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Radioactive waste tank Initial Pretreatment Module (IPM) technology development and selection
The processing of nuclear materials at the Hanford Site has resulted in the accumulation of radioactive wastes stored in 177 single- and double-shell tanks (SSTs and DSTs). Fifty-four of the 177 tanks are currently on a tank watch list because organic chemicals and ferrocyanide compounds in the tanks present a potential fire or explosion hazard. In addition, one additional SST is under consideration for placement on the watch list because of high organic concentration. Seventeen of the watch list tanks require pretreatment, and two DST complexant concentrate waste tanks not on the watch list may also need pretreatment. The proposed Initial Pretreatment Module (IPM) is expected to resolve the safety concerns by destroying the organics and ferrocyanide compounds in the tank wastes. The primary objective of the IPM is to destroy or modify constituents that cause safety concerns in the watch list tanks. A secondary objective is to enhance the cost effectiveness of processing the wastes by performing additional processing. Overall, IPM will achieve organic/ferrocyanide destruction (the primary goal) and will assist in the separation of cesium, strontium, and technetium from the tank wastes
The Albedo, Size, and Density of Binary Kuiper Belt Object (47171) 1999 TC36
We measured the system-integrated thermal emission of the binary Kuiper Belt
Object 1999 TC36 at wavelengths near 24 and 70 microns using the Spitzer space
telescope. We fit these data and the visual magnitude using both the Standard
Thermal Model and thermophysical models. We find that the effective diameter of
the binary is 405 km, with a range of 350 -- 470 km, and the effective visible
geometric albedo for the system is 0.079 with a range of 0.055 -- 0.11. The
binary orbit, magnitude contrast between the components, and system mass have
been determined from HST data (Margot et al., 2004; 2005a; 2005b). Our
effective diameter, combined with that system mass, indicate an average density
for the objects of 0.5 g/cm3, with a range 0.3 -- 0.8 g/cm3. This density is
low compared to that of materials expected to be abundant in solid bodies in
the trans-Neptunian region, requiring 50 -- 75% of the interior of 1999 TC36 be
taken up by void space. This conclusion is not greatly affected if 1999 TC36 is
``differentiated'' (in the sense of having either a rocky or just a non-porous
core). If the primary is itself a binary, the average density of that
(hypothetical) triple system would be in the range 0.4 -- 1.1 g/cm3, with a
porosity in the range 15 -- 70%.Comment: ApJ, in press (May, 2006
Debris disks around Sun-like stars
We have observed nearly 200 FGK stars at 24 and 70 microns with the Spitzer
Space Telescope. We identify excess infrared emission, including a number of
cases where the observed flux is more than 10 times brighter than the predicted
photospheric flux, and interpret these signatures as evidence of debris disks
in those systems. We combine this sample of FGK stars with similar published
results to produce a sample of more than 350 main sequence AFGKM stars. The
incidence of debris disks is 4.2% (+2.0/-1.1) at 24 microns for a sample of 213
Sun-like (FG) stars and 16.4% (+2.8/-2.9) at 70 microns for 225 Sun-like (FG)
stars. We find that the excess rates for A, F, G, and K stars are statistically
indistinguishable, but with a suggestion of decreasing excess rate toward the
later spectral types; this may be an age effect. The lack of strong trend among
FGK stars of comparable ages is surprising, given the factor of 50 change in
stellar luminosity across this spectral range. We also find that the incidence
of debris disks declines very slowly beyond ages of 1 billion years.Comment: ApJ, in pres
Renal DCE-MRI model selection using Bayesian probability theory
The goal of this work was to demonstrate the utility of Bayesian probability theory-based model selection for choosing the optimal mathematical model from among 4 competing models of renal dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) data. DCE-MRI data were collected on 21 mice with high (n = 7), low (n = 7), or normal (n = 7) renal blood flow (RBF). Model parameters and posterior probabilities of 4 renal DCE-MRI models were estimated using Bayesian-based methods. Models investigated included (1) an empirical model that contained a monoexponential decay (washout) term and a constant offset, (2) an empirical model with a biexponential decay term (empirical/biexponential model), (3) the Patlak–Rutland model, and (4) the 2-compartment kidney model. Joint Bayesian model selection/parameter estimation demonstrated that the empirical/biexponential model was strongly favored for all 3 cohorts, the modeled DCE signals that characterized each of the 3 cohorts were distinctly different, and individual empirical/biexponential model parameter values clearly distinguished cohorts of low and high RBF from one another. The Bayesian methods can be readily extended to a variety of model analyses, making it a versatile and valuable tool for model selection and parameter estimation.</jats:p
Simulation of thermal conductivity and heat transport in solids
Using molecular dynamics (MD) with classical interaction potentials we
present calculations of thermal conductivity and heat transport in crystals and
glasses. Inducing shock waves and heat pulses into the systems we study the
spreading of energy and temperature over the configurations. Phonon decay is
investigated by exciting single modes in the structures and monitoring the time
evolution of the amplitude using MD in a microcanonical ensemble. As examples,
crystalline and amorphous modifications of Selenium and are
considered.Comment: Revtex, 8 pages, 11 postscript figures, accepted for publication in
PR
Spin-wave Scattering in the Effective Lagrangian Perspective
Nonrelativistic systems exhibiting collective magnetic behavior are analyzed
in the framework of effective Lagrangians. The method, formulating the dynamics
in terms of Goldstone bosons, allows to investigate the consequences of
spontaneous symmetry breaking from a unified point of view. Low energy theorems
concerning spin-wave scattering in ferro- and antiferromagnets are established,
emphasizing the simplicity of actual calculations. The present work includes
approximate symmetries and discusses the modification of the low energy
structure imposed by an external magnetic and an anisotropy field,
respectively. Throughout the paper, analogies between condensed matter physics
and Lorentz-invariant theories are pointed out, demonstrating the universal
feature of the effective Lagrangian technique.Comment: Published versio
Identification of Nuclear Relaxation Processes in a Gapped Quantum Magnet: Proton NMR in the S=1/2 Heisenberg Ladder Cu2(C5H12N2)2Cl4
The proton hyperfine shift K and NMR relaxation rate have been
measured as a function of temperature in the S=1/2 Heisenberg antiferromagnetic
ladder Cu2(C5H12N2)2Cl4. The presence of a spin gap in this strongly coupled ladder ()
is supported by the K and results. By comparing at two
different proton sites, we infer the evolution of the spectral functions
and . When the gap is significantly
reduced by the magnetic field, two different channels of nuclear relaxation,
specific to gapped antiferromagnets, are identified and are in agreement with
theoretical predictions.Comment: 4 pages, 4 figures, to be published in Phys. Rev. Letter
Metric tensor as the dynamical variable for variable cell-shape molecular dynamics
We propose a new variable cell-shape molecular dynamics algorithm where the
dynamical variables associated with the cell are the six independent dot
products between the vectors defining the cell instead of the nine cartesian
components of those vectors. Our choice of the metric tensor as the dynamical
variable automatically eliminates the cell orientation from the dynamics.
Furthermore, choosing for the cell kinetic energy a simple scalar that is
quadratic in the time derivatives of the metric tensor, makes the dynamics
invariant with respect to the choice of the simulation cell edges. Choosing the
densitary character of that scalar allows us to have a dynamics that obeys the
virial theorem. We derive the equations of motion for the two conditions of
constant external pressure and constant thermodynamic tension. We also show
that using the metric as variable is convenient for structural optimization
under those two conditions. We use simulations for Ar with Lennard-Jones
parameters and for Si with forces and stresses calculated from first-principles
of density functional theory to illustrate the applications of the method.Comment: 10 pages + 6 figures, Latex, to be published in Physical Review
Towards device-size atomistic models of amorphous silicon
The atomic structure of amorphous materials is believed to be well described
by the continuous random network model. We present an algorithm for the
generation of large, high-quality continuous random networks. The algorithm is
a variation of the "sillium" approach introduced by Wooten, Winer, and Weaire.
By employing local relaxation techniques, local atomic rearrangements can be
tried that scale almost independently of system size. This scaling property of
the algorithm paves the way for the generation of realistic device-size atomic
networks.Comment: 7 pages, 3 figure
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