240 research outputs found
Prescission neutron multiplicity and fission probability from Langevin dynamics of nuclear fission
A theoretical model of one-body nuclear friction which was developed earlier,
namely the chaos-weighted wall formula, is applied to a dynamical description
of compound nuclear decay in the framework of the Langevin equation coupled
with statistical evaporation of light particles and photons. We have used both
the usual wall formula friction and its chaos-weighted version in the Langevin
equation to calculate the fission probability and prescission neutron
multiplicity for the compound nuclei W, Pt, Pb,
Fr, Th, and Es. We have also obtained the contributions
of the presaddle and postsaddle neutrons to the total prescission multiplicity.
A detailed analysis of our results leads us to conclude that the chaos-weighted
wall formula friction can adequately describe the fission dynamics in the
presaddle region. This friction, however, turns out to be too weak to describe
the postsaddle dynamics properly. This points to the need for a suitable
explanation for the enhanced neutron emission in the postsaddle stage of
nuclear fission.Comment: RevTex, 14 pages including 5 Postscript figures, results improved by
using a different potential, conclusions remain unchanged, to appear in Phys.
Rev.
Systematic 1/S study of the 2D Hubbard model at half-filling
The 2D Hubbard model is extended by placing 2S orbitals at each lattice site
and studied in a systematic 1/S expansion. The 1/S results for the magnetic
susceptibility and the spectra of spin-wave excitations at half-filling are
consistent with the large S calculations for the Heisenberg antiferromagnet.
The 1/S corrections to the fermionic spectrum lift the degeneracy along the
edge of the magnetic Brillouin zone yielding minima at (+- pi/2, +- pi/2).
Relation to previous papers on the subject is discussed.Comment: 18 pages, emTex version 3.
Thermal decomposition as route for silver nanoparticles
Single crystalline silver nanoparticles have been synthesized by thermal decomposition of silver oxalate in water and in ethylene glycol. Polyvinyl alcohol (PVA) was employed as a capping agent. The particles were spherical in shape with size below 10 nm. The chemical reduction of silver oxalate by PVA was also observed. Increase of the polymer concentration led to a decrease in the size of Ag particles. Ag nanoparticle was not formed in the absence of PVA. Antibacterial activity of the Ag colloid was studied by disc diffusion method
Hydrodynamic Transport Coefficients in Relativistic Scalar Field Theory
Hydrodynamic transport coefficients may be evaluated from first principles in
a weakly coupled scalar field theory at arbitrary temperature. In a theory with
cubic and quartic interactions, the infinite class of diagrams which contribute
to the leading weak coupling behavior are identified and summed. The resulting
expression may be reduced to a single linear integral equation, which is shown
to be identical to the corresponding result obtained from a linearized
Boltzmann equation describing effective thermal excitations with temperature
dependent masses and scattering amplitudes. The effective Boltzmann equation is
valid even at very high temperature where the thermal lifetime and mean free
path are short compared to the Compton wavelength of the fundamental particles.
Numerical results for the shear and the bulk viscosities are presented.Comment: 116 pages. An error in printing the figures file is corrected. The
bulk viscosity sections are corrected and entirely rewritten. 38 figures in
post-script format. One style file is include
Using the past to constrain the future: how the palaeorecord can improve estimates of global warming
Climate sensitivity is defined as the change in global mean equilibrium
temperature after a doubling of atmospheric CO2 concentration and provides a
simple measure of global warming. An early estimate of climate sensitivity,
1.5-4.5{\deg}C, has changed little subsequently, including the latest
assessment by the Intergovernmental Panel on Climate Change.
The persistence of such large uncertainties in this simple measure casts
doubt on our understanding of the mechanisms of climate change and our ability
to predict the response of the climate system to future perturbations. This has
motivated continued attempts to constrain the range with climate data, alone or
in conjunction with models. The majority of studies use data from the
instrumental period (post-1850) but recent work has made use of information
about the large climate changes experienced in the geological past.
In this review, we first outline approaches that estimate climate sensitivity
using instrumental climate observations and then summarise attempts to use the
record of climate change on geological timescales. We examine the limitations
of these studies and suggest ways in which the power of the palaeoclimate
record could be better used to reduce uncertainties in our predictions of
climate sensitivity.Comment: The final, definitive version of this paper has been published in
Progress in Physical Geography, 31(5), 2007 by SAGE Publications Ltd, All
rights reserved. \c{opyright} 2007 Edwards, Crucifix and Harriso
Epidemic centrality - is there an underestimated epidemic impact of network peripheral nodes?
In the study of disease spreading on empirical complex networks in SIR model,
initially infected nodes can be ranked according to some measure of their
epidemic impact. The highest ranked nodes, also referred to as
"superspreaders", are associated to dominant epidemic risks and therefore
deserve special attention. In simulations on studied empirical complex
networks, it is shown that the ranking depends on the dynamical regime of the
disease spreading. A possible mechanism leading to this dependence is
illustrated in an analytically tractable example. In systems where the
allocation of resources to counter disease spreading to individual nodes is
based on their ranking, the dynamical regime of disease spreading is frequently
not known before the outbreak of the disease. Therefore, we introduce a
quantity called epidemic centrality as an average over all relevant regimes of
disease spreading as a basis of the ranking. A recently introduced concept of
phase diagram of epidemic spreading is used as a framework in which several
types of averaging are studied. The epidemic centrality is compared to
structural properties of nodes such as node degree, k-cores and betweenness.
There is a growing trend of epidemic centrality with degree and k-cores values,
but the variation of epidemic centrality is much smaller than the variation of
degree or k-cores value. It is found that the epidemic centrality of the
structurally peripheral nodes is of the same order of magnitude as the epidemic
centrality of the structurally central nodes. The implications of these
findings for the distributions of resources to counter disease spreading are
discussed
20 years of the Atlantic Meridional Transect - AMT
The AMT (www.amt-uk.org) is a multidisciplinary programme which undertakes biological, chemical, and physical oceanographic research during an annual voyage between the UK and a destination in the South Atlantic such as the Falkland Islands, South Africa, or Chile. This transect of >12,000 km crosses a range of ecosystems from subpolar to tropical, from euphotic shelf seas and upwelling systems, to oligotrophic mid-ocean gyres. The year 2015 has seen two milestones in the history of the AMT: the achievement of 20 years of this unique ocean going programme and the departure of the 25th cruise on the 15th of September. Both of these events were celebrated in June this year with an open science conference hosted by the Plymouth Marine Laboratory (PML) and will be further documented in a special issue of Progress in Oceanography which is planned for publication in 2016. Since 1995, the 25 research cruises have involved 242 sea-going scientists from 66 institutes representing 22 countries. AMT was designed from the outset to be a collaborative programme. It was originally conceived by Jim Aiken, Patrick Holligan, Roger Harris, and Dave Robins with Chuck McClain and Chuck Trees at NASA to test and ground truth satellite algorithms of ocean color. The opportunities offered by this initiative meant that this series of repeated biannual cruises rapidly developed into a coordinated study of ocean biodiversity, biogeochemistry, and ocean/atmosphere interactions
Eye-tracking the time‐course of novel word learning and lexical competition in adults and children
Lexical competition is a hallmark of proficient, automatic word recognition. Previous research suggests that there is a delay before a new spoken word becomes engaged in this process, with sleep playing an important role. However, data from one method--the visual world paradigm--consistently show competition without a delay. We trained 42 adults and 40 children (aged 7-8) on novel word-object pairings, and employed this paradigm to measure the time-course of lexical competition. Fixations to novel objects upon hearing existing words (e.g., looks to the novel object biscal upon hearing “click on the biscuit”) were compared to fixations on untrained objects. Novel word-object pairings learned immediately before testing and those learned the previous day exhibited significant competition effects, with stronger competition for the previous day pairings for children but not adults. Crucially, this competition effect was significantly smaller for novel than existing competitors (e.g., looks to candy upon hearing “click on the candle”), suggesting that novel items may not compete for recognition like fully-fledged lexical items, even after 24 hours. Explicit memory (cued recall) was superior for words learned the day before testing, particularly for children; this effect (but not the lexical competition effects) correlated with sleep-spindle density. Together, the results suggest that different aspects of new word learning follow different time courses: visual world competition effects can emerge swiftly, but are qualitatively different from those observed with established words, and are less reliant upon sleep. Furthermore, the findings fit with the view that word learning earlier in development is boosted by sleep to a greater degree
Field Theory Approaches to Nonequilibrium Dynamics
It is explained how field-theoretic methods and the dynamic renormalisation
group (RG) can be applied to study the universal scaling properties of systems
that either undergo a continuous phase transition or display generic scale
invariance, both near and far from thermal equilibrium. Part 1 introduces the
response functional field theory representation of (nonlinear) Langevin
equations. The RG is employed to compute the scaling exponents for several
universality classes governing the critical dynamics near second-order phase
transitions in equilibrium. The effects of reversible mode-coupling terms,
quenching from random initial conditions to the critical point, and violating
the detailed balance constraints are briefly discussed. It is shown how the
same formalism can be applied to nonequilibrium systems such as driven
diffusive lattice gases. Part 2 describes how the master equation for
stochastic particle reaction processes can be mapped onto a field theory
action. The RG is then used to analyse simple diffusion-limited annihilation
reactions as well as generic continuous transitions from active to inactive,
absorbing states, which are characterised by the power laws of (critical)
directed percolation. Certain other important universality classes are
mentioned, and some open issues are listed.Comment: 54 pages, 9 figures, Lecture Notes for Luxembourg Summer School
"Ageing and the Glass Transition", submitted to Springer Lecture Notes in
Physics (www.springeronline/com/series/5304/
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