74,500 research outputs found
Calculation of Radiative Corrections to E1 matrix elements in the Neutral Alkalis
Radiative corrections to E1 matrix elements for ns-np transitions in the
alkali metal atoms lithium through francium are evaluated. They are found to be
small for the lighter alkalis but significantly larger for the heavier alkalis,
and in the case of cesium much larger than the experimental accuracy. The
relation of the matrix element calculation to a recent decay rate calculation
for hydrogenic ions is discussed, and application of the method to parity
nonconservation in cesium is described
Equivalence of weak and strong modes of measures on topological vector spaces
A strong mode of a probability measure on a normed space can be defined
as a point such that the mass of the ball centred at uniformly
dominates the mass of all other balls in the small-radius limit. Helin and
Burger weakened this definition by considering only pairwise comparisons with
balls whose centres differ by vectors in a dense, proper linear subspace of
, and posed the question of when these two types of modes coincide. We show
that, in a more general setting of metrisable vector spaces equipped with
measures that are finite on bounded sets, the density of and a uniformity
condition suffice for the equivalence of these two types of modes. We
accomplish this by introducing a new, intermediate type of mode. We also show
that these modes can be inequivalent if the uniformity condition fails. Our
results shed light on the relationships between among various notions of
maximum a posteriori estimator in non-parametric Bayesian inference.Comment: 22 pages, 3 figure
Average-Atom Model for X-ray Scattering from Warm Dense Matter
A scheme for analyzing Thomson scattering of x-rays by warm dense matter,
based on the average-atom model, is developed. Emphasis is given to x-ray
scattering by bound electrons. Contributions to the scattered x-ray spectrum
from elastic scattering by electrons moving with the ions and from inelastic
scattering by free and bound electrons are evaluated using parameters (chemical
potential, average ionic charge, free electron density, bound and continuum
wave functions, and occupation numbers) taken from the average-atom model. The
resulting scheme provides a relatively simple diagnostic for use in connection
with x-ray scattering measurements. Applications are given to dense hydrogen,
beryllium, aluminum, titanium, and tin plasmas. At high momentum transfer,
contributions from inelastic scattering by bound electrons are dominant
features of the scattered x-ray spectrum for aluminum, titanium, and tin.Comment: 22 pages, 10 figures Presentation at Workshop IV: Computational
Challenges in Warm Dense Matter at IPAM (UCLA) May 21 - 25, 201
Using the X-FEL to understand X-ray Thomson scattering for partially ionized plasmas
For the last decade numerous researchers have been trying to develop
experimental techniques to use X-ray Thomson scattering as a method to measure
the temperature, electron density, and ionization state of high energy density
plasmas such as those used in inertial confinement fusion. With the advent of
the X-ray free electron laser (X-FEL) at the SLAC Linac Coherent Light Source
(LCLS) we now have such a source available in the keV regime. One challenge
with X-ray Thomson scattering experiments is understanding how to model the
scattering for partially ionized plasmas. Most Thomson scattering codes used to
model experimental data greatly simplify or neglect the contributions of the
bound electrons to the scattered intensity. In this work we take the existing
models of Thomson scattering that include elastic ion-ion scattering and the
electron-electron plasmon scattering and add the contribution of the bound
electrons in the partially ionized plasmas. Except for hydrogen plasmas almost
every plasma that is studied today has bound electrons and it is important to
understand their contribution to the Thomson scattering, especially as new
X-ray sources such as the X-FEL will allow us to study much higher Z plasmas.
Currently most experiments have looked at hydrogen or beryllium. We will first
look at the bound electron contributions to beryllium by analysing existing
experimental data. We then consider several higher Z materials such as Cr and
predict the existence of additional peaks in the scattering spectrum that
requires new computational tools to understand. For a Sn plasma we show that
the bound contributions changes the shape of the scattered spectrum in a way
that would change the plasma temperature and density inferred by the
experiment.Comment: 13th International Conference on X-ray Lasers Paris, France June 10,
2012 through June 15, 201
Physical modelling of amorphous thermoplastic polymer and numerical simulation of micro hot embossing process
Micro hot embossing process is considered as one of the most promising micro replication processes for manufacturing of polymeric components, especially for the high aspect ratio components and large surface structural components. A large number of hot embossing experimental results have been published, the material modelling and processes simulation to improve the quality of micro replication by hot embossing process are still lacking. This paper consists to 3D modelling of micro hot embossing process with amorphous thermoplastic polymers, including the mechanical characterisation of polymers properties, identification of the viscoelastic behaviour law of the polymers, numerical simulation and experimental investigation of micro hot embossing process. Static compression creep tests have been carried out to investigate the selected polymersâ viscoelastic properties. The Generalized Maxwell model has been proposed to describe the relaxation modulus of the polymers and good agreement has been observed. The numerical simulation of the hot embossing process in 3D has been achieved by taking into account the viscoelastic behaviour of the polymers. The microfluidic devices with the thickness of 2 mm have been elaborated by hot embossing process. The hot embossing process has been carried out using horizontal injection/compression moulding equipment, especially developed for this study. A complete compression mould tool, equipped with the heating system, the cooling system, the ejection system and the vacuum system, has been designed and elaborated in our research. Polymer-based microfluidic devices have been successfully replicated by the hot embossing process using the compression system developed. Proper agreement between the numerical simulation and the experimental elaboration has been observed. It shows strong possibility for the development of the 3D numerical model to optimize the micro hot embossing process in the future
Flavor and Spin Contents of the Nucleon in the Quark Model with Chiral Symmetry
A simple calculation in the framework of the chiral quark theory of Manohar
and Georgi yields results that can account for many of the ''failures'' of the
naive quark model: significant strange quark content in the nucleon as
indicated by the value of the -
asymmetry in the nucleon as measured by the deviation from Gottfried sum rule
and by the Drell-Yan process, as well as the various quark contributions to the
nucleon spin as measured by the deep inelastic polarized lepton-nucleon
scatterings.Comment: figure has been separated from tex file. No other changes. Preprint
CMU-HEP94-3
Instantons and the singlet-coupling in the chiral quark model
Chiral quark model with a broken-U(3) flavor symmetry can be interpreted as
the effective theory of the instanton-dominated non-perturbative QCD. This
naturally suggests the possibility of a negative singlet/octet coupling ratio,
which has been found, in a previous publication, to be compatible with the
phenomenological description of the nucleon spin-flavor structure.Comment: 9 page
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