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Renewal, Modulation, and Superstatistics in Times Series (open access)

Renewal, Modulation, and Superstatistics in Times Series

Article discussing two different approaches, referred to as renewal and modulation, to generate time series with a nonexponential distribution of waiting times.
Date: April 27, 2006
Creator: Allegrini, Paolo; Barbi, Francesco; Grigolini, Paolo & Paradisi, Paolo
System: The UNT Digital Library
Seismic imaging of reservoir flow properties: Resolving waterinflux and reservoir permeability (open access)

Seismic imaging of reservoir flow properties: Resolving waterinflux and reservoir permeability

Methods for geophysical model assessment, in particuale thecomputation of model parameter resolution, indicate the value and thelimitations of time-lapse data in estimating reservoir flow properties. Atrajectory-based method for computing sensitivities provides an effectivemeans to compute model parameter resolutions. We examine the commonsituation in which water encroaches into a resrvoir from below, as due tothe upward movement of an oil-water contact. Using straight-forwardtechniques we find that, by inclusing reflections off the top and bottomof a reservoir tens of meters thick, we can infer reservoir permeabilitybased upon time-lapse data. We find that, for the caseof water influxfrom below, using multiple time-lapse 'snapshots' does not necessarilyimprove the resolution of reservoir permeability. An application totime-lapse data from the Norne field illustrates that we can resolve thepermeability near a producing well using reflections from threeinterfaces associated with the reservoir.
Date: November 27, 2006
Creator: Vasco, D.W. & Keers, Henk
System: The UNT Digital Library
Do grain boundaries in nanophase metals slide? (open access)

Do grain boundaries in nanophase metals slide?

Nanophase metallic materials show a maximum in strength as grain size decreases to the nano scale, indicating a break down of the Hall-Petch relation. Grain boundary sliding, as a possible accommodation mechanisms, is often the picture that explain computer simulations results and real experiments. In a recent paper, Bringa et al. Science 309, 1838 (2005), we report on the observation of an ultra-hard behavior in nanophase Cu under shock loading, explained in terms of a reduction of grain boundary sliding under the influence of the shock pressure. In this work we perform a detailed study of the effects of hydrostatic pressure on nanophase Cu plasticity and find that it can be understood in terms of pressure dependent grain boundary sliding controlled by a Mohr-Coulomb law.
Date: October 27, 2006
Creator: Bringa, E M; Leveugle, E & Caro, A
System: The UNT Digital Library
The Heavy-Ion Approximation for Ambipolar Diffusion Calcuations for Weakly Ionized Plasmas (open access)

The Heavy-Ion Approximation for Ambipolar Diffusion Calcuations for Weakly Ionized Plasmas

Ambipolar diffusion redistributes magnetic flux in weakly ionized plasmas and plays a critical role in star formation. Simulations of ambipolar diffusion using explicit MHD codes are prohibitively expensive for the level of ionization observed in molecular clouds ({approx}< 10{sup -6}) since an enormous number of time steps is required to represent the dynamics of the dominant neutral component with a time step determined by the trace ion component. Here we show that ambipolar diffusion calculations can be significantly accelerated by the 'heavy-ion approximation', in which the mass density of the ions is increased and the collisional coupling constant with the neutrals decreased such that the product remains constant. In this approximation, the ambipolar diffusion time and the ambipolar magnetic Reynolds number remain unchanged. We present three tests of the heavy-ion approximation: C-type shocks, the Wardle instability, and the 1D collapse of a magnetized slab. We show that this approximation is quite accurate provided that (1) the square of the Alfven Mach number is small compared to the ambipolar diffusion Reynolds number for dynamical problems, and that (2) the ion mass density is negligible for quasi-static problems; a specific criterion is given for the magnetized slab problem. The first condition can …
Date: July 27, 2006
Creator: Li, P; McKee, C & Klein, R
System: The UNT Digital Library
Reversible expansion of gallium-stabilized (delta)-plutonium (open access)

Reversible expansion of gallium-stabilized (delta)-plutonium

It is shown that the transient expansion of plutonium-gallium alloys observed both in the lattice parameter as well as in the dimension of a sample held at ambient temperature can be explained by assuming incipient precipitation of Pu{sub 3}Ga. However, this ordered {zeta}-phase is also subject to radiation-induced disordering. As a result, the gallium-stabilized {delta}-phase, being metastable at ambient temperature, is driven towards thermodynamic equilibrium by radiation-enhanced diffusion of gallium and at the same time reverted back to its metastable state by radiation-induced disordering. A steady state is reached in which only a modest fraction of the gallium present is arranged in ordered {zeta}-phase regions.
Date: February 27, 2006
Creator: Wolfer, W G; Oudot, B & Baclet, N
System: The UNT Digital Library
Analysis of Order Formation in Block Copolymer Thin Films UsingResonant Soft X-Ray Scattering (open access)

Analysis of Order Formation in Block Copolymer Thin Films UsingResonant Soft X-Ray Scattering

The lateral order of poly(styrene-block-isoprene) copolymer(PS-b-PI) thin films is characterized by the emerging technique ofresonant soft X-ray scattering (RSOXS) at the carbon K edge and comparedto ordering in bulk samples of the same materials measured usingconventional small-angle X-ray scattering. We show resonance using theoryand experiment that the loss of scattering intensity expected with adecrease in sample volume in the case of thin films can be overcome bytuning X-rays to the pi* resonance of PS or PI. Using RSOXS, we study themicrophase ordering of cylinder- and phere-forming PS-b-PI thin films andcompare these results to position space data obtained by atomic forcemicroscopy. Our ability to examine large sample areas (~;9000 mu m2) byRSOXS enables unambiguous identification of the lateral lattice structurein the thin films. In the case of the sphere-forming copolymer thin film,where the spheres are hexagonally arranged, the average sphere-to-spherespacing is between the bulk (body-centered cubic) nearest neighbor andbulk unit cell spacings. In the case of the cylinder-forming copolymerthin film, the cylinder-to-cylinder spacing is within experimental errorof that obtained in the bulk.
Date: November 27, 2006
Creator: Virgili, Justin M.; Tao, Yuefei; Kortright, Jeffrey B.; Balsara,Nitash P. & Segalman, Rachel A.
System: The UNT Digital Library
Nuclear Resonance Fluorescence Excitations Near 2 MeV in 235U and 239Pu (open access)

Nuclear Resonance Fluorescence Excitations Near 2 MeV in 235U and 239Pu

A search for nuclear resonance fluorescence excitations in {sup 235}U and {sup 239}Pu within the energy range of 1.0- to 2.5-MeV was performed using a 4-MeV continuous bremsstrahlung source at the High Voltage Research Laboratory at the Massachusetts Institute of Technology. Measurements utilizing high purity Ge detectors at backward angles identified 9 photopeaks in {sup 235}U and 12 photopeaks in {sup 239}Pu in this energy range. These resonances provide unique signatures that allow the materials to be non-intrusively detected in a variety of environments including fuel cells, waste drums, vehicles and containers. The presence and properties of these states may prove useful in understanding the mechanisms for mixing low-lying collective dipole excitations with other states at low excitations in heavy nuclei.
Date: December 27, 2006
Creator: Bertozzi, W.; Caggiano, J. A.; Hensley, W. K.; Johnson, M. S.; Korbly, S. E.; Ledoux, R. J. et al.
System: The UNT Digital Library
An Efficient Vector Finite Element Method for Nonlinear Electromagnetic Modeling (open access)

An Efficient Vector Finite Element Method for Nonlinear Electromagnetic Modeling

We have developed a mixed Vector Finite Element Method (VFEM) for Maxwell's equations with a nonlinear polarization term. The method allows for discretization of complicated geometries with arbitrary order representations of the B and E fields. In this paper we will describe the method and a series of optimizations that significantly reduce the computational cost. Additionally, a series of test simulations will be presented to validate the method. Finally, a nonlinear waveguide mode mixing example is presented and discussed.
Date: June 27, 2006
Creator: Fisher, A. C.; White, D. A. & Rodrigue, G. H.
System: The UNT Digital Library
A Cone Jet-Finding Algorithm for Heavy-Ion Collisions at LHCEnergies (open access)

A Cone Jet-Finding Algorithm for Heavy-Ion Collisions at LHCEnergies

Standard jet finding techniques used in elementary particle collisions have not been successful in the high track density of heavy-ion collisions. This paper describes a modified cone-type jet finding algorithm developed for the complex environment of heavy-ion collisions. The primary modification to the algorithm is the evaluation and subtraction of the large background energy, arising from uncorrelated soft hadrons, in each collision. A detailed analysis of the background energy and its event-by-event fluctuations has been performed on simulated data, and a method developed to estimate the background energy inside the jet cone from the measured energy outside the cone on an event-by-event basis. The algorithm has been tested using Monte-Carlo simulations of Pb+Pb collisions at {radical}s = 5.5 TeV for the ALICE detector at the LHC. The algorithm can reconstruct jets with a transverse energy of 50 GeV and above with an energy resolution of {approx} 30%.
Date: July 27, 2006
Creator: Blyth, S.-L.; Horner, M.J.; Awes, T.C.; Cormier, T.; Gray, H.M.; Klay, J.L. et al.
System: The UNT Digital Library
Surface structure of CdSe Nanorods revealed by combined X-rayabsorption fine structure measurements and ab-initio calculations (open access)

Surface structure of CdSe Nanorods revealed by combined X-rayabsorption fine structure measurements and ab-initio calculations

We report orientation-specific, surface-sensitive structural characterization of colloidal CdSe nanorods with extended X-ray absorption fine structure spectroscopy and ab-initio density functional theory calculations. Our measurements of crystallographically-aligned CdSe nanorods show that they have reconstructed Cd-rich surfaces. They exhibit orientation-dependent changes in interatomic distances which are qualitatively reproduced by our calculations. These calculations reveal that the measured interatomic distance anisotropy originates from the nanorod surface.
Date: January 27, 2006
Creator: Aruguete, Deborah A.; Marcus, Matthew A.; Li, Liang-shi; Williamson, Andrew; Fakra, Sirine; Gygi, Francois et al.
System: The UNT Digital Library
Screening and Ranking Framework (SRF) for Geologic CO2 Storage Site Selection on the Basis of Hse Risk (open access)

Screening and Ranking Framework (SRF) for Geologic CO2 Storage Site Selection on the Basis of Hse Risk

A screening and ranking framework (SRF) has been developedto evaluate potential geologic carbon dioxide (CO2) storage sites on thebasis of health, safety, and environmental (HSE) risk arising from CO2leakage. The approach is based on the assumption that CO2 leakage risk isdependent on three basic characteristics of a geologic CO2 storage site:(1) the potential for primary containment by the target formation; (2)the potential for secondary containment if the primary formation leaks;and (3) the potential for attenuation and dispersion of leaking CO2 ifthe primary formation leaks and secondary containment fails. Theframework is implemented in a spreadsheet in which users enter numericalscores representing expert opinions or published information along withestimates of uncertainty. Applications to three sites in Californiademonstrate the approach. Refinements and extensions are possible throughthe use of more detailed data or model results in place of propertyproxies.
Date: November 27, 2006
Creator: Oldenburg, Curtis M.
System: The UNT Digital Library
d-alpha correlation functions and collective motion in Xe+Au collisions at E/A=50 MeV (open access)

d-alpha correlation functions and collective motion in Xe+Au collisions at E/A=50 MeV

The interplay of the effects of geometry and collective motion on d-{alpha} correlation functions is investigated for central Xe+Au collisions at E/A=50 MeV. The data cannot be explained with out collective motion, which could be partly along the beam axis. A semi-quantitative description of the data can be obtained using a Monte -Carlo model, where thermal emission is superimposed on collective motion. Both the emission volume and the competition between the thermal and collective motion influence significantly the shape of the correlation function, motivating new strategies for extending intensity interferometry studies to massive particles.
Date: July 27, 2006
Creator: Verde, G.; Danielewicz, P.; Lynch, W.; Chan, C.; Gelbke, C.; Kwong, L. et al.
System: The UNT Digital Library
Exact solutions in a model of vertical gas migration (open access)

Exact solutions in a model of vertical gas migration

This work is motivated by the growing interest in injectingcarbon dioxide into deep geological formations as a means of avoidingatmospheric emissions of carbon dioxide and consequent global warming.One of the key questions regarding the feasibility of this technology isthe potential rate of leakage out of the primary storage formation. Weseek exact solutions in a model of gas flow driven by a combination ofbuoyancy, viscous and capillary forces. Different combinations of theseforces and characteristic length scales of the processes lead todifferent time scaling and different types of solutions. In the case of athin, tight seal, where the impact of gravity is negligible relative tocapillary and viscous forces, a Ryzhik-type solution implies square-rootof time scaling of plume propagation velocity. In the general case, a gasplume has two stable zones, which can be described by travelling-wavesolutions. The theoretical maximum of the velocity of plume migrationprovides a conservative estimate for the time of vertical migration.Although the top of the plume has low gas saturation, it propagates witha velocity close to the theoretical maximum. The bottom of the plumeflows significantly more slowly at a higher gas saturation. Due to localheterogeneities, the plume can break into parts. Individual plumes alsocan coalesce and from larger plumes. The …
Date: June 27, 2006
Creator: Silin, Dmitriy B.; Patzek, Tad W. & Benson, Sally M.
System: The UNT Digital Library
Asymptotic Analysis of Time-Dependent Neutron Transport Coupled with Isotopic Depletion and Radioactive Decay (open access)

Asymptotic Analysis of Time-Dependent Neutron Transport Coupled with Isotopic Depletion and Radioactive Decay

We describe an asymptotic analysis of the coupled nonlinear system of equations describing time-dependent three-dimensional monoenergetic neutron transport and isotopic depletion and radioactive decay. The classic asymptotic diffusion scaling of Larsen and Keller [1], along with a consistent small scaling of the terms describing the radioactive decay of isotopes, is applied to this coupled nonlinear system of equations in a medium of specified initial isotopic composition. The analysis demonstrates that to leading order the neutron transport equation limits to the standard time-dependent neutron diffusion equation with macroscopic cross sections whose number densities are determined by the standard system of ordinary differential equations, the so-called Bateman equations, describing the temporal evolution of the nuclide number densities.
Date: September 27, 2006
Creator: Brantley, P S
System: The UNT Digital Library
Potential bias of model projected greenhouse warming in irrigated regions (open access)

Potential bias of model projected greenhouse warming in irrigated regions

Atmospheric general circulation models (GCMs) used to project climate responses to increased CO{sub 2} generally omit irrigation of agricultural land. Using the NCAR CAM3 GCM coupled to a slab-ocean model, we find that inclusion of an extreme irrigation scenario has a small effect on the simulated temperature and precipitation response to doubled CO{sub 2} in most regions, but reduced warming by as much as 1 C in some agricultural regions, such as Europe and India. This interaction between CO{sub 2} and irrigation occurs in cases where agriculture is a major fraction of the land surface and where, in the absence of irrigation, soil moisture declines are projected to provide a positive feedback to temperature change. The reduction of warming is less than 25% of the temperature increase modeled for doubled CO{sub 2} in most regions; thus greenhouse warming will still be dominant. However, the results indicate that land use interactions may be an important component of climate change uncertainty in some agricultural regions. While irrigated lands comprise only {approx}2% of the land surface, they contribute over 40% of global food production. Climate changes in these regions are therefore particularly important to society despite their relatively small contribution to average global …
Date: April 27, 2006
Creator: Lobell, D.; Bala, G.; Bonfils, C. & Duffy, P.
System: The UNT Digital Library
The effect of pulse duration on laser-induced damage by 1053-nm light in potassium dihydrogen phosphate crystals (open access)

The effect of pulse duration on laser-induced damage by 1053-nm light in potassium dihydrogen phosphate crystals

Laser induced damage in potassium dihydrogen phosphate (KDP) has previously been shown to depend significantly on pulse duration for 351-nm Gaussian pulses. In this work we studied the properties of damage initiated by 1053-nm temporally Gaussian pulses with 10ns and 3ns FWHM durations. Our results indicate that the number of damage sites induced by 1053-nm light scales with pulse duration ({tau}) as ({tau}{sub 1}/{tau}{sub 2}){sup 0.17} in contrast to the previously reported results for 351-nm light as ({tau}{sub 1}/{tau}{sub 2}){sup 0.35}. This indicates that damage site formation is significantly less probable at longer wavelengths for a given fluence.
Date: November 27, 2006
Creator: Cross, D A; Braunstein, M R & Carr, C W
System: The UNT Digital Library
Absolute Measurement of Electron Cloud Density in a Positively-Charged Particle Beam (open access)

Absolute Measurement of Electron Cloud Density in a Positively-Charged Particle Beam

Clouds of stray electrons are ubiquitous in particle accelerators and frequently limit the performance of storage rings. Earlier measurements of electron energy distribution and flux to the walls provided only a relative electron cloud density. We have measured electron accumulation using ions expelled by the beam. The ion energy distribution maps the depressed beam potential and gives the dynamic cloud density. Clearing electrode current reveals the static background cloud density, allowing the first absolute measurement of the time-dependent electron cloud density during the beam pulse.
Date: April 27, 2006
Creator: Kireeff Covo, Michel; Molvik, Arthur W.; Friedman, Alex; Vay, Jean-Luc; Seidl, Peter A.; Logan, Grant et al.
System: The UNT Digital Library
Reduction of Allowed Inventory When Chemicals are Located in Close Proximity with Explosives (open access)

Reduction of Allowed Inventory When Chemicals are Located in Close Proximity with Explosives

The objective of this report is to determine the allowed inventory of chemicals stored in the same bay, building or magazine, i.e., in close proximity, with high explosives (HE) that would, in the event of an accident, result in acceptable risks to colocated workers and the public.
Date: September 27, 2006
Creator: Chong, Y P & Nguyen, S N
System: The UNT Digital Library
Strong and Weak Lensing United III: Measuring the Mass Distribution of the Merging Galaxy Cluster 1E0657-56 (open access)

Strong and Weak Lensing United III: Measuring the Mass Distribution of the Merging Galaxy Cluster 1E0657-56

The galaxy cluster 1E0657-56 (z = 0.296) is remarkably well-suited for addressing outstanding issues in both galaxy evolution and fundamental physics. We present a reconstruction of the mass distribution from both strong and weak gravitational lensing data. Multi-color, high-resolution HST ACS images allow detection of many more arc candidates than were previously known, especially around the subcluster. Using the known redshift of one of the multiply imaged systems, we determine the remaining source redshifts using the predictive power of the strong lens model. Combining this information with shape measurements of ''weakly'' lensed sources, we derive a high-resolution, absolutely-calibrated mass map, using no assumptions regarding the physical properties of the underlying cluster potential. This map provides the best available quantification of the total mass of the central part of the cluster. We also confirm the result from Clowe et al. (2004, 2006a) that the total mass does not trace the baryonic mass.
Date: September 27, 2006
Creator: Bradac, Marusa; Clowe, Douglas; Gonzalez, Anthony H.; Marshall, Phil; Forman, William; Jones, Christine et al.
System: The UNT Digital Library
Galaxies, Black Holes & Laboratories: Studies of interstellar medium materials in energetic environments (open access)

Galaxies, Black Holes & Laboratories: Studies of interstellar medium materials in energetic environments

None
Date: June 27, 2006
Creator: vanBreugel, W; Bringa, E & Tielens, A
System: The UNT Digital Library
Edge Localized Mode Control in DIII-D Using Magnetic Perturbation-Induced Pedestal Transport Changes (open access)

Edge Localized Mode Control in DIII-D Using Magnetic Perturbation-Induced Pedestal Transport Changes

Edge localized mode (ELM) control is a critical issue for ITER because the impulsive power loading from ELMs is predicted to limit the divertor lifetime to only a few hundred full-length pulses. Consequently, a technique that replaces the ELM-induced transport with more continuous transport while preserving the H-mode pedestal height and core performance would significantly improve the viability of ITER. One approach is to use edge resonant magnetic perturbations (RMPs) to enhance pedestal transport enough to reduce the pedestal pressure gradient {del}p{sub ped} below the stability limit for Type I ELMs. In DIII-D, n = 3 RMPs have been used to eliminate Type I ELMs when the edge safety factor is in the resonant window q95 {approx} 3.5 without degrading confinement in H-modes with ITER-relevant pedestal collisionalities v*{sub e} {approx} 0.2. The RMP reduces {del}p{sub ped} as expected, with {del}p{sub ped} controlled by the RMP amplitude. Linear peeling-ballooning (P-B) stability analysis indicates that the ELMs are suppressed by reducing {del}p{sub ped} below the P-B stability limit. The {del}p{sub ped} reduction results primarily from an increase in particle transport, not electron thermal transport. This result is inconsistent with estimates based on quasi-linear stochastic diffusion theory based on the vacuum field (no …
Date: September 27, 2006
Creator: Moyer, R. A.; Burrell, K. H.; Evans, T. E.; Fenstermacher, M. E.; Joseph, I.; Osborne, T. H. et al.
System: The UNT Digital Library
Quantitative experiments with electrons in a positively chargedBeam (open access)

Quantitative experiments with electrons in a positively chargedBeam

None
Date: October 27, 2006
Creator: Molvik, A. W.; Vay, J. L.; Kireef Covo, M.; Cohen, R.; Baca, D.; Bieniosek, F. et al.
System: The UNT Digital Library
Accretion Column Structure of Magnetic Cataclysmic Variables from X-ray Spectroscopy (open access)

Accretion Column Structure of Magnetic Cataclysmic Variables from X-ray Spectroscopy

Using Chandra HETG data we present light curves for individual spectral lines of Mg XI and Mg XII for EX Hydrae, an intermediate-polar type cataclysmic variable. The Mg XI light curve, folded on the white dwarf spin period, shows two spikes that are not seen in the Mg XII or broad-band light curves. Occultation of the accretion column by the body of the white dwarf would produce such spikes for an angle between the rotation axis and the accretion columns of {alpha} = 18{sup o} and a height of the Mg XI emission above the white dwarf surface of {approx}< 0.0004 white dwarf radii or {approx}< 4 km. The absence of spikes in the Mg XII and broad-band light curves could then be explained if the bulk of its emission forms at much larger height, > 0.004 white dwarf radii or > 40 km, above the white dwarf surface. The technique described in this letter demonstrates that high signal-to-noise ratio and high spectral resolution X-ray spectra can be used to map the temperature and density structure of accretion flows in magnetic cataclysmic variables. The Mg XI and Mg XII light curves are not consistent with the temperature and density structure …
Date: February 27, 2006
Creator: Hoogerwerf, R; Brickhouse, N S & Mauche, C W
System: The UNT Digital Library
A Direct Empirical Proof of the Existence of Dark Matter (open access)

A Direct Empirical Proof of the Existence of Dark Matter

We present new weak lensing observations of 1E0657-558 (z = 0.296), a unique cluster merger, that enable a direct detection of dark matter, independent of assumptions regarding the nature of the gravitational force law. Due to the collision of two clusters, the dissipationless stellar component and the fluid-like X-ray emitting plasma are spatially segregated. By using both wide-field ground based images and HST/ACS images of the cluster cores, we create gravitational lensing maps which show that the gravitational potential does not trace the plasma distribution, the dominant baryonic mass component, but rather approximately traces the distribution of galaxies. An 8{sigma} significance spatial offset of the center of the total mass from the center of the baryonic mass peaks cannot be explained with an alteration of the gravitational force law, and thus proves that the majority of the matter in the system is unseen.
Date: September 27, 2006
Creator: Clowe, Douglas; Bradac, Marusa; Gonzalez, Anthony H.; Markevitch, Maxim; Randall, Scott W.; Jones, Christine et al.
System: The UNT Digital Library