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Articles 33331 - 33360 of 36553

Full-Text Articles in Physical Sciences and Mathematics

Radiation Risk Predictions For Space Station Freedom Orbits, Francis A. Cucinotta, William Atwell, Mark Weyland, Alva C. Hardy, John W. Wilson, Lawrence W. Townsend, Judy L. Shinn, Robert Katz Jan 1991

Radiation Risk Predictions For Space Station Freedom Orbits, Francis A. Cucinotta, William Atwell, Mark Weyland, Alva C. Hardy, John W. Wilson, Lawrence W. Townsend, Judy L. Shinn, Robert Katz

Robert Katz Publications

Risk-assessment calculations are presented for the preliminary proposed solar minimum and solar maximum orbits for Space Station Freedom (SSF). Integral linear energy transfer (LET) fluence spectra are calculated for the trapped-proton and galactic cosmic ray (GCR) environments. Organ-dose calculations are discussed using the Computerized Anatomical Man model. The cellular track model of Katz is applied to calculate cell survival, transformation, and mutation rates for various aluminum shields. Comparisons between relative biological effectiveness (RBE) and quality factors (QF) for SSF orbits are made, and fluence-dependent effects are discussed.

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On The Normalized Yield (Events/Rad/Dalton) Of Biological Molecules Irradiated With Energetic Heavy Ions, Robert Katz Jan 1991

On The Normalized Yield (Events/Rad/Dalton) Of Biological Molecules Irradiated With Energetic Heavy Ions, Robert Katz

Robert Katz Publications

When irradiated with electrons or γ-rays, a number of biological molecules display an approximately linear relationship between the reciprocal of the D37 dose and molecular weight. This is conventionally plotted as a relation between target molecular weight and true molecular weight, where the target molecular weight Mt is given as: Mt × (D37) = α (1) where α is approximately 1010 Da-rad/eV of the energy assumed to be “deposited in the target” in a single “hit” for expression of the end-point. We emphasize that D37 is expressed in rads for we will later …

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Cross Sections For Single And Double Strand Breaks In Sv-40 Virus In Eo Buffer After Heavy Ion Irradiation: Experiment And Theory, Robert Katz, S. Wesely Jan 1991

Cross Sections For Single And Double Strand Breaks In Sv-40 Virus In Eo Buffer After Heavy Ion Irradiation: Experiment And Theory, Robert Katz, S. Wesely

Robert Katz Publications

Measured cross sections after heavy ion bombardment, for both single and double strand breaks of SV-40 virus in EO buffer (which emphasizes indirect effects), are consistent with the theory of Butts and Katz for 1-hit detectors.

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Cluster As Solitons On The Nuclear Surface, Andrei Ludu Jan 1991

Cluster As Solitons On The Nuclear Surface, Andrei Ludu

Andrei Ludu

No abstract provided.

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The Slater–Pauling Curve: First Principles Calculations Of The Moments Of Fe1 − C Ni C And V1 − C Fe C, Duane Johnson, F. Pinski, J. Staunton Jan 1991

The Slater–Pauling Curve: First Principles Calculations Of The Moments Of Fe1 − C Ni C And V1 − C Fe C, Duane Johnson, F. Pinski, J. Staunton

Duane D. Johnson

We have performed calculations of the electronic structure of the random substitutional alloys Fe1−c Ni c and V1−c Fe c using the spin‐polarized, self‐consistent Korringa–Kohn–Rostoker coherent‐potential approximation (KKR‐CPA) method. This is a first principles method based on spin density functionaltheory and a local spin density approximation for the exchange and correlation functional. For fcc Fe1−c Ni c , a range of volumes were considered for 0.25

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Novel Ce Magnetism In Cedipnictide And Di‐Ce Pnictide Structures, Paul C. Canfield, J. D. Thompson, Z. Fisk Jan 1991

Novel Ce Magnetism In Cedipnictide And Di‐Ce Pnictide Structures, Paul C. Canfield, J. D. Thompson, Z. Fisk

Paul C. Canfield

Results of electrical resistance and magnetic susceptibility measurements on Ce2Bi, Ce2Sb, CeScGe, CeScSi and CeSb2 are presented. Ce2Bi and Ce2Sb have antiferromagnetictransitions at low temperatures, while CeSb2, CeScGe and CeScSi have ferromagnetictransitions, CeScGe having a T c = 46 K. The data are analyzed with respect to the similarities of the two crystal structure groups that these materials fall into: CeSb2 having the LaSb2structure and the other materials all having the La2Sb structure.

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Magnetism And Heavy Fermion‐Like Behavior In The Rbipt Series, Paul C. Canfield, J. D. Thompson, W. P. Beyermann, A. Lacerda, M. F. Hundley, E. Peterson, Z. Fisk, H. R. Ott Jan 1991

Magnetism And Heavy Fermion‐Like Behavior In The Rbipt Series, Paul C. Canfield, J. D. Thompson, W. P. Beyermann, A. Lacerda, M. F. Hundley, E. Peterson, Z. Fisk, H. R. Ott

Paul C. Canfield

Members of the RBiPt (R=Ce–Lu with the exceptions of Pm and Eu) series have been grown as single crystals.Magnetic susceptibility and electrical resistance have been measured on all members of the series, and specific heatmeasurements have been performed on representatives. The high temperature resistance uniformly changes from that of a small‐gap semiconductor or semimetal seen in NdBiPt to that of a heavy‐fermion metal seen in YbBiPt, which shows a linear coefficient of specific heat at low temperatures of 8 J/K2 mole. Further, the lighter rare earth members show an unusually sharp increase in their resistance associated with antiferromagnetic ordering at …

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Ion Irradiation Effects On Graphite With Scanning Tunneling Microscope, T. -C. Shen, R. T. Brockenbrough, J. R. Tucker, J. W. Lyding Jan 1991

Ion Irradiation Effects On Graphite With Scanning Tunneling Microscope, T. -C. Shen, R. T. Brockenbrough, J. R. Tucker, J. W. Lyding

T. -C. Shen

Scanning tunneling microscope is used to create local surface modifications by means of ion impact damage. Graphite has been used as a test case to demonstrate this local surface sputtering. Using 0.1-µs voltage pulse of - 30 to - 140 V applied to the sample in a rough vacuum of 10-2 Torr, a confined area of damage (typically about 100 Å in diameter) is usually obtained. The damaged area consists of several layers of terraces. Defects of the size of a few atoms can also be found. Electronic perturbations caused by defects can form superlattices with a spacing three times …

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Longitudinal Nmr Relaxation Of Degenerate 2-D 3he In 4he Films, Robert Hallock, D. T. Sprague, N. Alikacem Jan 1991

Longitudinal Nmr Relaxation Of Degenerate 2-D 3he In 4he Films, Robert Hallock, D. T. Sprague, N. Alikacem

Robert Hallock

The NMR spin-lattice relaxation time T1 of a degenerate submonolayer coverage of 3He in a thin adsorbed 4He film has been measured. As a function of 4He coverage, T1 exhibits a maximum and undergoes a change in temperature dependence at a coverage near that of the superfluid transition in the underlying 4He film. Two relaxation mechanisms contribute to the observed T1.

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Linear Tc Depression In Mg Doped Yba2cu3o7-X, Robert Hallock, W. M. Tiernan Jan 1991

Linear Tc Depression In Mg Doped Yba2cu3o7-X, Robert Hallock, W. M. Tiernan

Robert Hallock

We report resistivity-versus-temperature measurements performed on polycrystalline pellets of YBa2(Cu1-xMgx)3O7-δ made using the polymer-metal-complex precursor method. Tc was found to decrease linearly with increasing Mg doping in the range x=8.6×10-4 to 8.6×10-3 with slope -1400 K/x; the normal-state resistivity and the superconducting transition width showed little or no change. Mg was found to have a maximum solubility in YBa2(Cu1-xMgx)3O7-δ of x~=9×10-3.

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Nonequilibrium Processes In Polymers Undergoing Interchange Reactions. Part 2: Reaction-Diffusion Processes, Alejandro Garcia, J. Pojman, D. Kondepudi, C. Van De Broeck Jan 1991

Nonequilibrium Processes In Polymers Undergoing Interchange Reactions. Part 2: Reaction-Diffusion Processes, Alejandro Garcia, J. Pojman, D. Kondepudi, C. Van De Broeck

Faculty Publications

A reactiodffusion system of polymers undergoing interchange reactions is studied. The equation that describes the dynamics of the system is similar to the Boltzmann equation for a gas of hard spheres. We consider a one-dimensionsl system in which the average length and the concentrations at the boundaries are fixed. The resulting steady states are obtained analytically and with numerical integration of equations obtained by using a local equilibrium approximation. Monte Carlo simulations of experimentally realizable conditions were performed and compared. The results reveal a nonlinear distribution of molecular concentration and mass. The entropy of the polymer distributions is calculated as …

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Systematic Variation Of Transport And Thermodynamic Properties With Degree Of Reduction In Nd1.85ce0.15cuo4-, Nathanael A. Fortune, K. Murata, M. Ishibashi, Y. Yokoyama, Y. Nishihara Jan 1991

Systematic Variation Of Transport And Thermodynamic Properties With Degree Of Reduction In Nd1.85ce0.15cuo4-, Nathanael A. Fortune, K. Murata, M. Ishibashi, Y. Yokoyama, Y. Nishihara

Physics: Faculty Publications

Superconductivity only occurs in Nd1.85Ce0.15CuO4- after heat treatment in a reducing at- mosphere; yet we find the oxygen loss during reduction to be remarkably small. To better understand the physical role of reduction, we have systematically varied the degree of reduction in a series of ceramic samples by varying the partial pressures of oxygen and argon in the reducing atmosphere, keeping the total pressure constant. We find that small increases in the degree of reduction strongly increase the apparent carrier density and the superconducting Meissner fraction. We also find that there is an optimal degree of reduction to produce a …

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Nonequilibrium Processes In Polymers Undergoing Interchange Reactions. Part 2: Reaction-Diffusion Processes, Alejandro Garcia, J. Pojman, D. Kondepudi, C. Van De Broeck Jan 1991

Nonequilibrium Processes In Polymers Undergoing Interchange Reactions. Part 2: Reaction-Diffusion Processes, Alejandro Garcia, J. Pojman, D. Kondepudi, C. Van De Broeck

Alejandro Garcia

A reactiodffusion system of polymers undergoing interchange reactions is studied. The equation that describes the dynamics of the system is similar to the Boltzmann equation for a gas of hard spheres. We consider a one-dimensionsl system in which the average length and the concentrations at the boundaries are fixed. The resulting steady states are obtained analytically and with numerical integration of equations obtained by using a local equilibrium approximation. Monte Carlo simulations of experimentally realizable conditions were performed and compared. The results reveal a nonlinear distribution of molecular concentration and mass. The entropy of the polymer distributions is calculated as …

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Nonlinear-Interaction Of A Detonation Vorticity Wave, D. G. Lasseigne, T. L. Jackson, M. Y. Hussaini Jan 1991

Nonlinear-Interaction Of A Detonation Vorticity Wave, D. G. Lasseigne, T. L. Jackson, M. Y. Hussaini

Mathematics & Statistics Faculty Publications

The interaction of an oblique, overdriven detonation wave with a vorticity disturbance is investigated by a direct two-dimensional numerical simulation using a multidomain, finite-difference solution of the compressible Euler equations. The results are compared to those of linear theory, which predict that the effect of exothermicity on the interaction is relatively small except possibly near a critical angle where linear theory no longer holds. It is found that the steady-state computational results whenever obtained in this study agree with the results of linear theory. However, for cases with incident angle near the critical angle, moderate disturbance amplitudes, and/or sudden transient …

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Effects Of Sample Orientation On The Corrosion Of Zinc In Ammonium Sulfate And Sodium Hydroxide Solutions, D. Abayarathna, Edward Boyd Hale, Thomas J. O'Keefe, Y. M. Wang, D. Radovic Jan 1991

Effects Of Sample Orientation On The Corrosion Of Zinc In Ammonium Sulfate And Sodium Hydroxide Solutions, D. Abayarathna, Edward Boyd Hale, Thomas J. O'Keefe, Y. M. Wang, D. Radovic

Physics Faculty Research & Creative Works

The corrosion and electrochemical properties of three zinc single crystal surfaces with different orientations have been investigated. In near-neutral 1 M (NH4)2SO4, the corrosion rates on all three surfaces were found to be similar. However, the SEM morphologies of the corresponding corroded surfaces were markedly different from each other, but consistent with preferential attack of {1120} surfaces. In alkaline 0.5 N NaOH solution, the three sample orientations showed significantly different reactivities, with the (1120) surface exhibiting the highest reactivity and corrosion rate. Here, passivation ultimately occurred and the difference in the corrosion performance of …

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Raman Scattering And Lattice-Dynamical Calculations Of Alkali-Metal Sulfates, D. Liu, H. M. Lu, John R. Hardy, F. G. Ullman Jan 1991

Raman Scattering And Lattice-Dynamical Calculations Of Alkali-Metal Sulfates, D. Liu, H. M. Lu, John R. Hardy, F. G. Ullman

John R. Hardy Papers

Raman-scattering measurements on single crystals of K2SO4, Rb2SO4, and Cs2SO4 have been made at both room and liquid-nitrogen temperatures. Lattice-dynamical calculations, based on a rigid-ion model using the Gordon-Kim method to calculate the short-range potentials, were performed. The influence of the alkali-metal ions on the lattice-dynamical properties of the crystals is discussed.

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First-Principles Study Of Phase Transitions In Kno3, H. M. Lu, John R. Hardy Jan 1991

First-Principles Study Of Phase Transitions In Kno3, H. M. Lu, John R. Hardy

John R. Hardy Papers

We report a first-principles simulation study of phase transitions in KNO3, using our recently developed method for treating ionic molecular solids. With the interionic potentials calculated from ab initio electron charge densities of the ions, our structural static relaxation gave close fits to both the normal room-temperature (α-phase) and the ferroelectric (γ-phase) structures. Our supercell molecular-dynamics calculations closely simulated the transitions from the α phase and γ phase to the high-temperature disordered β phase, and successfully reproduced the abnormally large c-axis thermal expansion observed in experiment. Both transitions were found to be initiated by the rotations of the …

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First-Principles Study Of The Lattice Dynamics Of K2So4, D. Liu, H. M. Lu, F. G. Ullman, John R. Hardy Jan 1991

First-Principles Study Of The Lattice Dynamics Of K2So4, D. Liu, H. M. Lu, F. G. Ullman, John R. Hardy

John R. Hardy Papers

Using a newly developed first-principles approach to simulations of ionic molecular crystals, we performed static relaxation, molecular-dynamics simulation, and lattice-dynamics calculations, and measurements of the Raman spectrum, for the Pnam structure of K2SO4. It was found that the structure does not have the zone-center instability present in isomorphous K2SeO42 found in an earlier study. This difference between the two systems is attributed to the different charge distributions in the molecular ions. The calculated Raman-active zone-center frequencies for the Pnam structure of K2SO4 were found to be in general agreement with …

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Optical Property Measurements In Turbid Media Using Frequency Domain Photon Migration, Bruce J. Tromberg, Lars O. Svaasand, Tsong-Tseh Tsay, Richard C. Haskell, Michael W. Berns Jan 1991

Optical Property Measurements In Turbid Media Using Frequency Domain Photon Migration, Bruce J. Tromberg, Lars O. Svaasand, Tsong-Tseh Tsay, Richard C. Haskell, Michael W. Berns

All HMC Faculty Publications and Research

In frequency domain photon migration (FDPM), amplitude-modulated light is launched into a turbid medium, e.g. tissue, which results in the propagation of density waves of diffuse photons. Variations in the optical properties of the medium perturb the phase velocity and amplitude of the diffusing waves. These parameters can be determined by measuring the phase delay and demodulation amplitude of the waves with respect to the source. More specifically, the damped spherical wave solutions to the homogeneous form of the diffusion equation yield expressions for phase (φ) and demodulation (m) as a function of source distance, modulation frequency, absorption coefficient (β), …

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Modulated, Frequency-Locked, And Chaotic Cross-Waves, William B. Underhill, Seth Lichter, Andrew J. Bernoff Jan 1991

Modulated, Frequency-Locked, And Chaotic Cross-Waves, William B. Underhill, Seth Lichter, Andrew J. Bernoff

All HMC Faculty Publications and Research

Measurements were made of the wave height of periodic, quasi-periodic, and chaotic parametrically forced cross-waves in a long rectangular channel. In general, three frequencies (and their harmonics) may be observed: the subharmonic frequency and two slow temporal modulations — a one-mode instability associated with streamwise variation and a sloshing motion associated with spanwise variation. Their interaction, as forcing frequency, f, and forcing amplitude, a, were varied, produced a pattern of Arnold tongues in which two or three frequencies were locked. The overall picture of frequency-locked and -unlocked regions is explained in terms of the Arnold tongues predicted by …

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Cellular Track Model Of Biological Damage To Mammalian Cell Cultures From Galactic Cosmic Rays, Francis A. Cucinotta, Robert Katz, John W. Wilson, Lawrence W. Townsend, John E. Nealy, Judy L. Shinn Jan 1991

Cellular Track Model Of Biological Damage To Mammalian Cell Cultures From Galactic Cosmic Rays, Francis A. Cucinotta, Robert Katz, John W. Wilson, Lawrence W. Townsend, John E. Nealy, Judy L. Shinn

Robert Katz Publications

The quality factor (QF) as defined in International Commission on Radiological Protection report no. 26 (ICRP 26, ref. 1) or in International Commission on Radiation Units and Measurements report no. 40 (ICRU 40, ref. 2) is not expected to be a valid method for assessing the biological risk for deep space missions where the high-energy heavy ion (HZE) particles of the galactic cosmic rays (GCR) are of major concern. No human data for cancer induction from the HZE particles exist, and information on biological effectiveness is expected to be taken from experiments with animals and cultured cells (ref. 3). Experiments …

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Radial Distribution Of Dose, Robert Katz, Matesh N. Varma Jan 1991

Radial Distribution Of Dose, Robert Katz, Matesh N. Varma

Robert Katz Publications

The radial distribution of dose about the path of a heavy ion, principally from delta rays, is one of the central contributions of atomic physics to the systematization of high LET radiation effects in condensed matter, whether the detection arises in chemical, physical, or biological systems. In addition to the radial distribution of dose, we require knowledge of the response of the system to X-rays or gamma-rays or to beams of energetic electrons such that the electron slowing-down spectra from these radiations can approximate the slowing-down spectra from delta rays even at different radial distances from the ion's path. A …

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Double Tuned Cosine Coil For Nmr Imaging/Microscopy, Roger M. Hawk, Rao P. Gullapalli, Dikoma P. Shungu Jan 1991

Double Tuned Cosine Coil For Nmr Imaging/Microscopy, Roger M. Hawk, Rao P. Gullapalli, Dikoma P. Shungu

Journal of the Arkansas Academy of Science

The fabrication of a cosine coil having unevenly distributed struts is detailed. Placing the struts in such a manner enables a standing wave at the desired frequency and, hence, only one resonance frequency is obtained. This study details the fabrication of a cosine coil tuned to the LJ-7 frequency (77.76 MHz) and then double-tuned to the H-1 frequency (200.1 MHz) when operated at 4.7 Tesla. Double-tuning is attained by placing an LC trap in series with a capacitor used to single-tune the coil. Also, a method is suggested by which a cosine coil can be broadbanded in the lower frequency …

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The Venus Ultraviolet Aurora: A Soft Electron Source, Jane L. Fox, A. I. F. Stewart Jan 1991

The Venus Ultraviolet Aurora: A Soft Electron Source, Jane L. Fox, A. I. F. Stewart

Physics Faculty Publications

The Pioneer Venus Orbiter Ultraviolet Spectrometer has recorded continuous but variable emissions of atomic oxygen at 1304 and 1356 Å in images of the nightside of Venus. We show that the observed intensities are consistent with the presence of precipitation of soft electrons into the nightside thermosphere. Model calculations are presented in which upper and lower limits to the magnitude of the electron flux necessary to produce the observed intensities are derived. Constraints are imposed on the energy spectrum of the electrons by the measured ion densities and by the predicted intensities of other emissions that have not been detected.

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Microscopic Irreversibility And Gibbs Entropy, John D. Ramshaw Jan 1991

Microscopic Irreversibility And Gibbs Entropy, John D. Ramshaw

Physics Faculty Publications and Presentations

In a recent paper of the same title [J. Non-Equilib. Thermodyn., 15 (1990), 151], Liboff observed that the fine-grained Gibbs entropy of a canonical Hamiltonian system remains constant in time even for Hamiltonians that are not even in momenta and consequently violate time-reversal invariance (TRI). Here we extend this observation to non-canonical Hamiltonian systems, including systems with singular Poisson tensors and pseudo-Hamiltonian systems that violate the Jacobi identity. Necessary and sufficient conditions are given for the Gibbs entropy to be constant in such systems. The concept of TRI is not in general meaningful for such systems, but it is shown …

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The Quantum Magneto Effect Of Bismuth, Omar Kerem Durdag Jan 1991

The Quantum Magneto Effect Of Bismuth, Omar Kerem Durdag

Honors Theses, 1963-2015

Using a standard field modulation technique, the de Haas-van Alphen effect was measured in bismuth for various crystal orientations. Measurements performed using a d.c. magnetic field that was ramped up from -0.05 T to -0.6 T at 4.2 K with a w.01 T modulating field. Data shows a variation of dHvA period with orientation that is in good agreement with the results of Bhargava'.

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Modeling Radon Transport And Entry, Michael John Bergmann Jan 1991

Modeling Radon Transport And Entry, Michael John Bergmann

Honors Theses, 1963-2015

Techniques and devices for measuring the emanation power and permeabdity of soil samples are developed. A Monte Carlo simulation is used to model the transport of radon to 112 basements in Minnesota. Correlation between this model and the measured indoor radon concentration is compared to other leading models' correlations. This model has been found to have the strongest correlation.

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A Comparison Of Pressure Induced And Concentration Induced Smectic A-Nematic-Isotropic Triple Points, J. L. Walker, Peter J. Collings Jan 1991

A Comparison Of Pressure Induced And Concentration Induced Smectic A-Nematic-Isotropic Triple Points, J. L. Walker, Peter J. Collings

Physics & Astronomy Faculty Works

No abstract provided.

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Transient Heat Transfer, Eric Roth Jan 1991

Transient Heat Transfer, Eric Roth

Dissertations and Theses

With the advent of the new high Tc superconductors, liquid nitrogen will be one of the preferred cryogens used to cool these materials. Consequently, a more thorough understanding of the heat transfer characteristics of liquid nitrogen is required. In our investigations we examine the transient heating characteristics of liquid nitrogen to states of nucleate and film boiling under different liquid flow conditions. Using a platinum hot wire technique, it is verified that there is a premature transition to film boiling in the transient case at power levels significantly lower than under steady state nucleate boiling conditions. It is also shown …

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Onset Of Convection For Autocatalytic Reaction Fronts: Laterally Unbounded System, Boyd F. Edwards, J. W. Wilder, K. Showalter Jan 1991

Onset Of Convection For Autocatalytic Reaction Fronts: Laterally Unbounded System, Boyd F. Edwards, J. W. Wilder, K. Showalter

All Physics Faculty Publications

The linear stability of exothermic autocatalytic reaction fronts that convert unreacted fluid into a lighter reacted fluid is considered using the viscous thermohydrodynamic equations. For upward front propagation and a thin front, the discontinuous jump in density at the front is reminiscent of the Rayleigh-Taylor problem of an interface between two immiscible fluids, whereas the vertical thermal gradient near the front is reminiscent of the Rayleigh-Bénard problem of a fluid layer heated from below. The problem is also similar to flame propagation, except that here the front propagation speed is limited by catalyst diffusion rather than by activation kinetics. For …

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