798 research outputs found
Photon statistics of a random laser
A general relationship is presented between the statistics of thermal
radiation from a random medium and its scattering matrix S. Familiar results
for black-body radiation are recovered in the limit S to 0. The mean photocount
is proportional to the trace of 1-SS^dagger, in accordance with Kirchhoff's law
relating emissivity and absorptivity. Higher moments of the photocount
distribution are related to traces of powers of 1-SS^dagger, a generalization
of Kirchhoff's law. The theory can be applied to a random amplifying medium (or
"random laser") below the laser threshold, by evaluating the Bose-Einstein
function at a negative temperature. Anomalously large fluctuations are
predicted in the photocount upon approaching the laser threshold, as a
consequence of overlapping cavity modes with a broad distribution of spectral
widths.Comment: 26 pages, including 9 figure
A global method for coupling transport with chemistry in heterogeneous porous media
Modeling reactive transport in porous media, using a local chemical
equilibrium assumption, leads to a system of advection-diffusion PDE's coupled
with algebraic equations. When solving this coupled system, the algebraic
equations have to be solved at each grid point for each chemical species and at
each time step. This leads to a coupled non-linear system. In this paper a
global solution approach that enables to keep the software codes for transport
and chemistry distinct is proposed. The method applies the Newton-Krylov
framework to the formulation for reactive transport used in operator splitting.
The method is formulated in terms of total mobile and total fixed
concentrations and uses the chemical solver as a black box, as it only requires
that on be able to solve chemical equilibrium problems (and compute
derivatives), without having to know the solution method. An additional
advantage of the Newton-Krylov method is that the Jacobian is only needed as an
operator in a Jacobian matrix times vector product. The proposed method is
tested on the MoMaS reactive transport benchmark.Comment: Computational Geosciences (2009)
http://www.springerlink.com/content/933p55085742m203/?p=db14bb8c399b49979ba8389a3cae1b0f&pi=1
More than a century of bathymetric observations and present-day shallow sediment characterization in Belfast Bay, Maine, USA: implications for pockmark field longevity
This paper is not subject to U.S. copyright. The definitive version was published in Geo-Marine Letters 31 (2011): 237-248, doi:10.1007/s00367-011-0228-0.Mechanisms and timescales responsible for
pockmark formation and maintenance remain uncertain,
especially in areas lacking extensive thermogenic fluid
deposits (e.g., previously glaciated estuaries). This study
characterizes seafloor activity in the Belfast Bay, Maine
nearshore pockmark field using (1) three swath bathymetry
datasets collected between 1999 and 2008, complemented
by analyses of shallow box-core samples for radionuclide
activity and undrained shear strength, and (2) historical
bathymetric data (report and smooth sheets from 1872,
1947, 1948). In addition, because repeat swath bathymetry
surveys are an emerging data source, we present a selected
literature review of recent studies using such datasets for
seafloor change analysis. This study is the first to apply the
method to a pockmark field, and characterizes macro-scale
(>5 m) evolution of tens of square kilometers of highly
irregular seafloor. Presence/absence analysis yielded no
change in pockmark frequency or distribution over a 9-year
period (1999–2008). In that time pockmarks did not
detectably enlarge, truncate, elongate, or combine. Historical
data indicate that pockmark chains already existed in
the 19th century. Despite the lack of macroscopic changes
in the field, near-bed undrained shear-strength values of
less than 7 kPa and scattered downcore 137Cs signatures
indicate a highly disturbed setting. Integrating these
findings with independent geophysical and geochemical
observations made in the pockmark field, it can be
concluded that (1) large-scale sediment resuspension and
dispersion related to pockmark formation and failure do not
occur frequently within this field, and (2) pockmarks can
persevere in a dynamic estuarine setting that exhibits
minimal modern fluid venting. Although pockmarks are
conventionally thought to be long-lived features maintained
by a combination of fluid venting and minimal sediment
accumulation, this suggests that other mechanisms may be
equally active in maintaining such irregular seafloor
morphology. One such mechanism could be upwelling
within pockmarks induced by near-bed currents.Graduate support for Brothers came from a
Maine Economic Improvement Fund Dissertation Fellowship
Primary Language and Receipt of Recommended Health Care Among Hispanics in the United States
BackgroundDisparities in health care services between Hispanics and whites in the United States are well documented.ObjectiveThe objective of the study was to determine whether language spoken at home identifies Hispanics at risk for not receiving recommended health care services.DesignThe design of the study was cross-sectional, nationally representative survey of households.PatientsThe patients were non-Hispanic white and Hispanic adults participating in the 2003 Medical Expenditure Panel Survey.MeasurementsWe compared receipt of ten recommended health care services by ethnicity and primary language adjusting for demographic and socioeconomic characteristics, health status, and access to care.ResultsThe sample included 12,706 whites and 5,500 Hispanics. In bivariate comparisons, 57.0% of whites received all eligible health care services compared to 53.6% for Hispanics who spoke English at home, 44.9% for Hispanics who did not speak English at home but who were comfortable speaking English, and 35.0% for Hispanics who did not speak English at home and were uncomfortable speaking English (p < .001). In multivariate logistic models, compared to non-Hispanic whites, Hispanics who did not speak English at home were less likely to receive all eligible health care services, whether they were comfortable speaking English (risk ratio [RR] 0.88, 95% confidence interval [CI] 0.74-0.97) or not (RR 0.84, 95% CI 0.68-0.95).ConclusionsSpeaking a language other than English at home identified Hispanics at risk for not receiving recommended health care services, whether they were comfortable in speaking English or not. Identifying the mechanism for disparities by language usage may lead to interventions to reduce ethnic disparities
Automatic Physiological Waveform Processing for fMRI Noise Correction and Analysis
Functional MRI resting state and connectivity studies of brain focus on neural fluctuations at low frequencies which share power with physiological fluctuations originating from lung and heart. Due to the lack of automated software to process physiological signals collected at high magnetic fields, a gap exists in the processing pathway between the acquisition of physiological data and its use in fMRI software for both physiological noise correction and functional analyses of brain activation and connectivity. To fill this gap, we developed an open source, physiological signal processing program, called PhysioNoise, in the python language. We tested its automated processing algorithms and dynamic signal visualization on resting monkey cardiac and respiratory waveforms. PhysioNoise consistently identifies physiological fluctuations for fMRI noise correction and also generates covariates for subsequent analyses of brain activation and connectivity
Measurement of Beam-Spin Asymmetries for Deep Inelastic Electroproduction
We report the first evidence for a non-zero beam-spin azimuthal asymmetry in
the electroproduction of positive pions in the deep-inelastic region. Data have
been obtained using a polarized electron beam of 4.3 GeV with the CLAS detector
at the Thomas Jefferson National Accelerator Facility (JLab). The amplitude of
the modulation increases with the momentum of the pion relative to
the virtual photon, , with an average amplitude of for range.Comment: 5 pages, RevTEX4, 3 figures, 2 table
Measurement of the Polarized Structure Function for in the Resonance Region
The polarized longitudinal-transverse structure function
has been measured in the resonance region at and 0.65
GeV. Data for the reaction were taken at Jefferson Lab
with the CEBAF Large Acceptance Spectrometer (CLAS) using longitudinally
polarized electrons at an energy of 1.515 GeV. For the first time a complete
angular distribution was measured, permitting the separation of different
non-resonant amplitudes using a partial wave analysis. Comparison with previous
beam asymmetry measurements at MAMI indicate a deviation from the predicted
dependence of using recent phenomenological
models.Comment: 5 pages, LaTex, 4 eps figures: to be published in PRC/Rapid
Communications. Version 2 has revised Q^2 analysi
Two-Nucleon Momentum Distributions Measured in 3He(e,e'pp)n
We have measured the 3He(e,e'pp)n reaction at 2.2 GeV over a wide kinematic
range. The kinetic energy distribution for `fast' nucleons (p > 250 MeV/c)
peaks where two nucleons each have 20% or less, and the third nucleon has most
of the transferred energy. These fast pp and pn pairs are back-to-back with
little momentum along the three-momentum transfer, indicating that they are
spectators. Experimental and theoretical evidence indicates that we have
measured distorted two-nucleon momentum distributions by striking the third
nucleon and detecting the spectator correlated pair.Comment: 6 pages, 5 figures, submitted to PR
Survey of A_LT' asymmetries in semi-exclusive electron scattering on He4 and C12
Single spin azimuthal asymmetries A_LT' were measured at Jefferson Lab using
2.2 and 4.4 GeV longitudinally polarized electrons incident on He4 and C12
targets in the CLAS detector. A_LT' is related to the imaginary part of the
longitudinal-transverse interference and in quasifree nucleon knockout it
provides an unambiguous signature for final state interactions (FSI).
Experimental values of A_LT' were found to be below 5%, typically |A_LT'| < 3%
for data with good statistical precision. Optical Model in Eikonal
Approximation (OMEA) and Relativistic Multiple-Scattering Glauber Approximation
(RMSGA) calculations are shown to be consistent with the measured asymmetries.Comment: 9 pages, 5 figure
First Measurement of Transferred Polarization in the Exclusive e p --> e' K+ Lambda Reaction
The first measurements of the transferred polarization for the exclusive ep
--> e'K+ Lambda reaction have been performed in Hall B at the Thomas Jefferson
National Accelerator Facility using the CLAS spectrometer. A 2.567 GeV electron
beam was used to measure the hyperon polarization over a range of Q2 from 0.3
to 1.5 (GeV/c)2, W from 1.6 to 2.15 GeV, and over the full center-of-mass
angular range of the K+ meson. Comparison with predictions of hadrodynamic
models indicates strong sensitivity to the underlying resonance contributions.
A non-relativistic quark model interpretation of our data suggests that the
s-sbar quark pair is produced with spins predominantly anti-aligned.
Implications for the validity of the widely used 3P0 quark-pair creation
operator are discussed.Comment: 6 pages, 4 figure
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