2,209 research outputs found
Effect of an electric field on superfluid helium scintillation produced by alpha-particle sources
We report a study of the intensity and time dependence of scintillation
produced by weak alpha particle sources in superfluid helium in the presence of
an electric field (0 - 45 kV/cm) in the temperature range of 0.2 K to 1.1 K at
the saturated vapor pressure. Both the prompt and the delayed components of the
scintillation exhibit a reduction in intensity with the application of an
electric field. The reduction in the intensity of the prompt component is well
approximated by a linear dependence on the electric field strength with a
reduction of 15% at 45 kV/cm. When analyzed using the Kramers theory of
columnar recombination, this electric field dependence leads to the conclusion
that roughly 40% of the scintillation results from species formed from atoms
originally promoted to excited states and 60% from excimers created by
ionization and subsequent recombination with the charges initially having a
cylindrical Gaussian distribution about the alpha track of 60 nm radius. The
intensity of the delayed component of the scintillation has a stronger
dependence on the electric field strength and on temperature. The implications
of these data on the mechanisms affecting scintillation in liquid helium are
discussed.Comment: 17 pages, 23 figure
Interoperability in the OpenDreamKit Project: The Math-in-the-Middle Approach
OpenDreamKit --- "Open Digital Research Environment Toolkit for the
Advancement of Mathematics" --- is an H2020 EU Research Infrastructure project
that aims at supporting, over the period 2015--2019, the ecosystem of
open-source mathematical software systems. From that, OpenDreamKit will deliver
a flexible toolkit enabling research groups to set up Virtual Research
Environments, customised to meet the varied needs of research projects in pure
mathematics and applications.
An important step in the OpenDreamKit endeavor is to foster the
interoperability between a variety of systems, ranging from computer algebra
systems over mathematical databases to front-ends. This is the mission of the
integration work package (WP6). We report on experiments and future plans with
the \emph{Math-in-the-Middle} approach. This information architecture consists
in a central mathematical ontology that documents the domain and fixes a joint
vocabulary, combined with specifications of the functionalities of the various
systems. Interaction between systems can then be enriched by pivoting off this
information architecture.Comment: 15 pages, 7 figure
The contrasting oceanography of the Rhodes Gyre and the Central Black Sea
The Rhodes Gyre, a prominent feature of the oceanography of the eastern Mediterranean, is modelled as a vertical, continuous flow, cylindrical reactor illuminated during the day at its upper end. If the Gyre is supposed to be in a steady state whilst the concentrations, C, of a chemical are being measured, the nett rate of formation or consumption of the chemical is given by -w d C/d z + u d C/d r, where w is the upward velocity of the water in the vertical, z , direction and u is the velocity of the water in the radial, r, direction. The behaviour of w and u is analysed to show that the Gyre may be used as a field laboratory in which rates of chemical change may be derived from depth profiles together with values of the surface velocities of the Gyre waters. In contrast, the central Black Sea is modelled as an ideal, strongly stratified sea in which the nett rates of formation or consumption of chemicals under steady state conditions are given by Ds d2C/ds 2, where s is the water density and Ds is an eddy diffusion coefficient. Computations reveal that, given better knowledge of its eddy diffusion coefficients, the Black Sea can also be treated as a field laboratory where rates of reaction mediated by bacteria may be derived from depth profiles
Small-angle X-ray scattering from GaN nanowires on Si(111): facet truncation rods, facet roughness and Porod's law
Small-angle X-ray scattering from GaN nanowires grown on Si(111) is measured in the grazing-incidence geometry and modelled by means of a Monte Carlo simulation that takes into account the orientational distribution of the faceted nanowires and the roughness of their side facets. It is found that the scattering intensity at large wavevectors does not follow Porod's law I(q) ∝ q-4. The intensity depends on the orientation of the side facets with respect to the incident X-ray beam. It is maximum when the scattering vector is directed along a facet normal, reminiscent of surface truncation rod scattering. At large wavevectors q, the scattering intensity is reduced by surface roughness. A root-mean-square roughness of 0.9 nm, which is the height of just 3-4 atomic steps per micrometre-long facet, already gives rise to a strong intensity reduction. open access
The Role of Plastic Flow in Processes of High-speed Sintering of Ceramic Materials under Pressure
A model to describe the kinetics of the compaction of conductive nitride ceramics using electropulse technologies is developed. The relationship between density and pressure is established on the basis of three components of the geometric, plastic and stressed state, which is affects the contact area between the particles. The model takes into account the change in the relative area of the interpartial contacts under the action oftwo mechanisms of mass transfer-diffusion and plastic flow. It is shown that a decrease in the particle size of the powder leads to an in-crease in the diffusion contribution and a decrease in the plastic flow, at all other conditions being equal. And for the case of nano-sized particles, diffusion mass transfer is predominant.Increasing in the heating rate leads to a decrease in the contribution of dif-fusion mass transfer at equal temperatures, as well as to an increase in the temperature of the beginning of shrinkage.The processes of plasma-plasma sintering, high-voltage electro-pulsed consolidation and hot pressing control the same mechanisms, plastic flow and diffusion mass transfer, which do not require, in the first approximation, the influence of the electric current on the properties of materials.
Keywords: spark-plasma sintering, high-voltage electrodischarge consolidation, sintering kinetic
Search for EC and ECEC processes in Sn
Limits on EC (here EC denotes electron capture) and ECEC processes
in Sn have been obtained using a 380 cm HPGe detector and an
external source consisting of 53.355 g enriched tin (94.32% of Sn). A
limit with 90% C.L. on the Sn half-life of y for
the ECEC(0) transition to the excited state in Cd (1871.0
keV) has been established. This transition is discussed in the context of a
possible enhancement of the decay rate by several orders of magnitude given
that the ECEC process is nearly degenerate with an excited state in the
daughter nuclide. Prospects for investigating such a process in future
experiments are discussed. The limits on other EC and ECEC processes
in Sn were obtained on the level of y at the
90% C.L.Comment: 14 pages, 4 figure
The environment effect on operation of in-vessel mirrors for plasma diagnostics in fusion devices
First mirrors will be the plasma facing components of optical diagnostic
systems in ITER. Mirror surfaces will undergo modification caused by erosion
and re-deposition processes [1,2]. As a consequence, the mirror performance may
be changed and may deteriorate [3,4]. In the divertor region it may also be
obscured by deposition [5-7]. The limited access to in-vessel components of
ITER calls for testing the mirror materials in present day devices in order to
gather information on the material damage and degradation of the mirror
performance, i.e. reflectivity. A dedicated experimental programme, First
Mirror Test (FMT), has been initiated at the JET tokamak within the framework
Tritium Retention Studies (TRS).Comment: 12th International Congress on Plasma Physics, 25-29 October 2004,
Nice (France).Submitted by B. Schunke on behalf of V. Voytseny
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