rapid heating to relativistic velocities and resulting decompression of plasma electrons in an ultra-thin target foil can result in the target becoming relativistically transparent to the laser light during the interaction. Ion acceleration in this regime is strongly affected by the transition from an opaque to a relativistically transparent plasma. By spatially resolving the laser-accelerated proton beam at near-normal laser incidence and at an incidence angle of 30°, we identify characteristic features both experimentally and in particle-in-cell simulations which are consistent with the onset of three distinct ion acceleration mechanisms: sheath acceleration; radiation pressure acceleration; and transparency-enhanced acceleration. The latter mechanism occurs late in the interaction and is mediated by the formation of a plasma jet extending into the expanding ion population. The effect of laser incident angle on the plasma jet is explored

    Proton acceleration enhanced by a plasma jet in expanding foils undergoing relativistic transparency

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    Ion acceleration driven by the interaction of an ultraintense (2x10^20 Wcm^-2) laser pulse with an ultrathin (40nm) foil target is experimentally and numerically investigated. Protons accelerated by sheath fields and via laser radiation pressure are angularly separated and identified based on their directionality and signature features (e.g. transverse instabilities) in the measured spatial-intensity distribution. A low divergence, high energy proton component is also detected when the heated target electrons expand and the target becomes relativistically transparent during the interaction. 2D and 3D particle-in-cell (PIC) simulations indicate that under these conditions a plasma jet is formed at the target rear, supported by a self-generated azimuthal magnetic field, which extends into the expanded layer of sheath-accelerated protons. Electrons trapped within this jet are directly accelerated to super-thermal energies by the portion of the laser pulse transmitted through the target. The resulting streaming of the electrons into the ion layers enhances the energy of protons in the vicinity of the jet. Through the addition of a controlled prepulse, the maximum energy of these protons is demonstrated experimentally and numerically to be sensitive to the picosecond rising edge prole of the laser pulse

    Aluminium content of spanish infant formula

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    Aluminium toxicity has been relatively well documented in infants with impaired renal function and premature neonates. The aims of this study were to analyse the concentration of aluminium in the majority of infant formulae sold commercially in Spain, to determine the influence of aluminium content in the tap water in reconstituted powder formulae and to estimate the theoretical toxic aluminium intake in comparison with the PTWI, and lastly, to discuss the possible interactions of certain essential trace elements added to formulation with aluminium according to type or main protein based infant formula. A total of 82 different infant formulae from 9 different manufacturers were studied. Sample digestion was simulated in a closed acid-decomposition microwave system. Aluminium concentration was determined by atomic absorption spectrophotometry with graphite furnace. In general, the infant formulae studied provide an aluminium level higher than that found in human milk, especially in the case of soya, preterm or hydrolysed casein-based formulae. Standard formulae provide lower aluminium intakes amounting to about 4 % PTWI. Specialised and preterm formulae result in moderate intake (11 – 12 % and 8 – 10 % PTWI, respectively). Soya formulae contribute the highest intake (15 % PTWI). Aluminium exposure from drinking water used for powder formula reconstitution is not considered a clear potential risk. In accordance with the present state of knowledge about aluminium toxicity, it seems prudent to call for continued efforts to standardise routine quality control and reduce aluminium levels in infant formula as well as to keep the aluminium concentration under 300 g l-1 for all infant formulae, most specifically those formulae for premature and low birth neonates

    Phase structure of ceramics Al[2]O[3]-ZrW[2]O[8]

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    In the course of work the phase composition of the ceramic composite material Al[2]O[3] - 50 % ZrW[2]O[8] was studied. On the polished surface of the Al[2]O[3]-ZrW[2]O[8] sintered composite can be allocated three areas: dark gray matrix, white inclusions of an irregular form and spherical inclusions. The average size of spherical inclusions was equal to 15 [mu]m. The phase structure of ceramic composite was presented by trigonal modification of aluminum oxide, cubic zirconium tungstate, monoclinic modification of zirconium oxide and tungsten oxide

    Physical and mechanical properties and deformation behavior of porous ceramics based on plasma chemical powders Al[2]O[3], ZrO[2](MgO)

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    This work investigates the behavior of porous alumina and zirconia stabilized with magnesium oxide (within the porosity range of 18% to 70%) ceramics when subjected to deformation by compression and shearing. The analysis of strain-deformation curves showed that there was a transition from a typically brittle state for relatively dense ceramics, to a pseudo-plastic one with a high rate of porosity. The values of the effective elasticity modulus, effective shear modulus and Poisson's ratio decrease with an increase in volume in the pore space of ceramics, which correlates with the appearance of plural cracking during the deformation of ceramics with a high level of porosity. There was made analysis of the nature of the destruction of ceramics with the same level of porosity. The difference in the deformation behavior of the ZrO[2]-MgO compared to the Al[2]O[3] ceramic was the fact that the transition from a typically brittle to tively dense ceramics to the pseudo -plastic at a high level of porosity is implemented with a lower level of porosity
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