25,006 research outputs found
Shield weight optimization using Monte Carlo transport calculations
Outlines are given of the theory used in FASTER-3 Monte Carlo computer program for the transport of neutrons and gamma rays in complex geometries. The code has the additional capability of calculating the minimum weight layered unit shield configuration which will meet a specified dose rate constraint. It includes the treatment of geometric regions bounded by quadratic and quardric surfaces with multiple radiation sources which have a specified space, angle, and energy dependence. The program calculates, using importance sampling, the resulting number and energy fluxes at specified point, surface, and volume detectors. Results are presented for sample problems involving primary neutron and both primary and secondary photon transport in a spherical reactor shield configuration. These results include the optimization of the shield configuration
Information-theoretic bound on the entropy production to maintain a classical nonequilibrium distribution using ancillary control
There are many functional contexts where it is desirable to maintain a
mesoscopic system in a nonequilibrium state. However, such control requires an
inherent energy dissipation. In this article, we unify and extend a number of
works on the minimum energetic cost to maintain a mesoscopic system in a
prescribed nonequilibrium distribution using ancillary control. For a variety
of control mechanisms, we find that the minimum amount of energy dissipation
necessary can be cast as an information-theoretic measure of distinguishability
between the target nonequilibrium state and the underlying equilibrium
distribution. This work offers quantitative insight into the intuitive idea
that more energy is needed to maintain a system farther from equilibrium.Comment: 6 pages, 2 figure
THE IMPACT OF LABELS AND BRAND NAMES ON CONSUMER CHOICE AT THE RETAIL MARKET
Consumer/Household Economics,
Mean Field Theory for Sigmoid Belief Networks
We develop a mean field theory for sigmoid belief networks based on ideas
from statistical mechanics. Our mean field theory provides a tractable
approximation to the true probability distribution in these networks; it also
yields a lower bound on the likelihood of evidence. We demonstrate the utility
of this framework on a benchmark problem in statistical pattern
recognition---the classification of handwritten digits.Comment: See http://www.jair.org/ for any accompanying file
Constraining the Sub-AU-Scale Distribution of Hydrogen and Carbon Monoxide Gas around Young Stars with the Keck Interferometer
We present Keck Interferometer observations of T Tauri and Herbig Ae/Be stars
with a spatial resolution of a few milliarcseconds and a spectral resolution of
~2000. Our observations span the K-band, and include the Br gamma transition of
Hydrogen and the v=2-0 and v=3-1 transitions of carbon monoxide. For several
targets we also present data from Keck/NIRSPEC that provide higher spectral
resolution, but a seeing-limited spatial resolution, of the same spectral
features. We analyze the Br gamma emission in the context of both disk and
infall/outflow models, and conclude that the Br gamma emission traces gas at
very small stellocentric radii, consistent with the magnetospheric scale.
However some Br gamma-emitting gas also seems to be located at radii of >0.1
AU, perhaps tracing the inner regions of magnetically launched outflows. CO
emission is detected from several objects, and we generate disk models that
reproduce both the KI and NIRSPEC data well. We infer the CO spatial
distribution to be coincident with the distribution of continuum emission in
most cases. Furthermore the Br gamma emission in these objects is roughly
coincident with both the CO and continuum emission. We present potential
explanations for the spatial coincidence of continuum, Br gamma, and CO
overtone emission, and explore the implications for the low occurrence rate of
CO overtone emission in young stars. Finally, we provide additional discussion
of V1685 Cyg, which is unusual among our sample in showing large differences in
emitting region size and spatial position as a function of wavelength.Comment: Accepted for publication in MNRA
The designer of the 90's: A live demonstration
A survey of design tools to be used by the aircraft designer is given. Structural reliability, maintainability, cost and predictability, and acoustics expert systems are discussed, as well as scheduling, drawing, engineering systems, sizing functions, and standard parts and materials data bases
Phase-resolved far-ultraviolet HST spectroscopy of the peculiar magnetic white dwarf RE J0317-853
We present phase resolved FUV HST FOS spectra of the rapidly rotating, highly
magnetic white dwarf RE J0317-853. Using these data, we construct a new model
for the magnetic field morphology across the stellar surface. From an expansion
into spherical harmonics, we find the range of magnetic field strengths present
is 180-800MG. For the first time we could identify an absorption feature
present at certain phases at 1160A as a ``forbidden'' 1s_0 -> 2s_0 component,
due to the combined presence of an electric and magnetic field.Comment: 15 pages including 4 figures. Accepted for publication in ApJ Letter
Gap theory of rectification in ballistic three-terminal conductors
We introduce a model for rectification in three-terminal ballistic
conductors, where the central connecting node is modeled as a chaotic cavity.
For bias voltages comparable to the Fermi energy, a strong nonlinearity is
created by the opening of a gap in the transport window. Both noninteracting
cavity electrons at arbitrary temperature as well as the hot electron regime
are considered. Charging effects are treated within the transmission formalism
using a self-consistent analysis. The conductance of the third lead in a
voltage probe configuration is varied to also model inelastic effects. We find
that the basic transport features are insensitive to all of these changes,
indicating that the nonlinearity is robust and well suited to applications such
as current rectification in ballistic systems. Our findings are in broad
agreement with several recent experiments.Comment: 8 pages, 6 figure
Thin and lumpy: an experimental investigation of water quality trading
Water quality trading schemes in the United States can predominantly be characterized by low trading volumes. In this paper we utilize laboratory economics experiments to explore the extent to which the technology through which pollution abatement is achieved influences market outcomes. Mirroring the majority of water quality trading markets, the sessions utilize small trading groups composed of six participants. To understand the extent to which abatement technology influences trading behavior, the experimental treatments vary the degree of heterogeneity in initial abatement costs and the potential for long-lived investments in cost-reducing abatement technology.Environmental Economics and Policy,
- …