3,726 research outputs found
Dielectric properties measurements of brown and white adipose tissue in rats from 0.5 to 10 GHz
Brown adipose tissue (BAT) plays an important role in whole body metabolism and with appropriate stimulus could potentially mediate weight gain and insulin sensitivity. Although imaging techniques are available to detect subsurface BAT, there are currently no viable methods for continuous acquisition of BAT energy expenditure. Microwave (MW) radiometry is an emerging technology that allows the quantification of tissue temperature variations at depths of several centimeters. Such temperature differentials may be correlated with variations in metabolic rate, thus providing a quantitative approach to monitor BAT metabolism. In order to optimize MW radiometry, numerical and experimental phantoms with accurate dielectric properties are required to develop and calibrate radiometric sensors. Thus, we present for the first time, the characterization of relative permittivity and electrical conductivity of brown (BAT) and white (WAT) adipose tissues in rats across the MW range 0.5-10GHz. Measurements were carried out in situ and post mortem in six female rats of approximately 200g. A Cole-Cole model was used to fit the experimental data into a parametric model that describes the variation of dielectric properties as a function of frequency. Measurements confirm that the dielectric properties of BAT (Δr = 14.0-19.4, Ï = 0.3-3.3S/m) are significantly higher than those of WAT (Δr = 9.1-11.9, Ï = 0.1-1.9S/m), in accordance with the higher water content of BAT
Coronal energy release by MHD avalanches. Effects on a structured, active region, multi-threaded coronal loop
A possible key element for large-scale energy release in the solar corona is
an MHD kink instability in a single twisted magnetic flux tube. An initial
helical current sheet fragments in a turbulent way into smaller-scale sheets,
similarly to a nanoflare storm. As the loop expands in the radial direction
during the relaxation process, an unstable loop can disrupt nearby stable loops
and trigger an MHD avalanche. Exploratory investigations have been conducted in
previous works with relatively simplified loop configurations. Here, we address
a more realistic environment that comprehensively accounts for most of the
physical effects involved in a stratified atmosphere, typical of an active
region. The question is whether the avalanche process will be triggered, with
what timescales, and how it will develop, as compared with the original,
simpler approach. Three-dimensional MHD simulations describe the interaction of
magnetic flux tubes, which have a stratified atmosphere, including
chromospheric layers, the thin transition region to the corona, and the related
transition from high-beta to low-beta regions. The model also includes the
effects of thermal conduction and of optically thin radiation. Our simulations
address the case where one flux tube among a few is twisted at the footpoints
faster than its neighbours. We show that this flux tube becomes kink unstable
first, in conditions in agreement with those predicted by analytical models. It
rapidly involves nearby stable tubes, instigating significant magnetic
reconnection and dissipation of energy as heat. The heating determines the
development of chromospheric evaporation, while the temperature rises up to
about 10 MK, close to microflares observations. This work confirms that
avalanches are a viable mechanism for the storing and release of magnetic
energy in plasma confined in closed coronal loops, as a result of photospheric
motions.Comment: 16 pages, 16 figure
Performance and enteric methane emissions from housed beef cattle fed silage produced on pastures with different forage profiles
Methane (CH4) produced by ruminants is a significant source of greenhouse gases (GHG) from agriculture in the United Kingdom (UK), accounting for approximately 50% of the emissions in this sector. Ration modification is linked to changes in rumen fermentation and can be an effective means of CH4 abatement. In temperate climate countries, forage silage represents a major feed component for cattle during the housing period. The objective of this study was, therefore, to compare enteric CH4 emission from cattle offered silage produced from different types of grassland. Beef cattle, steers (n = 89) and heifers (n = 88) with average liveweight (LW) of 328 ± 57.1 kg, were evaluated during two housing seasons (2016-2017 and 2017-2018) from November to April, at the Rothamsted Research North Wyke Farm Platform (NWFP, UK). The treatments corresponded to three diet types, comprising silage harvested from three different pastures: MRG, monoculture of perennial ryegrass (PRG, Lolium perenne L.cv. AberMagic), bred to express the high sugar phenotype; RG-WC, a mixed sward comprised of the same perennial ryegrass cultivar with white clover (Trifolium repens L.) with a target clover proportion of 30% as land cover; and PP, permanent pasture dominated by PRG and a small number of non-introduced species. MRG and PP received 160-200 kg N/ha/year. Cattle were weighed every 30 days and the enteric CH4 emission was determined using GreenFeed automated systems. No significant differences in enteric CH4 emission per head or per kg LW were observed between treatments. However, emission expressed per average daily gain (ADG) in LW was greater (p<0.001) for MRG compared with RG-WC and PP, at 270, 248 and 235 g CH4/kg ADG, respectively. This related to a lower ADG (p=0.041) for the animals fed MRG silage compared with RG-WC and PP which were similar, with respective values of 0.67, 0.71 and 0.74 kg/day. The forages compared in this study showed little or no potential to reduce enteric CH4 emission when fed as silage to growing beef cattle during the winter housing period
Holonomic quantum computation with neutral atoms
We propose an all-geometric implementation of quantum computation using
neutral atoms in cavity QED. We show how to perform generic single- and
two-qubit gates, the latter by encoding a two-atom state onto a single,
many-level atom. We compare different strategies to overcome limitations due to
cavity imperfections.Comment: 14 pages, 9 figure
Probing the single-particle character of rotational states in F using a short-lived isomeric beam
A beam containing a substantial component of both the ,
ns isomeric state of F and its , 109.77-min ground
state has been utilized to study members of the ground-state rotational band in
F through the neutron transfer reaction , in inverse kinematics.
The resulting spectroscopic strengths confirm the single-particle nature of the
13/2 band-terminating state. The agreement between shell-model
calculations, using an interaction constructed within the shell, and our
experimental results reinforces the idea of a single-particle/collective
duality in the descriptions of the structure of atomic nuclei
Pliocene Te Aute limestones, New Zealand: Expanding concepts for cool-water shelf carbonates
Acceptance of a spectrum of warm- through cold-water shallow-marine carbonate facies has become of fundamental importance for correctly interpreting the origin and significance of all ancient platform limestones. Among other attributes, properties that have become a hallmark for characterising many Cenozoic non-tropical occurrences include: (1) the presence of common bryozoan and epifaunal bivalve skeletons; (2) a calcite-dominated mineralogy; (3) relatively thin deposits exhibiting low rates of sediment accumulation; (4) an overall destructive early diagenetic regime; and (5) that major porosity destruction and lithification occur mainly in response to chemical compaction of calcitic skeletons during moderate to deep burial. The Pliocene Te Aute limestones are non-tropical skeletal carbonates formed at paleolatitudes near 40-42°S under the influence of commonly strong tidal flows along the margins of an actively deforming and differentially uplifting forearc basin seaway, immediately inboard of the convergent Pacific-Australian plate boundary off eastern North Island, New Zealand. This dynamic depositional and tectonic setting strongly influenced both the style and subsequent diagenetic evolution of the limestones. Some of the Te Aute limestones exhibit the above kinds of "normal" non-tropical characteristics, but others do not. For example, many are barnacle and/or bivalve dominated, and several include attributes that at least superficially resemble properties of certain tropical carbonates. In this regard, a number of the limestones are infaunal bivalve rich and dominated by an aragonite over a calcite primary mineralogy, with consequently relatively high diagenetic potential. Individual limestone units are also often rather thick (e.g., up to 50-300 m), with accumulation rates from 0.2 to 0.5 m/ka, and locally as high as 1 m/ka. Moreover, there can be a remarkable array of diagenetic features in the limestones, involving grain alteration and/or cementation to widely varying extents within any, or some combination of, the marine phreatic, burial, and meteoric diagenetic environments, including locally widespread development of meteoric cement sourced from aragonite dissolution. The message is that non-tropical shelf carbonates include a more diverse array of geological settings, of skeletal and mineralogical facies, and of diagenetic features than current sedimentary models mainly advocate. While several attributes positively distinguish tropical from non-tropical limestones, continued detailed documentation of the wide spectrum of shallow-marine carbonate deposits formed outside tropical regions remains an important challenge in carbonate sedimentology
Initiation and propagation of coronal mass ejections
This paper reviews recent progress in the research on the initiation and
propagation of CMEs. In the initiation part, several trigger mechanisms are
discussed; In the propagation part, the observations and modelings of EIT
waves/dimmings, as the EUV counterparts of CMEs, are described.Comment: 8 pages, 1 figure, an invited review, to appear in J. Astrophys.
Astro
The effect of soil type on yield and micronutrient content of pasture species
The use of multispecies swards on livestock farms is growing due to the wide range of benefits they bring, such as improved biomass yield and animal performance. Preferential uptake of micronutrients by some plant species means the inclusion of legumes and forbs in grass-dominated pasture swards could improve micronutrient provision to livestock via careful species selection. However, although soil properties affect plant micronutrient concentrations, it is unknown whether choosing âbest-performingâ species, in terms of their micronutrient content, needs to be soil-specific or whether the recommendations can be more generic.
To address this question, we carried out an experiment with 15 common grass, forb and legume species grown on four soils for five weeks in a controlled environment. The soils were chosen to have contrasting properties such as texture, organic matter content and micronutrient concentrations. The effect of soil pH was tested on two soils (pH 5.4 and 7.4) chosen to minimise other confounding variables.
Yield was significantly affected by soil properties and there was a significant interaction with botanical group but not species within a botanical group (grass, forb or legume). There were differences between botanical groups and between species in both their micronutrient concentrations and total uptake. Micronutrient herbage concentrations often, but not always, reflected soil micronutrient concentrations. There were soil-botanical group interactions for micronutrient concentration and uptake by plants, but the interaction between plant species (within a botanical group) and soil was significant only for forbs, and predominantly occurred when considering micronutrient uptake rather than concentration. Generally, plants had higher yields and micronutrient contents at pH 5.4 than 7.4.
Forbs tended to have higher concentrations of micronutrients than other botanical groups and the effect of soil on micronutrient uptake was only significant for forbs
Self-healing diffusion quantum Monte Carlo algorithms: methods for direct reduction of the fermion sign error in electronic structure calculations
We develop a formalism and present an algorithm for optimization of the trial
wave-function used in fixed-node diffusion quantum Monte Carlo (DMC) methods.
We take advantage of a basic property of the walker configuration distribution
generated in a DMC calculation, to (i) project-out a multi-determinant
expansion of the fixed-node ground-state wave function and (ii) to define a
cost function that relates the fixed-node ground-state and the non-interacting
trial wave functions. We show that (a) locally smoothing out the kink of the
fixed-node ground-state wave function at the node generates a new trial
wave-function with better nodal structure and (b) we argue that the noise in
the fixed-node wave-function resulting from finite sampling plays a beneficial
role, allowing the nodes to adjust towards the ones of the exact many-body
ground state in a simulated annealing-like process. We propose a method to
improve both single determinant and multi-determinant expansions of the trial
wave-function. We test the method in a model system where benchmark
configuration interaction calculations can be performed. Comparing the DMC
calculations with the exact solutions, we find that the trial wave-function is
systematically improved. The overlap of the optimized trial wave function and
the exact ground state converges to 100% even starting from wave-functions
orthogonal to the exact ground state. In the optimization process we find an
optimal non-interacting nodal potential of density-functional-like form whose
existence was predicted earlier[Phys.Rev. B {\bf 77}, 245110 (2008)]. We obtain
the exact Kohn-Sham effective potential from the DMC data.Comment: Final version of the paper accepted in Physical Review B. The review
reports and replies are included in the sourc
Optimization of inhomogeneous electron correlation factors in periodic solids
A method is presented for the optimization of one-body and inhomogeneous
two-body terms in correlated electronic wave functions of Jastrow-Slater type.
The most general form of inhomogeneous correlation term which is compatible
with crystal symmetry is used and the energy is minimized with respect to all
parameters using a rapidly convergent iterative approach, based on Monte Carlo
sampling of the energy and fitting energy fluctuations. The energy minimization
is performed exactly within statistical sampling error for the energy
derivatives and the resulting one- and two-body terms of the wave function are
found to be well-determined. The largest calculations performed require the
optimization of over 3000 parameters. The inhomogeneous two-electron
correlation terms are calculated for diamond and rhombohedral graphite. The
optimal terms in diamond are found to be approximately homogeneous and
isotropic over all ranges of electron separation, but exhibit some
inhomogeneity at short- and intermediate-range, whereas those in graphite are
found to be homogeneous at short-range, but inhomogeneous and anisotropic at
intermediate- and long-range electron separation.Comment: 23 pages, 15 figures, 1 table, REVTeX4, submitted to PR
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