47,115 research outputs found
Quantifying the Effects of Measures to Control Highly Pathogenic Avian Influenza H5N1 in Poultry in Southeast Asia
Despite the ongoing efforts to contain its spread, H5N1 is now considered endemic within
poultry in various settings worldwide, threatening both the livelihoods of those involved in
poultry production in affected countries and posing a continuous public health risk. The
reasons for the varying levels of success in controlling H5N1 in Southeast Asia need to be
better understood. In this thesis, various different methods of quantifying the effects of
individual control measures, using the types of data available in various different contexts,
are discussed and applied. In the first half of this thesis a spatio-temporal survival model is
fitted to H5N1 outbreak surveillance data from Vietnam and Thailand using a Bayesian
framework in order to account for unobserved infection times. Following vaccination in
Vietnam it was found that transmissibility had been successfully reduced but, during a wave
of outbreaks in 2007, that this coincided with a reduction in the rate of at which outbreaks
were reported following the introduction of infection, limiting the overall impact this
reduction in transmissibility had on the total epidemic size. In Thailand, active surveillance
was found to be successful in contributing to the control of infection. Furthermore,
backyard producers, whilst responsible for the majority of outbreaks, were, on average, less
likely to transmit infection than those involved in more intensive production. In the second
half of the thesis, the use of final size methods to assess the effectiveness of vaccination
from trial data is explored. This involved an investigation into the effects of different
assumptions regarding the action by which vaccination confers immunity and fitting
estimates of transmissibility to data collected from outbreak investigations in the context of
a field trial of vaccination in Indonesia, where, making strong assumptions about the
underlying infection process, a reduction in both within and between flock transmissibility
was detected for outbreaks occurring in areas where vaccination was being carried out
Method for deducing anisotropic spin-exchange rates
Using measured spin-transfer rates from alkali atoms to He-3, combined with
spin-relaxation rates of the alkali atoms due to He-3 and He-4, it should be
possible to differentiate between isotropic and anisotropic spin-exchange. This
would give a fundamental limit on the He-3 polarization attainable in
spin-exchange optical pumping. For K-He, we find the limit to be 0.90+-0.11
Circular Dichroism of RbHe and RbN Molecules
We present measurements of the circular dichroism of optically pumped Rb
vapor near the D1 resonance line. Collisions with the buffer gases He and
N reduce the transparency of the vapor, even when fully polarized. We use
two methods to measure this effect, show that the He results can be understood
from RbHe potential curves, and show how this effect conspires with the
spectral profile of the optical pumping light to increase the laser power
demands for optical pumping of very optically thick samples
Effects of Nitrogen Quenching Gas on Spin-Exchange Optical Pumping of He-3
We consider the degree of conservation of nuclear spin polarization in the
process of optical pumping under typical spin-exchange optical pumping
conditions. Previous analyses have assumed that negligible nuclear spin
precession occurs in the brief periods of time the alkali-metal atoms are in
the excited state after absorbing photons and before undergoing quenching
collisions with nitrogen molecules. We include excited-state hyperfine
interactions, electronic spin relaxation in collisions with He and N_2,
spontaneous emission, quenching collisions, and a simplified treatment of
radiation trapping
Deterministic entanglement of two neutral atoms via Rydberg blockade
We demonstrate the first deterministic entanglement of two individually
addressed neutral atoms using a Rydberg blockade mediated controlled-NOT gate.
Parity oscillation measurements reveal an entanglement fidelity of
, which is above the entanglement threshold of , without
any correction for atom loss, and after correcting for
background collisional losses. The fidelity results are shown to be in good
agreement with a detailed error model.Comment: 4 figure
Stellar Mixing and the Primordial Lithium Abundance
We compare the properties of recent samples of the lithium abundances in halo
stars to one another and to the predictions of theoretical models including
rotational mixing, and we examine the data for trends with metal abundance. We
find from a KS test that in the absence of any correction for chemical
evolution, the Ryan, Norris, & Beers (1999} sample is fully consistent with
mild rotational mixing induced depletion and, therefore, with an initial
lithium abundance higher than the observed value. Tests for outliers depend
sensitively on the threshold for defining their presence, but we find a
1045% probability that the RNB sample is drawn from the rotationally mixed
models with a 0.2 dex median depletion (with lower probabilities corresponding
to higher depletion factors). When chemical evolution trends (Li/H versus Fe/H)
are treated in the linear plane we find that the dispersion in the RNB sample
is not explained by chemical evolution; the inferred bounds on lithium
depletion from rotational mixing are similar to those derived from models
without chemical evolution. We find that differences in the equivalent width
measurements are primarily responsible for different observational conclusions
concerning the lithium dispersion in halo stars. The standard Big Bang
Nucleosynthesis predicted lithium abundance which corresponds to the deuterium
abundance inferred from observations of high-redshift, low-metallicity QSO
absorbers requires halo star lithium depletion in an amount consistent with
that from our models of rotational mixing, but inconsistent with no depletion.Comment: 39 pages, 9 figures; submitted Ap
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