1,814 research outputs found
Factors influencing the stable carbon isotopic composition of suspended and sinking organic matter in the coastal Antarctic sea ice environment
A high resolution time-series analysis of stable carbon isotopic signatures in particulate organic carbon (&delta;<sup>13</sup>C<sub>POC</sub>) and associated biogeochemical parameters in sea ice and surface waters provides an insight into the factors affecting &delta;<sup>13</sup>C<sub>POC</sub> in the coastal western Antarctic Peninsula sea ice environment. The study covers two austral summer seasons in Ryder Bay, northern Marguerite Bay between 2004 and 2006. A shift in diatom species composition during the 2005/06 summer bloom to near-complete biomass dominance of <i>Proboscia inermis</i> is strongly correlated with a large ~10 &permil; negative isotopic shift in &delta;<sup>13</sup>C<sub>POC</sub> that cannot be explained by a concurrent change in concentration or isotopic signature of CO<sub>2</sub>. We hypothesise that the &delta;<sup>13</sup>C<sub>POC</sub> shift may be driven by the contrasting biochemical mechanisms and utilisation of carbon-concentrating mechanisms (CCMs) in different diatom species. Specifically, very low &delta;<sup>13</sup>C<sub>POC</sub> in <i>P. inermis</i> may be caused by the lack of a CCM, whilst some diatom species abundant at times of higher &delta;<sup>13</sup>C<sub>POC</sub> may employ CCMs. These short-lived yet pronounced negative &delta;<sup>13</sup>C<sub>POC</sub> excursions drive a 4 &permil; decrease in the seasonal average &delta;<sup>13</sup>C<sub>POC</sub> signal, which is transferred to sediment traps and core-top sediments and consequently has the potential for preservation in the sedimentary record. This 4 &permil; difference between seasons of contrasting sea ice conditions and upper water column stratification matches the full amplitude of glacial-interglacial Southern Ocean &delta;<sup>13</sup>C<sub>POC</sub> variability and, as such, we invoke phytoplankton species changes as a potentially important factor influencing sedimentary &delta;<sup>13</sup>C<sub>POC</sub>. We also find significantly higher &delta;<sup>13</sup>C<sub>POC</sub> in sea ice than surface waters, consistent with autotrophic carbon fixation in a semi-closed environment and possible contributions from post-production degradation, biological utilisation of HCO<sub>3</sub><sup>&minus;</sup> and production of exopolymeric substances. This study demonstrates the importance of surface water diatom speciation effects and isotopically heavy sea ice-derived material for &delta;<sup>13</sup>C<sub>POC</sub> in Antarctic coastal environments and underlying sediments, with consequences for the utility of diatom-based &delta;<sup>13</sup>C<sub>POC</sub> in the sedimentary record
Isospin symmetry breaking in an algebraic pairing Sp(4) model
An exactly solvable sp(4) algebraic approach extends beyond the traditional
isospin conserving nuclear interaction to bring forward effects of isospin
symmetry breaking and isospin mixing resulting from a two-body nuclear
interaction that includes proton-neutron (pn) and like-particle isovector
pairing correlations plus significant isoscalar pn interactions. The model
yields an estimate for the extent to which isobaric analog 0+ states in light
and medium mass nuclei may mix with one another and reveals possible, but still
extremely weak, non-analog beta-decay transitions.Comment: 8 pages, 2 figure
Informed consent - a survey of doctors' practices in South Africa
Objective. To examine doctors' practices with regard to informed consent.Design. Cross-sectional, descriptive survey.Participants 'and setting. All full-time consultants and registrars in the Departments of Medicine, Obstetrics and Gynaecology, Paediatrics and Child Health, Paediatric Surgery and Surgery at the University of Cape Town were included. The overall response rate was 63% (160/254).Measurement. Data were collected by means of selfadministered, semi-structured questionnaires.Results. Most doctors (79%) felt it was their responsibility to ensure that patients and parents were fully informed about diagnostic and therapeutic interventions. Many (62%) supported a patient-centred standard for determining the type and amount of information to disclose. Doctors disclose most of the legally required information except for information about alternative forms of treatment and remote serious risks. They almost never provide information on medical costs. The most common reasons for not obtaining informed consent were the doctors' tendency to 'tell' patients! parents what they intend doing and their belief that patients/parents expect doctors to know what is medicallybest for them. Language, inadequate communication skills and lack of time were, surprisingly, seldom viewed as obstacles to the obtaining of informed consent. Findings were independent of discipline (medical or surgical) and doctors' status (consultant or registrar). Doctors who treat children were significantly less likely to obtain consent forcertain interventions.Conclusion. Doctors meet many, but not all, of the legal requirements for informed consent. The findings question whether informed consent as envisioned by the law existsin reality. Cross-cultural research is needed to clarify patients' and parents' expectations of informed consent
Synchronization and Coarsening (without SOC) in a Forest-Fire Model
We study the long-time dynamics of a forest-fire model with deterministic
tree growth and instantaneous burning of entire forests by stochastic lightning
strikes. Asymptotically the system organizes into a coarsening self-similar
mosaic of synchronized patches within which trees regrow and burn
simultaneously. We show that the average patch length grows linearly with
time as t-->oo. The number density of patches of length L, N(L,t), scales as
^{-2}M(L/), and within a mean-field rate equation description we find
that this scaling function decays as e^{-1/x} for x-->0, and as e^{-x} for
x-->oo. In one dimension, we develop an event-driven cluster algorithm to study
the asymptotic behavior of large systems. Our numerical results are consistent
with mean-field predictions for patch coarsening.Comment: 5 pages, 4 figures, 2-column revtex format. To be submitted to PR
Phason elasticity of a three-dimensional quasicrystal: transfer-matrix method
We introduce a new transfer matrix method for calculating the thermodynamic
properties of random-tiling models of quasicrystals in any number of
dimensions, and describe how it may be used to calculate the phason elastic
properties of these models, which are related to experimental measurables such
as phason Debye-Waller factors, and diffuse scattering wings near Bragg peaks.
We apply our method to the canonical-cell model of the icosahedral phase,
making use of results from a previously-presented calculation in which the
possible structures for this model under specific periodic boundary conditions
were cataloged using a computational technique. We give results for the
configurational entropy density and the two fundamental elastic constants for a
range of system sizes. The method is general enough allow a similar calculation
to be performed for any other random tiling model.Comment: 38 pages, 3 PostScript figures, self-expanding uuencoded compressed
tar file, LaTeX using RevTeX macros and epsfig.st
- mixing and spin dependent CSV potential
We construct the charge symmetry violating (CSV) nucleon-nucleon potential
induced by the -\o mixing due to the neutron-proton mass difference
driven by the loop. Analytical expression for for the two-body CSV
potential is presented containing both the central and non- central
interaction. We show that the tensor interaction can significantly
enhance the charge symmetry violating interaction even if momentum
dependent off-shell - mixing amplitude is considered. It is
also shown that the inclusion of form factors removes the divergence arising
out of the contact interaction. Consequently, we see that the precise size of
the computed scattering length difference depends on how the short range
aspects of the CSV potential are treated.Comment: Accepted for publication in Phys. Rev.
Ab initio Ti-Zr-Ni phase diagram predicts stability of icosahedral TiZrNi quasicrystals
The ab initio phase diagram determines the energetic stability of the icosahedral TiZrNi quasicrystal. The complete ab initio zero-temperature ternary phase diagram is constructed from the calculated energies of the elemental, binary and ternary Ti-Zr-Ni phases. For this, the icosahedral i-TiZrNi quasicrystal is approximated by periodic structures of up to 123 atoms/unit cell, based on a decorated-tiling model [R. G. Hennig, K. F. Kelton, A. E. Carlsson, and C. L. Henley, Phys. Rev. B 67, 134202 (2003)]. The approximant structures containing the 45-atom Bergman cluster are nearly degenerate in energy, and are all energetically stable against the competing phases. It is concluded that i-TiZrNi is a ground-state quasicrystal, as it is experimentally the low-temperature phase for its composition
Scaling laws and simulation results for the self--organized critical forest--fire model
We discuss the properties of a self--organized critical forest--fire model
which has been introduced recently. We derive scaling laws and define critical
exponents. The values of these critical exponents are determined by computer
simulations in 1 to 8 dimensions. The simulations suggest a critical dimension
above which the critical exponents assume their mean--field values.
Changing the lattice symmetry and allowing trees to be immune against fire, we
show that the critical exponents are universal.Comment: 12 pages, postscript uuencoded, figures included, to appear in Phys.
Rev.
Evaluation of the digital diabetes prevention programme pilot: Uncontrolled mixed-methods study protocol
This is the final version. Available from the publisher via the DOI in this record.Introduction The prevalence of type 2 diabetes is rising steeply. National Health Service England (NHSE) is exploring the potential of a digital diabetes prevention programme (DDPP) and has commissioned a pilot with embedded evaluation. Methods and analysis This study aims to determine whether, and if so, how, should NHSE implement a national DDPP, using a mixed-methods pretest and post-test design, underpinned by two theoretical frameworks: the Coventry, Aberdeen and London - Refined (CALO-RE) taxonomy of behavioural change techniques for the digital interventions and the Consolidated Framework for Implementation Research (CFIR) for implementation processes. In eight pilot areas across England, adults with non-diabetic hyperglycaemia (NDH) (glycated haemoglobin (HbA1c) 42-47 mmol/mol or fasting plasma glucose 5.5-6.9 mmol/L) and adults without NDH who are overweight (body mass index (BMI) >25 kg/m 2) or obese (BMI >30 kg/m 2) will be referred to one of five digitally delivered diabetes prevention interventions. The primary outcomes are reduction in HbA1c and weight (for people with NDH) and reduction in weight (for people who are overweight or obese) at 12 months. Secondary outcomes include use of the intervention, satisfaction, physical activity, patient activation and resources needed for successful implementation. Quantitative data will be collected at baseline, 6 months and 12 months by the digital intervention providers. Qualitative data will be collected through semistructured interviews with commissioners, providers, healthcare professionals and patients. Quantitative data will be analysed descriptively and using generalised linear models to determine whether changes in outcomes are associated with demographic and intervention factors. Qualitative data will be analysed using framework analysis, with data pertaining to implementation mapped onto the CFIR. Ethics and dissemination The study has received ethical approval from the Public Health England Ethics and Research Governance Group (reference R&D 324). Dissemination will include a report to NHSE to inform future policy and publication in peer-reviewed journals.National Institute for Health Research (NIHR
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