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Articles 7621 - 7650 of 8870

Full-Text Articles in Astrophysics and Astronomy

Sontrac—A Scintillating Plastic Fiber Tracking Detector For Neutron And Proton Imaging Spectroscopy, James M. Ryan, John R. Macri, Mark L. Mcconnell, R S. Miller Jan 2001

Sontrac—A Scintillating Plastic Fiber Tracking Detector For Neutron And Proton Imaging Spectroscopy, James M. Ryan, John R. Macri, Mark L. Mcconnell, R S. Miller

Space Science Center

SONTRAC (SOlar Neutron TRACking imager and spectrometer) is a conceptual instrument intended to measure the energy and incident direction of 20–150 MeV neutrons produced in solar flares. The intense neutron background in a low-Earth orbit requires that imaging techniques be employed to maximize an instrument’s signal-to-noise ratio. The instrument is comprised of mutually perpendicular, alternating layers of parallel, scintillating, plastic fibers that are viewed by optoelectronic devices. Two stereoscopic views of recoil proton tracks are necessary to determine the incident neutron’s direction and energy. The instrument can also be used as a powerful energetic proton imager. Data from a fully …

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Comptel Observations Of The Blazars 3c 454.3 And Cta 102, S Zhang, W Collmar, V Schonfelder, H Bloemen, W Hermsen, Mark L. Mcconnell, K Bennett, O R. Williams Jan 2001

Comptel Observations Of The Blazars 3c 454.3 And Cta 102, S Zhang, W Collmar, V Schonfelder, H Bloemen, W Hermsen, Mark L. Mcconnell, K Bennett, O R. Williams

Space Science Center

We have analyzed the two blazars of 3C 454.3 and CTA 102 using all available COMPTEL data from 1991 to 1999. In the 10–30 MeV band, emission from the general direction of the sources is found at the 4σ-level, being consistent with contributions from both sources. Below 10 MeV only 3C 454.3 is significantly detected, with the strongest evidence (5.6 σ) in the 3–10 MeV band. Significant flux variability is not observed for both sources, while a low emission is seen most of the years in the 3–10 MeV light curve for 3C 454.3. Its time-averaged MeV spectrum suggests a …

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Comptel Observations Of The Virgo Blazars 3c 273 And 3c 279, W Collmar, V Schonfelder, S Zhang, H Bloemen, W Hermsen, Mark L. Mcconnell, K Bennett, O R. Williams Jan 2001

Comptel Observations Of The Virgo Blazars 3c 273 And 3c 279, W Collmar, V Schonfelder, S Zhang, H Bloemen, W Hermsen, Mark L. Mcconnell, K Bennett, O R. Williams

Space Science Center

We report the main MeV properties (detections, light curves, spectra) of the Virgo blazars 3C 273 and 3C 279 which were derived from a consistent analysis of all COMPTEL Virgo observations between 1991 and 1997.

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Bayesian Multiscale Deconvolution Applied To Gamma-Ray Spectroscopy, C A. Young, A Connors, E Kolaczyk, Mark L. Mcconnell, G Rank, James M. Ryan, V Schonfelder Jan 2001

Bayesian Multiscale Deconvolution Applied To Gamma-Ray Spectroscopy, C A. Young, A Connors, E Kolaczyk, Mark L. Mcconnell, G Rank, James M. Ryan, V Schonfelder

Space Science Center

A common task in gamma-ray astronomy is to extract spectral information, such as model constraints and incident photon spectrum estimates, given the measured energy deposited in a detector and the detector response. This is the classic problem of spectral “deconvolution” or spectral inversion. The methods of forward folding (i.e., parameter fitting) and maximum entropy “deconvolution” (i.e., estimating independent input photon rates for each individual energy bin) have been used successfully for gamma-ray solar flares (e.g., Rank, 1997; Share and Murphy, 1995). These methods have worked well under certain conditions but there are situations were they don’t apply. These are: 1) …

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Gamma-Ray Spectral Variability Of Cygnus X-1, Mark L. Mcconnell, K Bennett, H Bloemen, W Collmar, W Hermsen, L Kuiper, W Paciesas, B F. Phlips, J Poutanen, James M. Ryan, V Schonfelder, H Steinle, A W. Strong, A A. Zdziarski Jan 2001

Gamma-Ray Spectral Variability Of Cygnus X-1, Mark L. Mcconnell, K Bennett, H Bloemen, W Collmar, W Hermsen, L Kuiper, W Paciesas, B F. Phlips, J Poutanen, James M. Ryan, V Schonfelder, H Steinle, A W. Strong, A A. Zdziarski

Space Science Center

We have used observations from CGRO to study the variation in the MeV emission of Cygnus X-1 between its low and high X-ray states. These data provide a measurement of the spectral variability above 1 MeV. The high state MeV spectrum is found to be much harder than that of the low state MeV spectrum. In particular, the power-law emission seen at hard X-ray energies in the high state spectrum (with a photon spectral index of 2.6) is found to extend out to at least 5 MeV, with no evidence for any cutoff. Here we present the data and describe …

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Energetic Proton Spectra In The 11 June 1991 Solar Flare, C A. Young, K Bennett, A Connors, R Diehl, Mark L. Mcconnell, G Rank, James M. Ryan, R Suleiman, V Schonfelder, C Winkler Jan 2001

Energetic Proton Spectra In The 11 June 1991 Solar Flare, C A. Young, K Bennett, A Connors, R Diehl, Mark L. Mcconnell, G Rank, James M. Ryan, R Suleiman, V Schonfelder, C Winkler

Space Science Center

The June 11, 1991 gamma-ray flare seen by the Compton Gamma-ray Observatory (CGRO) displays several features that make it a dynamic and rich event. It is a member of a class of long duration gamma-ray events with both 2.223 MeV and greater than 8 MeV emission for hours after the impulsive phase. It also contains an inter-phase between the impulsive and extended phases that presents a challenge to the standard gamma-ray line (GRL) flare picture. This phase has strong 2.223 MeV emission and relatively weak 4.44 MeV emission indicative of a very hard parent proton spectrum. However, this would indicate …

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X- And Gamma-Ray Observations Of The 15 November 1991 Solar Flare, M B. Arndt, A Connors, J Lockwood, Mark L. Mcconnell, R Suleiman, James M. Ryan, C A. Young, G Rank, V Schonfelder, H Debrunner, K Bennett, O R. Williams, C Winkler Jan 2001

X- And Gamma-Ray Observations Of The 15 November 1991 Solar Flare, M B. Arndt, A Connors, J Lockwood, Mark L. Mcconnell, R Suleiman, James M. Ryan, C A. Young, G Rank, V Schonfelder, H Debrunner, K Bennett, O R. Williams, C Winkler

Space Science Center

This work expands the current understanding of the 15 November 1991 Solar Flare. The flare was a well observed event in radio to gamma-rays and is the first flare to be extensively studied with the benefit of detailed soft and hard X-ray images. In this work, we add data from all four instruments on the Compton Gamma Ray Observatory. Using these data we determined that the accelerated electron spectrum above 170 keV is best fit with a power law with a spectral index of −4.6, while the accelerated proton spectrum above 0.6 MeV is fit with a power law of …

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The Milagro Gamma-Ray Observatory, J E. Mcenery, R Atkins, W Benbow, D Berley, M L. Chen, D G. Coyne, B L. Dingus, D E. Dorfan, R W. Ellsworth, D Evans, A Falcone, L Fleysher, R Fleysher, G Gisler, J A. Goodman, T J. Haines, C M. Hoffman, S Hugenberger, L A. Kelley, I Leonor, Mark L. Mcconnell, J F. Mccullough, R S. Miller, A I. Mincer, M F. Morales, P Nemethy, James M. Ryan, B Shen, A Shoup, C Sinnis, A J. Smith, G W. Sullivan, O T. Tumer, K Wang, M O. Wascko, S Westerhoff, D A. Williams, G B. Yodh Jan 2001

The Milagro Gamma-Ray Observatory, J E. Mcenery, R Atkins, W Benbow, D Berley, M L. Chen, D G. Coyne, B L. Dingus, D E. Dorfan, R W. Ellsworth, D Evans, A Falcone, L Fleysher, R Fleysher, G Gisler, J A. Goodman, T J. Haines, C M. Hoffman, S Hugenberger, L A. Kelley, I Leonor, Mark L. Mcconnell, J F. Mccullough, R S. Miller, A I. Mincer, M F. Morales, P Nemethy, James M. Ryan, B Shen, A Shoup, C Sinnis, A J. Smith, G W. Sullivan, O T. Tumer, K Wang, M O. Wascko, S Westerhoff, D A. Williams, G B. Yodh

Space Science Center

The Milagro water Cherenkov detector began full operation in January 2000. This detector is capable of monitoring the Northern sky at energies above 500 GeV for sources of equivalent strength to the Crab Nebula over one year of integration. We report on the current performance and sensitivity of Milagro.

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Hα Imaging With Hubble Space Telescope-Nicmos Of An Elusive Damped Lyα Cloud At Z = 0.61, Nicolas Bouché, James D. Lowenthal, Jane C. Charlton, Matthew A. Bershady, Christopher W. Churchill, Charles C. Steidel Jan 2001

Hα Imaging With Hubble Space Telescope-Nicmos Of An Elusive Damped Lyα Cloud At Z = 0.61, Nicolas Bouché, James D. Lowenthal, Jane C. Charlton, Matthew A. Bershady, Christopher W. Churchill, Charles C. Steidel

Astronomy: Faculty Publications

Despite previous intensive ground-based imaging and spectroscopic campaigns and wideband Hubble Space Telescope (HST) imaging of the z = 0.927 QSO 3C 336 field, the galaxy that hosts the damped Lyα system along this line of sight has eluded detection. We present a deep narrowband Hα image of the field of this zabs = 0.656 damped Lyα absorber, obtained through the F108N filter of NICMOSI on board the HST. The goal of this project was to detect any Hα emission 10 times closer than previous studies to unveil the damped absorber. We do not detect Hα emission between 0".05 and …

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Multiband Microvariability Observations Of Bl Lacertae During The Outburst Of 1997, Sandra D. Clements, Michael T. Carini Jan 2001

Multiband Microvariability Observations Of Bl Lacertae During The Outburst Of 1997, Sandra D. Clements, Michael T. Carini

Aerospace, Physics, and Space Science Faculty Publications

BL Lac was observed in the V and R bands on 11 nights in 1997 July during a major optical outburst. It varied continuously, exhibiting nightly variations ranging from 0.1 to 0.6 mag. Over the course of the 11 nights, BL Lac was seen to vary by 1.8 R magnitudes. Color variations were also observed, with BL Lac becoming bluer as it brightened. The influence of variable seeing conditions on the observed variations was investigated. No correlation was found between the observed microvariability and the local seeing conditions.

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Designing Adaptive Low-Dissipative High Order Schemes For Long-Time Integrations, Helen Yee, Bjorn Sjögreen Jan 2001

Designing Adaptive Low-Dissipative High Order Schemes For Long-Time Integrations, Helen Yee, Bjorn Sjögreen

United States National Aeronautics and Space Administration: Publications

A general framework for the design of adaptive low-dissipative high order schemes is presented. It encompasses a rather complete treatment of the numerical approach based on four integrated design criteria: (1) For stability considerations, condition the governing equations before the application of the appropriate numerical scheme whenever it is possible. (2) For consistency, compatible schemes that possess stability properties, including physical and numerical boundary condition treatments, similar to those of the discrete analogue of the continuum are preferred. (3) For the minimization of numerical dissipation contamination, efficient and adaptive numerical dissipation control to further improve nonlinear stability and accuracy should …

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Corrected Article: Exclusive Electroproduction Of ᵠ Mesons At 4.2 Gev [Physical. Rev. C 63, 065205, (2001)], M. Bektasoglu, L. Ciciani, G. E. Dodge, T. A. Forest, C. E. Hyde-Wright, S. E. Kuhn, L. M. Qin, F. Sabatié, L. B. Weinstein, Et Al., The Clas Collaboration Jan 2001

Corrected Article: Exclusive Electroproduction Of ᵠ Mesons At 4.2 Gev [Physical. Rev. C 63, 065205, (2001)], M. Bektasoglu, L. Ciciani, G. E. Dodge, T. A. Forest, C. E. Hyde-Wright, S. E. Kuhn, L. M. Qin, F. Sabatié, L. B. Weinstein, Et Al., The Clas Collaboration

Physics Faculty Publications

We studied the exclusive reaction ep → e' p' ᵠ using the ᵠ →K+K- decay mode. The data were collected using a 4.2 GeV incident electron beam and the CEBAF Large Acceptance Spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility. Our experiment covers the range in Q2 from 0.7 to 2.2 GeV2and W from 2.0 to 2.6 GeV. Taken together with all previous data, we find a consistent picture of ᵠ production on the proton. Our measurement shows the expected decrease of the t slope with the vector-meson formation time c Δ t …

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Effective Field Theory For The Small-X Evolution, I. Balitsky Jan 2001

Effective Field Theory For The Small-X Evolution, I. Balitsky

Physics Faculty Publications

The small-x behavior of structure functions in the saturation region is determined by the non-linear generalization of the BFKL equation. I suggest the effective field theory for the small-x evolution which solves formally this equation. The result is the 2+1 functional integral for the structure functions at small x.

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Exclusive Electroproduction Of ᵠ Mesons At 4.2 Gev, M. Bektasoglu, L. Ciciani, G. E. Dodge, T. A. Forest, C. E. Hyde-Wright, S. E. Kuhn, L. M. Qin, F. Sabatié, L. B. Weinstein, Et Al., The Clas Collaboration Jan 2001

Exclusive Electroproduction Of ᵠ Mesons At 4.2 Gev, M. Bektasoglu, L. Ciciani, G. E. Dodge, T. A. Forest, C. E. Hyde-Wright, S. E. Kuhn, L. M. Qin, F. Sabatié, L. B. Weinstein, Et Al., The Clas Collaboration

Physics Faculty Publications

We studied the exclusive reaction ep → e' p' ᵠ using the ᵠ →K+K- decay mode. The data were collected using a 4.2 GeV incident electron beam and the CEBAF Large Acceptance Spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility. Our experiment covers the range in Q2 from 0.7 to 2.2 GeV2and W from 2.0 to 2.6 GeV. Taken together with all previous data, we find a consistent picture of ᵠ production on the proton. Our measurement shows the expected decrease of the t slope with the vector-meson formation time c Δ t …

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White Dwarfs In Common Proper Motion Binary Systems: Mass Distribution And Kinematics, Nicole M. Silvestri, Terry D. Oswalt, Matt A. Wood, Jonathan Allyn Smith, Iain Neill Reid, Edward M. Sión Jan 2001

White Dwarfs In Common Proper Motion Binary Systems: Mass Distribution And Kinematics, Nicole M. Silvestri, Terry D. Oswalt, Matt A. Wood, Jonathan Allyn Smith, Iain Neill Reid, Edward M. Sión

Aerospace, Physics, and Space Science Faculty Publications

We present the mass distribution, gravitational redshifts, radial velocities, and space motions of white dwarf stars in common proper motion binary systems. The mass distribution we derive for the 41 DA white dwarfs in this study has a mean of 0.68 ± 0.04 M⊙. This distribution has a slightly higher mean and larger dispersion than most previous white dwarf studies. We hypothesize that this is due to a higher fraction of cool (average Teff ~ 10,000 K), hence old, white dwarfs in our sample. Our results indicate that samples made up of predominantly cool, old white dwarf stars tend to …

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Multi-Phase Transport Study Of Relativistic Nuclear Collisions, Bin Zhang, C. M. Ko, Bao-An Li, Ziwei Lin, Subrata Pal Jan 2001

Multi-Phase Transport Study Of Relativistic Nuclear Collisions, Bin Zhang, C. M. Ko, Bao-An Li, Ziwei Lin, Subrata Pal

Journal of the Arkansas Academy of Science

A multi-phase transport model (AMPT) is developed for the study of hot and dense matter produced in relativistic nuclear collisions. This model includes both initial partonic and final hadronic scattering. Using the AMPT model, we study the momentum distributions of charged particles such as protons, antiprotons, pions, and kaons in central heavy ion collisions at Super Proton Synchrotron (SPS) and Relativistic Heavy Ion Collider (RHIC) energies. The results are consistent with experimental data at these energies. They indicate a significant nuclear shadowing but a relative weak jet quenching in the initial dense matter. Antiproton to proton ratio at mid-rapidity increases …

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Verification Of A Distortion In The Microstructure Of Gan Detected By Exafs Using Ab Initio Density Functional Theory Calculations, Nicholas Dimakis, Grant Bunker, M. Katsikini, E. C. Paloura Jan 2001

Verification Of A Distortion In The Microstructure Of Gan Detected By Exafs Using Ab Initio Density Functional Theory Calculations, Nicholas Dimakis, Grant Bunker, M. Katsikini, E. C. Paloura

Physics & Astronomy Faculty Publications

X-ray absorption fine structure (XAFS) measurements on a series of epitaxially grown GaN samples have shown a distortion in the microstructure of GaN. More specifically the central N atom is 4-fold coordinated but the four Ga atoms are not equidistant. It has been shown that 2.9 to 3.5 of them (depending on the growth conditions) are found in the expected from XRD distance of 1.94 A and the remaining are at a distance longer by approximately 15%. Second derivative calculation of the conformation energy using the Density Functional Theory (DFT) is used to investigate if the symmetric GaN cluster as …

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Building Blocks For Reliable Complex Nonlinear Numerical Simulations, Helen Yee Jan 2001

Building Blocks For Reliable Complex Nonlinear Numerical Simulations, Helen Yee

United States National Aeronautics and Space Administration: Publications

This chapter describes some of the building blocks to ensure a higher level of confidence in the predictability and reliability (PAR) of numerical simulation of multiscale complex nonlinear problems. The focus is on relating PAR of numerical simulations with complex nonlinear phenomena of numerics. To isolate sources of numerical uncertainties, the possible discrepancy between the chosen partial differential equation (PDE) model and the real physics and/or experimental data is set aside. The discussion is restricted to how well numerical schemes can mimic the solution behavior of the underlying PDE model for finite time steps and grid spacings. The situation is …

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Entropy Splitting For High Order Numerical Simulation Of Vortex Sound At Low Mach Numbers, B. Müller, H.C. Yee Jan 2001

Entropy Splitting For High Order Numerical Simulation Of Vortex Sound At Low Mach Numbers, B. Müller, H.C. Yee

United States National Aeronautics and Space Administration: Publications

A method of minimizing numerical errors, and improving nonlinear stability and accuracy associated with low Mach number computational aeroacoustics (CAA) is proposed. The method consists of two levels.

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Comparison Of The Enear Peculiar Velocities With The Pscz Gravity Field, A Nusser, L. N. Da Costa, E Branchini, M. Bernardi, M V. Alonso, G Wegner Jan 2001

Comparison Of The Enear Peculiar Velocities With The Pscz Gravity Field, A Nusser, L. N. Da Costa, E Branchini, M. Bernardi, M V. Alonso, G Wegner

Dartmouth Scholarship

We present a comparison between the peculiar velocity field measured from the ENEAR all-sky Dn−σ catalogue and that derived from the galaxy distribution of the IRAS Point Source Catalog Redshift Survey (PSCz). The analysis is based on a modal expansion of these data in redshift space by means of spherical harmonics and Bessel functions. The effective smoothing scale of the expansion is almost linear with redshift reaching 1500kms−1 at 3000kms−1. The general flow patterns in the filtered ENEAR and PSCz velocity fields agree well within 6000kms−1, assuming a linear biasing relation between the mass and …

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Dynamics Of Stars And Globular Clusters In M87, Aaron J. Romanowsky, C. S. Kochanek Jan 2001

Dynamics Of Stars And Globular Clusters In M87, Aaron J. Romanowsky, C. S. Kochanek

Faculty Publications

We examine the dynamics of the stars and globular clusters in the nearby giant elliptical galaxy M87 and constrain the mass distribution, using all the available data over a large range of radii, including higher order moments of the stellar line-of-sight velocity distributions and the discrete velocities of over 200 globular clusters. We introduce an extension of spherical orbit modeling methods that makes full use of all the information in the data and provides very robust constraints on the mass models. We conclusively rule out a constant mass-to-light ratio model and infer that the radial density profile of the galaxy's …

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Break Out Session: The Future Of White Dwarf Observing, Ted Von Hippel Jan 2001

Break Out Session: The Future Of White Dwarf Observing, Ted Von Hippel

Publications

On Thursday, June 15, 2000 the conference participants joined on of four afternoon break out sessions. Our break out session was charged with addressing the the following questions: Astronomy is changing. Eight to ten meter telescopes looking towards the edge of the Universe will dominate the astronomical landscape. Queue scheduling, large surveys, and the current emphasis towards the red end of the spectrum will also affect us. What consequences do these changes imply for our field? How can we convince telescope assignment committees to grand observing time to proposers from our community?

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White Dwarfs In Open Clusters: New Tests Of Stellar Evolution And The Age Of The Galaxy, Ted Von Hippel Jan 2001

White Dwarfs In Open Clusters: New Tests Of Stellar Evolution And The Age Of The Galaxy, Ted Von Hippel

Publications

White dwarf cooling theory and very deep observations in star clusters provide a new tool to test stellar evolution theory and time scales. In particular, white dwarf cooling theory is now testing the degree of enhanced core mixing in stars with turnoff ages of 1 to 2 Gyr. More generally, I show the good overall agreement between white dwarf and modern isochrone ages over the range 0.1 to 4 Gyr.

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White Dwarfs In Common Proper Motion Binary Systems: Mass Distribution And Kinematics, Nicole M. Silvestri, Terry D. Oswalt, Matt A. Wood, J. Allyn Smith, I. Neill Reid, Edward M. Sion Jan 2001

White Dwarfs In Common Proper Motion Binary Systems: Mass Distribution And Kinematics, Nicole M. Silvestri, Terry D. Oswalt, Matt A. Wood, J. Allyn Smith, I. Neill Reid, Edward M. Sion

Publications

We present the mass distribution, gravitational redshifts, radial velocities, and space motions of white dwarf stars in common proper motion binary systems. The mass distribution we derive for the 41 DA white dwarfs in this study has a mean of 0.68 ± 0.04 MꙨ. This distribution has a slightly higher mean and larger dispersion than most previous white dwarf studies. We hypothesize that this is due to a higher fraction of cool (average Teff ̴ 10,000 K), hence old, white dwarfs in our sample. Our results indicate that samples made up of predominantly cool, old white dwarf stars …

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Single-Pulse Characteristics Of The Millisecond Radio Pulsar Psr B1937 + 21 At 430 Mhz, F. A. Jenet, S. B. Anderson, T. A. Prince Jan 2001

Single-Pulse Characteristics Of The Millisecond Radio Pulsar Psr B1937 + 21 At 430 Mhz, F. A. Jenet, S. B. Anderson, T. A. Prince

Physics & Astronomy Faculty Publications

The single-pulse characteristics of the millisecond pulsar PSR B1937 + 21 are studied using the recently installed Caltech baseband recorder at the Arecibo Radio Observatory in Puerto Rico. This is the first such analysis of this object that includes both average intensity pulses as well as \"giant pulses.\" Pulse ensemble-averaging techniques are developed in order to study the characteristics of PSR B1937 + 21's single pulses since the high time resolution signal-to-noise ratio is less than unity. This analysis reveals that the non-giant pulse radio emission is extremely stable. All observed fluctuations are consistent with diffractive interstellar scattering. Such intrinsic …

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Probing The Strong Boundary Shape Dependence Of The Casimir Force, Thorsten Emig, Andreas Hanke, Ramin Golestanian, Mehran Kardar Jan 2001

Probing The Strong Boundary Shape Dependence Of The Casimir Force, Thorsten Emig, Andreas Hanke, Ramin Golestanian, Mehran Kardar

Physics & Astronomy Faculty Publications

We study the geometry dependence of the Casimir energy for deformed metal plates by a path integral quantization of the electromagnetic field. For the first time, we give a complete analytical result for the deformation induced change in Casimir energy δE in an experimentally testable, nontrivial geometry, consisting of a flat and a corrugated plate. Our results show an interesting crossover for δE as a function of the ratio of the mean plate distance H, to the corrugation length λ: For λ≪H we find a slower decay ∼H−4, compared to the H−5 behavior predicted by the commonly used pairwise summation …

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Polymer Depletion Interaction Between Two Parallel Repulsive Walls, F. Schlesener, Andreas Hanke, R. Kimpel, S. Dietrich Jan 2001

Polymer Depletion Interaction Between Two Parallel Repulsive Walls, F. Schlesener, Andreas Hanke, R. Kimpel, S. Dietrich

Physics & Astronomy Faculty Publications

The depletion interaction between two parallel repulsive walls confining a dilute solution of long and flexible polymer chains is studied by field-theoretic methods. Special attention is paid to self-avoidance between chain monomers relevant for polymers in a good solvent. Our direct approach avoids the mapping of the actual polymer chains on effective hard or soft spheres. We compare our results with recent Monte Carlo simulations [A. Milchev and K. Binder, Eur. Phys. J. B 3, 477 (1998)] and with experimental results for the depletion interaction between a spherical colloidal particle and a planar wall in a dilute solution of nonionic …

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Modified Critical Correlations Close To Modulated And Rough Surfaces, Andreas Hanke, Mehran Kardar Jan 2001

Modified Critical Correlations Close To Modulated And Rough Surfaces, Andreas Hanke, Mehran Kardar

Physics & Astronomy Faculty Publications

Correlation functions are sensitive to the presence of a boundary. Surface modulations give rise to modified near surface correlations, which can be measured by scattering probes. To determine these correlations, we develop a perturbative calculation in deformations in height from a flat surface. The results, combined with a renormalization group around four dimensions, are also used to predict critical behavior near a self-affine rough surface. We find that a large enough roughness exponent can modify surface critical behavior.

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Chemical Transferability Of Single- And Multiple-Scattering Exafs Debye-Waller Factors, Nicholas Dimakis, Grant Bunker Jan 2001

Chemical Transferability Of Single- And Multiple-Scattering Exafs Debye-Waller Factors, Nicholas Dimakis, Grant Bunker

Physics & Astronomy Faculty Publications

Single- and multiple-scattering EXAFS Debye-Waller factors are amplitude reduction parameters that appear in the EXAFS x(k) equation accounting for the structural and thermal disorder of a given sample. These parameters must be known accurately in order to obtain quantitative agreement between theory and experiment. Since experimental data can only support a limited number of fitted parameters these factors must be known from another source. Although various approaches have been considered in the past with a variety of results, the self-consistent ab initio Density functional theory stands for the most accurate and reliable method regardless of molecular symmetry or other specific …

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Apex Version 2.0: Latest Version Of The Cross-Platform Analysis Program For Exafs, Nicholas Dimakis, Grant Bunker Jan 2001

Apex Version 2.0: Latest Version Of The Cross-Platform Analysis Program For Exafs, Nicholas Dimakis, Grant Bunker

Physics & Astronomy Faculty Publications

This report describes recent progress on APEX, a free, open source, cross platform set of EXAFS data analysis software. In a previous report we described APEX 1.0 (Dimakis, N. and Bunker, G., 1999), a free and open source code suite of basic X-Ray Absorption Fine Structure (XAFS) data analysis programs for classical data reduction and single scattering analysis. The first version of APEX was the only cross platform (linux/irix/windows/MacOS) EXAFS analysis program to our knowledge, but it lacked important features like multiple scattering fitting, generic format conversion from ASCII to University of Washington (UW) binary-type files, and user friendly interactive …

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