Showing papers by "Argonne National Laboratory published in 1986"

Layered magnetic structures: Evidence for antiferromagnetic coupling of Fe layers across Cr interlayers.

Abstract: We investigated exchange coupling of Fe layers across Au and Cr interlayers by means of light scattering from spin waves. For Au interlayers we find a continuous decrease of this coupling to zero as the Au thickness is increased from 0 to \ensuremath{\cong}20 \AA{}. For Cr interlayers of proper thickness we find antiferromagnetic coupling of the Fe layers. In small external fields such double layers order antiparallel with their magnetization perpendicular to the external field, in analogy to the spin-flop phase of antiferromagnets.

The fourth stage of the epidemiologic transition: the age of delayed degenerative diseases.

Abstract: Gains in longevity in the United States since the mid-nineteenth century occurred as a result of an epidemiologic transition: deaths from infectious diseases were replaced by deaths from degenerative diseases. Recent trends in cause-specific mortality suggest a distinct new stage, one of postponement of degenerative diseases. Projections based on these data must be applied cautiously; their implication for health and social policies are likely to be profound.

Focusing of electric fields in the active site of Cu‐Zn superoxide dismutase: Effects of ionic strength and amino‐acid modification

Abstract: In this paper we report the implementation of a finite-difference algorithm which solves the linearized Poisson-Boltzmann equation for molecules of arbitrary shape and charge distribution and which includes the screening effects of electrolytes. The microcoding of the algorithm on an ST-100 array processor allows us to obtain electrostatic potential maps in and around a protein, including the effects of ionic strength, in about 30 minutes. We have applied the algorithm to a dimer of the protein Cu-Zn superoxide dismutase (SOD) and compared our results to those obtained from uniform dielectric models based on coulombic potentials. We find that both the shape of the protein-solvent boundary and the ionic strength of the solvent have a profound effect on the potentials in the solvent. For the case of SOD, the cluster of positive charge at the bottom of the active site channel produces a strongly enhanced positive potential due to the focusing of field lines in the channel—a result that cannot be obtained with any uniform dielectric model. The remainder of the protein is surrounded by a weak negative potential. The electrostatic potential of the enzyme seems designed to provide a large cross-sectional area for productive collisions. Based on the ionic strength dependence of the size of the positive potential region emanating from the active site and the repulsive negative potential barrier surrounding the protein, we are able to suggest an explanation for the ionic strength dependence of the activity of the native and chemically modified forms of the enzyme.

Thermodynamic analysis of ordered liquid solutions by a modified quasichemical approach—Application to silicate slags

Abstract: A system of semi-empirical equations has been developed for the analysis of the thermodynamic properties of ordered liquid solutions such as slags. The equations, which are based upon modifications of the quasichemical theory, take into account the concentration and temperature dependence of the solution properties of ordered systems and thus enhance the reliability of interpolations and extrapolations of data. For binary systems, these equations have been coupled with an optimization computer program to analyze simultaneously all available thermodynamic data including phase diagrams, Gibbs energies and enthalpies of formation of compounds, activities, enthalpies of mixing, entropies of fusion, miscibility gaps,etc. In this manner, data for several binary slag systems have been analyzed. In the present article, analyses for the CaO-SiO2, FeO-SiO2, and CaO-FeO systems are presented. The resulting equations represent all the binary data, including the phase diagrams, essentially within experimental error limits. The calculations have been extended to ternary systems, thereby permitting ternary thermodynamic properties to be approximated solely from data from the subsidiary binary systems. Results for the SiO2-CaO-FeO system are in excellent agreement with measured ternary data.

Structure of copper microclusters isolated in solid argon.

Abstract: Extended x-ray-absorption fine structure has been used in conjunction with the aggregation technique to investigate the structure of copper microclusters (7 to 15 \AA{} mean diameter). The crystallographic structure of the microclusters is consistent with an fcc lattice. A small contraction of the nearest-neighbor distance was measured as a function of cluster size.

Fundamental aspects of ion beam mixing

Abstract: Irradiation of solids with energetic particles results in the reorganization of constituent target atoms, i.e., ion beam mixing (IM). At low temperatures, IM is characterized by prompt (10 −10 s) diffusion processes which are localized in the vicinity of the displacement cascade. Mixing at low temperatures can cause the system to depart far from the equilibrium state. At elevated temperatures, the diffusion of radiation-induced defects extends the mixing to longer times and greater distances. These delayed IM processes tend to return the system toward equilibrium. Recent experimental progress has led to a qualitative understanding of the fundamental aspects of IM in both temperature regimes. This has been achieved through systematic measurements of the influence of temperature, dose, dose-rate, cascade energy density and chemical interactions on IM. The results of these experiments will be reviewed and compared to IM models based on collisional, thermal spike and radiation-enhanced diffusion processes. The relation of IM to other fundamental radiation damage effects will also be discussed.

Sensitivity analysis for variational inequalities

Abstract: Sensitivity analysis results for variational inequalities are presented which give conditions for existence and equations for calculating the derivatives of solution variables with respect to perturbation parameters. The perturbations are of both the variational inequality function and the feasible region. Results for the special case of nonlinear complementarity are also presented. A numerical example demonstrates the results for variational inequalities.

Nonlinear Fracture Properties from Size Effect Tests

Abstract: The previously derived size effect law for blunt fracture is exploited for determining the parameters of the R-curve of the crack band model, and of Hillerborg’s fictitious crack model. No measurements of the crack length or of the unloading compliance are needed. It suffices to measure only the maximum load values for a set of geometrically similar specimens of different sizes. The parameters of the size effect law can then be identified by linear regression. The inverse slope of the regression line yields the fracture energy. The regression also has a twofold benefit: it smoothes statistically scattered data, and it extends the range of the data, so that one can do with fewer tests. From the experimentally calibrated size effect law, the R-curve may then be obtained as the envelope of the family of fracture equilibrium curves for different specimen sizes. A simple algebraic formula for this envelope is presented. The size effect regression plot makes it also possible to determine crack band model parameters, particularly the fracture energy, the crack band width, and the strait-softening modulus. The same is made possible for Hillerborg’s model.

Job-shop scheduling using automated reasoning: a case study of the car-sequencing problem

Abstract: The ‘job-shop scheduling problem’ is known to be NP-complete. The version of interest in this paper concerns an assembly line designed to produce various cars, each of which requires a (possibly different) set of options. The combinatorics of the problem preclude seeking a maximal solution. Nevertheless, because of the underlying economic considerations, an approach that yields a ‘good’ sequence of cars, given the specific required options for each, would be most valuable. In this paper, we focus on an environment for seeking, studying, and evaluating approaches for yielding good sequences. The environment we discuss relies on the automated reasoning program ITP. Automated reasoning programs of this type offer a wide variety of ways to reason, strategies for controlling the reasoning, and auxiliary procedures that contribute to the effective study of problems of this kind. We view the study presented in this paper as a prototype of what can be accomplished with the assistance of an automated reasoning program.

Mapping of serpentinites in the Eastern Desert of Egypt by using Landsat thematic mapper data

Abstract: Serpentinites in the Eastern Desert of Egypt were mapped from Landsat thematic mapper (TM) data by using procedures that take advantage of the distinctive spectral reflectance of these rocks caused by the abundance of antigorite, lizardite, clinochrysotile, and magnetite. The method employs a threshold classifier based on three reflectance ratios: (1) band 5/7 for estimating the abundance of hydroxyl-bearing phases, (2) band 5/1 for magnetite content, and (3) the calculated value of reflectance for band 4, based on a linear interpolation between bands 3 and 5, divided by the observed band-4 reflectance. The third ratio was used to identify rocks in iron-bearing aluminosilicates and thereby to distinguish mafic rocks containing substantial amounts of magnetite and hydroxyl-bearing phases from serpentinites. The method was first successfully tested over the Meatiq dome and Wadi Ghadir areas, where serpentinites and ophiolitic melanges dominated by serpentinites have been mapped during the course of field work. A TM-based map was then generated; the map covered about 60,000 km/sup 2/ in the Eastern Desert. Results demonstrate that TM data can be used with reliability to distinguish serpentinites from surrounding rocks in arid regions and to generate detailed maps over wide regions by using quantitative, reproducible mapping criteria.more » Possibilities for locating suture zones over the less well known parts of arid continents are clear.« less

Octupole deformation in neutron-rich barium isotopes.

Abstract: Partial decay schemes for the very neutron-rich nuclei $^{142}$,144Ba and $^{146}\mathrm{Ba}$ have been determined by the study of \ensuremath{\gamma}-\ensuremath{\gamma} coincidences in $^{252}\mathrm{Cf}$ fission fragments. Interlaced positive- and negative-parity levels connected by fast electric dipole transitions are observed in $^{144}\mathrm{Ba}$ and $^{146}\mathrm{Ba}$ above spin 7\ensuremath{\Elzxh}. This is similar to the situation in some light actinide nuclei, where the data have been interpreted in terms of reflection-asymmetric shapes.

Structure of a one-component plasma in an external field: A molecular-dynamics study of particle arrangement in a heavy-ion storage ring.

Abstract: A one-component plasma has been studied by molecular dynamics calculations to simulate the behavior of charged particles in heavy-ion storage rings. The Hamiltonian used confines the plasma in the directions lateral to the direction of travel in the ring in the frame of reference which is moving with the beam. The results show an unexpected stratification of density in the lateral direction, and a tendency towards a first-neighbor coordination of 14(8+6) seems incipient. On each shell we observe a triangular pattern of particle arrangement.

Acoustic Remote Sensing

Abstract: The use of conventional instrumentation to measure mean winds, temperature, humidity, and turbulence in the atmospheric boundary layer, coupled with advances in numerical modeling methods, has led to great improvements in our understanding of this region of the atmosphere. Coincident with these advances has been the development of a variety of remote-sensing devices. Exploiting the interaction of acoustic, radio, and light waves with the atmosphere’s turbulent microstructure, these devices have provided a new means to measure and visualize the physical processes that govern the structure and evolution of the lower atmosphere. In this article, we describe the development and application of acoustic sounding methods to these studies.

On the compensation effect

Abstract: Arrhenius equation is popularly used to relate the change in the rate constant with temperature. The linearized form of Arrhenius equation has often been used to establish a relation between InA andE for a series of related reactions. Occurrence of such a compensating behavior between InA andE has been widely reported in the literature. The validity of the compensation effect has been heatedly debated. Compensation effect can arise from computational artifact or can arise from various chemical factors. This depends entirely on the data set used to establish the compensation behavior. This article stresses the importance of statistical analysis and the use of simple Arrhenius plots to draw conclusions regarding the occurrence of compensation effect. With the help of a few examples, some checkpoints to draw conclusions regarding the occurrence of true or false compensation effect are suggested.

Observation of unexpected density effects in muon-catalyzed d-t fusion.

Abstract: New experimental results on muon-catalyzed d-t fusion are reported. Unexpected target-density effects have been discovered both in dt\ensuremath{\mu} molecular formation in t\ensuremath{\mu}+${\mathrm{D}}_{2}$ collisions and in the effective sticking probability. The dt\ensuremath{\mu} formation rate is significantly enhanced at high density, presumably because of a strong three-body contribution. The origin of the observed reduction of the sticking probability at high density remains unclear. Both effects increase the number of fusions per muon that can be achieved; 150\ifmmode\pm\else\textpm\fi{}4(stat.)\ifmmode\pm\else\textpm\fi{}20(syst.) fusions per muon have been observed.

Anisotropy of transverse sound in the heavy fermion superconductor UPt3.

Abstract: We report the first measurements of the attenuation of ultrasound in the basal plane of superconducting UPt3. Transverse sound propagating along the b axis shows a marked anisotropy in its temperature dependence when the polarization is rotated in and out of the basal plane. For polarization in the basal plane the attenuation varies linearly with temperature down to 35 mK and the slope scales as the square of the frequency. Our results appear to indicate the presence of an additional attenuation mechanism when compared with recent theories of anisotropic superconductors in the dirty limit.

Cumulative disorder and x-ray line broadening in multilayers

Abstract: We have measured the high-angle x-ray diffraction from crystalline Pb-amorphous Ge multilayers. The experimental data in conjunction with a model calculation for the fluctuation in amorphous-layer thickness show tha this fluctuation is larger than about 5%. A smaller thickness variation implies the existence of high-angle peaks which have not been observed in any amorphous-crystalline multilayers.