22,449 research outputs found
Hospital and Physician Capacity Update
Offers an alternative view of healthcare costs by examining trends in hospital capacity and healthcare labor across regions. Outlines how effective management of healthcare capacity would enable affordable quality care that meets patient needs and wants
A Radon Progeny Deposition Model
The next generation low-background detectors operating underground aim for
unprecedented low levels of radioactive backgrounds. Although the radioactive
decays of airborne radon (particularly Rn-222) and its subsequent progeny
present in an experiment are potential backgrounds, also problematic is the
deposition of radon progeny on detector materials. Exposure to radon at any
stage of assembly of an experiment can result in surface contamination by
progeny supported by the long half life (22 y) of Pb-210 on sensitive locations
of a detector. An understanding of the potential surface contamination from
deposition will enable requirements of radon-reduced air and clean room
environments for the assembly of low background experiments. It is known that
there are a number of environmental factors that govern the deposition of
progeny onto surfaces. However, existing models have not explored the impact of
some environmental factors important for low background experiments. A test
stand has been constructed to deposit radon progeny on various surfaces under a
controlled environment in order to develop a deposition model. Results from
this test stand and the resulting deposition model are presented.Comment: Proceedings of the Topical Workshop in Low Radioactivity Techniques,
(Sudbury, Canada) August 28-29, 201
The generalized gradient approximation kernel in time-dependent density functional theory
A complete understanding of a material requires both knowledge of the excited
states as well as of the ground state. In particular, the low energy
excitations are of utmost importance while studying the electronic, magnetic,
dynamical, and thermodynamical properties of the material. Time-Dependent
Density Functional Theory (TDDFT), within the linear regime, is a successful
\textit{ab-initio} method to access the electronic charge and spin excitations.
However, it requires an approximation to the exchange-correlation (XC) kernel
which encapsulates the effect of electron-electron interactions in the
many-body system. In this work we derive and implement the spin-polarized XC
kernel for semi-local approximations such as the adiabatic Generalized Gradient
Approximation (AGGA). This kernel has a quadratic dependence on the wavevector,
{\bf q}, of the perturbation, however the impact of this on the electron energy
loss spectra (EELS) is small. Although the GGA functional is good in predicting
structural properties, it generality overestimates the exchange spin-splitting.
This leads to higher magnon energies, as compared to both ALDA and experiment.
In addition, interaction with the Stoner spin-flip continuum is enhanced by
AGGA, which strongly suppresses the intensity of spin-waves.Comment: 11 pages, 7 figure
Structural characteristics of positionally-disordered lattices: relation to the first sharp diffraction peak in glasses
Positional disorder has been introduced into the atomic structure of certain
crystalline lattices, and the orientationally-averaged structure factor S(k)
and pair-correlation function g(r) of these disordered lattices have been
studied. Analytical expressions for S(k) and g(r) for Gaussian positional
disorder in 2D and 3D are confirmed with precise numerical simulations. These
analytic results also have a bearing on the unsolved Gauss circle problem in
mathematics. As the positional disorder increases, high-k peaks in S(k) are
destroyed first, eventually leaving a single peak, that with the lowest-k
value. The pair-correlation function for lattices with such high levels of
positional disorder exhibits damped oscillations, with a period equal to the
separation between the furthest-separated (lowest-k) lattice planes. The last
surviving peak in S(k) is, for example for silicon and silica, at a wavevector
nearly identical to that of the experimentally-observed first sharp diffraction
peak (FSDP) in the amorphous phases of those materials. Thus, for these
amorphous materials at least, the FSDP can be regarded as arising from
scattering from atomic configurations equivalent to the single family of
positionally-disordered local Bragg planes having the furthest separation.Comment: v2: changes in response to referees' comments: Figure 2 made more
readable, improved discussion of height of peaks in S(k), other minor changes
4 pages, 3 figures, submitted to Physical Review
Compound nuclear decay and the liquid to vapor phase transition: a physical picture
Analyses of multifragmentation in terms of the Fisher droplet model (FDM) and
the associated construction of a nuclear phase diagram bring forth the problem
of the actual existence of the nuclear vapor phase and the meaning of its
associated pressure. We present here a physical picture of fragment production
from excited nuclei that solves this problem and establishes the relationship
between the FDM and the standard compound nucleus decay rate for rare particles
emitted in first-chance decay. The compound thermal emission picture is
formally equivalent to a FDM-like equilibrium description and avoids the
problem of the vapor while also explaining the observation of Boltzmann-like
distribution of emission times. In this picture a simple Fermi gas thermometric
relation is naturally justified and verified in the fragment yields and time
scales. Low energy compound nucleus fragment yields scale according to the FDM
and lead to an estimate of the infinite symmetric nuclear matter critical
temperature between 18 and 27 MeV depending on the choice of the surface energy
coefficient of nuclear matter.Comment: Five page two column pages, four figures, submitted to Phys. Rev.
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