41,176 research outputs found
Thermal neutron image intensifier tube provides brightly visible radiographic pattern
Vacuum-type neutron image intensifier tube improves image detection in thermal neutron radiographic inspection. This system converts images to an electron image, and with electron acceleration and demagnification between the input target and output screen, produces a bright image viewed through a closed circuit television system
Viscous Withdrawal of Miscible Liquid Layers
In viscous withdrawal, a converging flow imposed in an upper layer of viscous
liquid entrains liquid from a lower, stably stratified layer. Using the idea
that a thin tendril is entrained by a local straining flow, we propose a
scaling law for the volume flux of liquid entrained from miscible liquid
layers. A long-wavelength model including only local information about the
withdrawal flow is degenerate, with multiple tendril solutions for one
withdrawal condition. Including information about the global geometry of the
withdrawal flow removes the degeneracy while introducing only a logarithmic
dependence on the global flow parameters into the scaling law.Comment: 4 pages, 4 figure
Voltage-Controlled Spin Selection in a Magnetic Resonant Tunnelling Diode
We have fabricated all II-VI semiconductor resonant tunneling diodes based on
the (Zn,Mn,Be)Se material system, containing dilute magnetic material in the
quantum well, and studied their current-voltage characteristics. When subjected
to an external magnetic field the resulting spin splitting of the levels in the
quantum well leads to a splitting of the transmission resonance into two
separate peaks. This is interpreted as evidence of tunneling transport through
spin polarized levels, and could be the first step towards a voltage controlled
spin filter.Comment: To be published in Phys. Rev. Let
On reaction-subdiffusion equations
To analyze possible generalizations of reaction-diffusion schemes for the
case of subdiffusion we discuss a simple monomolecular conversion A --> B. We
derive the corresponding kinetic equations for local A and B concentrations.
Their form is rather unusual: The parameters of reaction influence the
diffusion term in the equation for a component A, a consequence of the
nonmarkovian nature of subdiffusion. The equation for a product contains a term
which depends on the concentration of A at all previous times. Our discussion
shows that reaction-subdiffusion equations may not resemble the corresponding
reaction-diffusion ones and are not obtained by a trivial change of the
diffusion operator for a subdiffusion one
Discovery of a Magnetic DZ White Dwarf with Zeeman-Split Lines of Heavy Elements
A spectroscopic survey of previously-unstudied Luyten Half Second proper
motion stars has resulted in the discoveries of two new cool magnetic white
dwarfs. One (LHS 2273) is a routine DA star, T= 6,500K, with Zeeman-split H
alpha and H beta, for which a simple model suggests a polar field strength of
18.5 MG viewed close to equator-on. However, the white dwarf LHS 2534 proves to
be the first magnetic DZ showing Zeeman-split Na I and Mg I components, as well
as Ca I and Ca II lines for which Zeeman components are blended. The Na I
splittings result in a mean surface field strength estimate of 1.92 MG. Apart
from the magnetic field, LHS 2534 is one of the most heavily-blanketed and
coolest DZ white dwarfs at T ~ 6,000K.Comment: 7 pages, Astrophysical Journal (Letters), in pres
On The Determination of MDI High-Degree Mode Frequencies
The characteristic of the solar acoustic spectrum is such that mode lifetimes
get shorter and spatial leaks get closer in frequency as the degree of a mode
increases for a given order. A direct consequence of this property is that
individual p-modes are only resolved at low and intermediate degrees, and that
at high degrees, individual modes blend into ridges. Once modes have blended
into ridges, the power distribution of the ridge defines the ridge central
frequency and it will mask the true underlying mode frequency. An accurate
model of the amplitude of the peaks that contribute to the ridge power
distribution is needed to recover the underlying mode frequency from fitting
the ridge.
We present the results of fitting high degree power ridges (up to l = 900)
computed from several two to three-month-long time-series of full-disk
observations taken with the Michelson Doppler Imager (MDI) on-board the Solar
and Heliospheric Observatory between 1996 and 1999.
We also present a detailed discussion of the modeling of the ridge power
distribution, and the contribution of the various observational and
instrumental effects on the spatial leakage, in the context of the MDI
instrument. We have constructed a physically motivated model (rather than some
ad hoc correction scheme) resulting in a methodology that can produce an
unbiased determination of high-degree modes, once the instrumental
characteristics are well understood.
Finally, we present changes in high degree mode parameters with epoch and
thus solar activity level and discuss their significance.Comment: 59 pages, 38 figures -- High-resolution version at
http://www-sgk.harvard.edu:1080/~sylvain/preprints/ -- Manuscript submitted
to Ap
Black (pyrogenic) carbon in boreal forests: a synthesis of current knowledge and uncertainties
International audienceThe carbon (C) cycle in boreal regions is strongly influenced by fire, which converts biomass and detrital C mainly to gaseous forms (CO2 and smaller proportions of CO and CH4), and some 1?7% of mass to pyrogenic C (PyC). PyC is mainly produced as solid charred residues, including visually-defined charcoal, and a black carbon (BC) fraction chemically defined by its resistance to laboratory oxidation, plus much lower proportions of volatile soot and polycyclic aromatic hydrocarbons (PAHs). All PyC is characterized by fused aromatic rings, but varying in cluster sizes, and presence of other elements (N, O) and functional groups. There are several reasons for current interest in defining more precisely the role of PyC in the C cycle of boreal regions. First, PyC is resistant to decomposition, and therefore contributes to very stable C pools in soils and sediments. Second, it influences soil processes, mainly through its sorption properties and cation exchange capacity, and third, soot aerosols absorb solar radiation and may contribute to global warming. However, there are large gaps in the basic information needed to address these topics. While charcoal is commonly defined by visual criteria, analytical methods for BC are mainly based on various measures of oxidation resistance, or on yield of benzenepolycarboxylic acids. These methods are still being developed, and capture different fractions of the PyC "continuum". There are few quantitative reports of PyC production and stocks in boreal forests (essentially none for boreal peatlands), and results are difficult to compare due to varying experimental goals and methods, as well as inconsistent terminology. There are almost no direct field measurements of BC aerosol production from boreal wildfires, and little direct information on rates and mechanisms for PyC loss. Structural characterization of charred biomass and forest floor from wildfires generally indicates a low level of thermal alteration, with the bulk of the material having H/C ratios still >0.2, and small aromatic cluster sizes. For the more chemically-recalcitrant BC fraction, a variety of mainly circumstantial evidence suggests very slow decomposition, with turnover on a millennium timescale (5000?10 000 y), depending on environmental conditions and PyC properties, but the main limitation to PyC storage in soil is likely consumption by subsequent fires. Degraded, functionalized PyC is also incorporated into humified soil organic matter, and is transported to sediments in dissolved and particulate form. Boreal production is estimated as 7?17 Tg BC y?1 of solid residues and 2?2.5 Tg BC y?1 as aerosols. Primary research needs include basic field data on PyC production and stocks in boreal forests and peatlands, suitable to support C budget modeling, and development of standardized analytical methods and of improved approaches to assess the chemical recalcitrance of typical chars from boreal wildfires. To accomplish these goals effectively will require much greater emphasis on interdisciplinary cooperation
Probing fractal magnetic domains on multiple length scales in Nd2Fe14B
Using small-angle neutron scattering, we demonstrate that the complex
magnetic domain patterns at the surface of Nd2Fe14B, revealed by quantitative
Kerr and Faraday microscopy, propagate into the bulk and exhibit structural
features with dimensions down to 6 nm, the domain wall thickness. The observed
fractal nature of the domain structures provides an explanation for the
anomalous increase in the bulk magnetization of Nd2Fe14B below the
spin-reorientation transition. These measurements open up a rich playground for
studies of fractal structures in highly anisotropic magnetic systems.Comment: Accepted for publication in Phys. Rev. Lett. (4 pages, 4 figures
Correlation-function spectroscopy of inelastic lifetime in heavily doped GaAs heterostructures
Measurements of resonant tunneling through a localized impurity state are
used to probe fluctuations in the local density of states of heavily doped
GaAs. The measured differential conductance is analyzed in terms of correlation
functions with respect to voltage. A qualitative picture based on the scaling
theory of Thouless is developed to relate the observed fluctuations to the
statistics of single particle wavefunctions. In a quantitative theory
correlation functions are calculated. By comparing the experimental and
theoretical correlation functions the effective dimensionality of the emitter
is analyzed and the dependence of the inelastic lifetime on energy is
extracted.Comment: 41 pages, 14 figure
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