224 research outputs found
A liquid helium target system for a measurement of parity violation in neutron spin rotation
A liquid helium target system was designed and built to perform a precision
measurement of the parity-violating neutron spin rotation in helium due to the
nucleon-nucleon weak interaction. The measurement employed a beam of low energy
neutrons that passed through a crossed neutron polarizer--analyzer pair with
the liquid helium target system located between them. Changes between the
target states generated differences in the beam transmission through the
polarizer--analyzer pair. The amount of parity-violating spin rotation was
determined from the measured beam transmission asymmetries. The expected
parity-violating spin rotation of order rad placed severe constraints
on the target design. In particular, isolation of the parity-odd component of
the spin rotation from a much larger background rotation caused by magnetic
fields required that a nonmagnetic cryostat and target system be supported
inside the magnetic shielding, while allowing nonmagnetic motion of liquid
helium between separated target chambers. This paper provides a detailed
description of the design, function, and performance of the liquid helium
target system.Comment: V2: 29 pages, 14 figues, submitted to Nucl. Instrum. Meth. B. Revised
to address reviewer comment
Sticky stuff : redefining bedform prediction in modern and ancient environments
This work was funded by the UK Natural Environment Research Council (NERC) under the COHBED project (NE/1027223/1). Paterson was funded by the Marine Alliance for Science and Technology for Scotland (MASTS).The dimensions and dynamics of subaqueous bedforms are well known for cohesionless sediments. However, the effect of physical cohesion imparted by cohesive clay within mixed sand-mud substrates has not been examined, despite its recognized influence on sediment stability. Here we present a series of controlled laboratory experiments to establish the influence of substrate clay content on subaqueous bedform dynamics within mixtures of sand and clay exposed to unidirectional flow. The results show that bedform dimensions and steepness decrease linearly with clay content, and comparison with existing predictors of bedform dimensions, established within cohesionless sediments, reveals significant over-prediction of bedform size for all but the lowermost clay contents examined. The profound effect substrate clay content has on bedform dimensions has a number of important implications for interpretation in a range of modern and ancient environments, including reduced roughness and bedform heights in estuarine systems and the often cited lack of large dune cross-sets in turbidites. The results therefore offer a step change in our understanding of bedform formation and dynamics in these, and many other, sedimentary environments.Publisher PDFPeer reviewe
Modelling J/psi production and absorption in a microscopic nonequilibrium approach
Charmonium production and absorption in heavy ion collisions is studied with
the Ultrarelativisitic Quantum Molecular Dynamics model. We compare the
scenario of universal and time independent color-octet dissociation cross
sections with one of distinct color-singlet J/psi, psi' and chi_c states,
evolving from small, color transparent configurations to their asymptotic
sizes. The measured J/psi production cross sections in pA and AB collisions at
SPS energies are consistent with both - purely hadronic - scenarios. The
predicted rapidity dependence of J/psi suppression can be used to discriminate
between the two experimentally. The importance of interactions with secondary
hadrons and the applicability of thermal reaction kinetics to J/psi absorption
are investigated. We discuss the effect of nuclear stopping and the role of
leading hadrons. The dependence of the psi' to J/psi ratio on the model
assumptions and the possible influence of refeeding processes is also studied.Comment: 35 pages, 16 figure
The systematic study of the influence of neutron excess on the fusion cross sections using different proximity-type potentials
Using different types of proximity potentials, we have examined the trend of
variations of barrier characteristics (barrier height and its position) as well
as fusion cross sections for 50 isotopic systems including various collisions
of C, O, Mg, Si, S, Ca, Ar, Ti and Ni nuclei with condition
for compound systems. The results of our studies reveal that the relationships
between increase of barrier positions and decrease of barrier heights are both
linear with increase of ratio. Moreover, fusion cross sections also
enhance linearly with increase of this ratio.Comment: 28 pages, 7 figures, 5 Table
On manifolds with nonhomogeneous factors
We present simple examples of finite-dimensional connected homogeneous spaces
(they are actually topological manifolds) with nonhomogeneous and nonrigid
factors. In particular, we give an elementary solution of an old problem in
general topology concerning homogeneous spaces
Peculiarities of the stochastic motion in antiferromagnetic nanoparticles
Antiferromagnetic (AFM) materials are widely used in spintronic devices as
passive elements (for stabilization of ferromangetic layers) and as active
elements (for information coding). In both cases switching between the
different AFM states depends in a great extent from the environmental noise. In
the present paper we derive the stochastic Langevin equations for an AFM vector
and corresponding Fokker-Planck equation for distribution function in the phase
space of generalised coordinate and momentum. Thermal noise is modeled by a
random delta-correlated magnetic field that interacts with the dynamic
magnetisation of AFM particle. We analyse in details a particular case of the
collinear compensated AFM in the presence of spin-polarised current. The energy
distribution function for normal modes in the vicinity of two equilibrium
states (static and stationary) in sub- and super-critical regimes is found. It
is shown that the noise-induced dynamics of AFM vector has pecuilarities
compared to that of magnetisation vector in ferromagnets.Comment: Submitted to EPJ ST, presented at the 4-th Conference on Statistical
Physics, Lviv, Ukraine, 201
A self-consistent treatment of non-equilibrium spin torques in magnetic multilayers
It is known that the transfer of spin angular momenta between current
carriers and local moments occurs near the interface of magnetic layers when
their moments are non-collinear. However, to determine the magnitude of the
transfer, one should calculate the spin transport properties far beyond the
interface regions. Based on the spin diffusion equation, we present a
self-consistent approach to evaluate the spin torque for a number of layered
structures. One of the salient features is that the longitudinal and transverse
components of spin accumulations are inter-twined from one layer to the next,
and thus, the spin torque could be significantly amplified with respect to
treatments which concentrate solely on the transport at the interface due to
the presence of the much longer longitudinal spin diffusion length. We conclude
that bare spin currents do not properly estimate the spin angular momentum
transferred between to the magnetic background; the spin transfer that occurs
at interfaces should be self-consistently determined by embedding it in our
globally diffuse transport calculations.Comment: 21 pages, 6 figure
Trying to solve the ‘worst situation’ together: participatory autism research
The importance of participatory autism research is discussed in relation to a project involving six autistic researchers and five non-autistic university researchers collaborating to investigate anxiety in autistic adolescents. The paper describes the process of establishing a research partnership and the values and philosophy behind this inclusive method of research. Lessons were learnt about neurodivergent thinking and the benefits it brings to the development of research questions and analysis of data
Signatures of Thermal Dilepton Radiation at RHIC
The properties of thermal dilepton production from heavy-ion collisions in
the RHIC energy regime are evaluated for invariant masses ranging from 0.5 to 3
GeV. Using an expanding thermal fireball to model the evolution through both
quark-gluon and hadronic phases various features of the spectra are addressed.
In the low-mass region, due to an expected large background, the focus is on
possible medium modifications of the narrow resonance structures from
and mesons, whereas in the intermediate-mass region the old idea of
identifying QGP radiation is reiterated including effects of chemical
under-saturation in the early stages of central Au+Au collisions.Comment: 17 pages ReVTeX including 16 figure
Current- and Wave-Generated Bedforms on Mixed Sand–Clay Intertidal Flats: A New Bedform Phase Diagram and Implications for Bed Roughness and Preservation Potential
The effect of bedforms on frictional roughness felt by the overlying flow is crucial to the regional modelling of estuaries and coastal seas. Bedforms are also a key marker of palaeoenvironments. Experiments have shown that even modest biotic and abiotic cohesion in sand inhibits bedform formation, modifies bedform size, and slows bedform development, but this has rarely been tested in nature. The present study used a comprehensive dataset recorded over a complete spring–neap cycle on an intertidal flat to investigate bedform dynamics controlled by a wide range of wave and current conditions, including the effects of wave–current angle and bed cohesion. A detailed picture of different bedform types and their relationship to the flow, be they equilibrium, non-equilibrium, or relict, was produced, and captured in a phase diagram that integrates wave-dominated, current-dominated, and combined wave–current bedforms. This bedform phase diagram incorporates a substantially wider range of flow conditions than previous phase diagrams, including bedforms related to near-orthogonal wave–current angles, such as ladderback ripples. Comparison with laboratory-derived bedform phase diagrams indicates that washed-out ripples, lunate interference ripples and upper-stage plane beds replace the subaqueous dune field; such bedform distributions may be a key characteristic of intertidal flats. The field data also provide a means of predicting the dimensions of these bedforms, which can be transferred to other areas and grain sizes. We show that an equation for the prediction of equilibrium bedform size is sufficient to predict the roughness, even though the bedforms are highly variable in character and only in equilibrium with the flow for approximately half the time. Whilst the effect of cohesive clay is limited under more active spring conditions, clay does play a role in reducing the bedform dimensions under more quiescent neap conditions. We also investigated which combinations of waves, currents, and bed clay contents in the intertidal zone have the highest potential for bedform preservation in the geological record. This shows that combined wave–current bedforms have the lowest preservation potential and equilibrium current ripples have the highest preservation potential, even in the presence of moderate and storm waves. Hence, the absence of wave ripples and combined-flow bedforms and their primary stratification in sedimentary successions cannot be taken as evidence that waves were absent at the time of deposition
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