Showing papers by "National Institute of Standards and Technology published in 1998"

Real-space multiple-scattering calculation and interpretation of x-ray-absorption near-edge structure

Abstract: A self-consistent real-space multiple-scattering (RSMS) approach for calculations of x-ray-absorption near-edge structure (XANES) is presented and implemented in an ab initio code applicable to arbitrary aperiodic or periodic systems This approach yields a quantitative interpretation of XANES based on simultaneous, self-consistent-field (SCF) calculations of local electronic structure and x-ray absorption spectra, which include full multiple scattering from atoms within a small cluster and the contributions of high-order MS from scatterers outside that cluster In addition, the code includes a SCF estimate of the Fermi energy and an account of orbital occupancy and charge transfer We also present a qualitative, scattering-theoretic interpretation of XANES Sample applications are presented for cubic BN, ${\mathrm{UF}}_{6},$ Pu hydrates, and distorted ${\mathrm{PbTiO}}_{3}$ Limitations and various extensions are also discussed

Electrochemical Quantitation of DNA Immobilized on Gold

Abstract: We have developed an electrochemical method to quantify the surface density of DNA immobilized on gold. The surface density of DNA, more specifically the number of nucleotide phosphate residues, is calculated from the amount of cationic redox marker measured at the electrode surface. DNA was immobilized on gold by forming mixed monolayers of thiol-derivitized, single-stranded oligonucleotide and 6-mercapto-1-hexanol. The saturated amount of charge-compensating redox marker in the DNA monolayer, determined using chronocoulometry, is directly proportional to the number of phosphate residues and thereby the surface density of DNA. This method permits quantitative determination of both single- and double-stranded DNA at electrodes. Surface densities of single-stranded DNA were precisely varied in the range of (1−10) × 1012 molecules/cm2, as determined by the electrochemical method, using mixed monolayers. We measured the hybridization efficiency of immobilized single-stranded DNA to complementary strands as a...

Radiochromic film dosimetry: Recommendations of AAPM Radiation Therapy Committee Task Group 55

Abstract: Recommendations of the American Association of Physicists in Medicine (AAPM) for the radiochromic film dosimetry are presented. These guidelines were prepared by a task group of the AAPM Radiation Therapy Committee and have been reviewed and approved by the AAPM Science Council.

Stringlike cooperative motion in a supercooled liquid

Abstract: Extensive molecular dynamics simulations are performed on a glass-forming Lennard-Jones mixture to determine the nature of the cooperative motions occurring in this model fragile liquid. We observe stringlike cooperative molecular motion (``strings'') at temperatures well above the glass transition. The mean length of the strings increases upon cooling, and the string length distribution is found to be nearly exponential.

Dynamics of component separation in a binary mixture of Bose-Einstein condensates

Abstract: We describe the first experiments that study in a controlled way the dynamics of distinguishable and interpenetrating bosonic quantum fluids. We work with a two-component system of Bose-Einstein condensates in the $|{F\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1,m}_{f}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ensuremath{-}1〉$ and $|2,1〉$ spin states of ${}^{87}\mathrm{Rb}$. The two condensates are created with complete spatial overlap, and in subsequent evolution they undergo complex relative motions that tend to preserve the total density profile. The motions quickly damp out, leaving the condensates in a steady state with a non-negligible (and adjustable) overlap region.

Using Self-Assembly To Control the Structure of DNA Monolayers on Gold: A Neutron Reflectivity Study

Abstract: Neutron reflectivity was used to determine the concentration profiles of oligomeric DNA monolayers on gold in high salt concentrations (1 M NaCl). These monolayers are of interest as models for DNA probe systems used in diagnostic devices. To facilitate its attachment, the DNA was functionalized at the 5‘ end with a thiol group connected to the oligonucleotide by a hexamethylene linker. Concentration profiles determined from neutron reflectivity indicate that adsorbed layers of single-stranded DNA (HS-ssDNA) on bare gold are compact, suggesting the presence of multiple contacts between each DNA strand and the surface. After treatment with mercaptohexanol, a short alkanethiol with a terminal hydroxy group, the DNA “stands up” and extends farther into the solvent phase. These changes are consistent with the DNA remaining attached through its thiol end group while contacts between DNA backbones and the surface are prevented by the formation of a mercaptohexanol monolayer. The end-tethered HS-ssDNA layer read...

The 30 m Small-Angle Neutron Scattering Instruments at the National Institute of Standards and Technology

Abstract: Two high-resolution, general-purpose, small-angle neutron scattering instruments have been constructed at the National Institute of Standards and Technology's Center for Neutron Research. The instruments are 30 m long and utilize mechanical velocity selectors, pinhole collimation and high-data-rate two-dimensional position-sensitive neutron detectors. The incident wavelength, wavelength resolution and effective length of the instruments are independently variable, under computer control, and provide considerable flexibility in optimizing beam intensity and resolution. The measurement range of the instruments extends from 0.0015 to 0.6 A−1 in scattering wavevector, corresponding to structure in materials from 10 A to nearly 4000 A. The design and characteristics of the instruments, and their modes of operation, are described, and data are presented which demonstrate their performance.

Spinodal Dewetting of Thin Polymer Films

Abstract: Dewetting of polystyrene films on a silicon substrate is investigated as a function of film thickness $h$. We observe the nucleation of holes in the early stage of dewetting for relatively thick films $(hg100\AA{})$, as observed previously, but the breakup of thinner films occurs through the growth of uniformly distributed surface undulations (``spinodal dewetting''). The average amplitude $\ensuremath{\delta}h$ of these undulations increases exponentially up to the film rupture point where $\ensuremath{\delta}h$ becomes comparable to $h$, as predicted by a capillary wave instability model.

Indentation Damage and Mechanical Properties of Human Enamel and Dentin

Abstract: Understanding the mechanical properties of human teeth is important to clinical tooth preparation and to the development of "tooth-like" restorative materials. Previous studies have focused on the macroscopic fracture behavior of enamel and dentin. In the present study, we performed indentation studies to understand the microfracture and deformation and the microcrackmicrostructure interactions of teeth. It was hypothesized that crack propagation would be influenced by enamel rods and the dentino-enamel junction (DEJ), and the mechanical properties would be influenced by enamel rod orientation and tooth-to-tooth variation. Twenty-eight human third molars were used for the measurement of hardness, fracture toughness, elastic modulus, and energy absorbed during indentation. We examined the effect of enamel rod orientation by propagating cracks in the occlusal surface, and in the axial section in directions parallel and perpendicular to the occlusal surface. The results showed that the cracks in the enamel a...

Experimental Issues in Coherent Quantum-State Manipulation of Trapped Atomic Ions

Abstract: Methods for, and limitations to, the generation of entangled states of trapped atomic ions are examined. As much as possible, state manipulations are described in terms of quantum logic operations since the conditional dynamics implicit in quantum logic is central to the creation of entanglement. Keeping with current interest, some experimental issues in the proposal for trappedion quantum computation by J. I. Cirac and P. Zoller (University of Innsbruck) are discussed. Several possible decoherence mechanisms are examined and what may be the more important of these are identified. Some potential applications for entangled states of trapped-ions which lie outside the immediate realm of quantum computation are also discussed.

Experimental models for validating technology

Abstract: Experimentation helps determine the effectiveness of proposed theories and methods. However, computer science has not developed a concise taxonomy of methods for demonstrating the validity of new techniques. Experimentation is a crucial part of attribute evaluation and can help determine whether methods used in accordance with some theory during product development will result in software being as effective as necessary. By looking at multiple examples of technology validation, the authors develop a taxonomy for software engineering experimentation that describes twelve different experimental approaches.

Variations in relevance judgments and the measurement of retrieval effectiveness

Abstract: Test collections have traditionally been used by information retrieval researchers to improve their retrieval strategies. To be viable as a laboratory tool, a collection must reliably rank diAerent retrieval variants according to their true eAectiveness. In particular, the relative eAectiveness of two retrieval strategies should be insensitive to modest changes in the relevant document set since individual relevance assessments are known to vary widely. The test collections developed in the TREC workshops have become the collections of choice in the retrieval research community. To verify their reliability, NIST investigated the eAect changes in the relevance assessments have on the evaluation of retrieval results. Very high correlations were found among the rankings of systems produced using diAerent relevance judgment sets. The high correlations indicate that the comparative evaluation of retrieval performance is stable despite substantial diAerences in relevance judgments, and thus reaArm the use of the TREC collections as laboratory tools. Published by Elsevier Science Ltd.

Deterministic Entanglement of Two Trapped Ions

Abstract: We have prepared the internal states of two trapped ions in both the Bell-like singlet and triplet entangled states. In contrast to all other experiments with entangled states of either massive particles or photons, we do this in a deterministic fashion, producing entangled states on demand without selection. The deterministic production of entangled states is a crucial prerequisite for large-scale quantum computation.

BoneSource™ hydroxyapatite cement: A novel biomaterial for craniofacial skeletal tissue engineering and reconstruction

Abstract: BoneSource-hydroxyapatite cement is a new self-setting calcium phosphate cement biomaterial. Its unique and innovative physical chemistry coupled with enhanced biocompatibility make it useful for craniofacial skeletal reconstruction. The general properties and clinical use guidelines are reviewed. The biomaterial and surgical applications offer insight into improved outcomes and potential new uses for hydroxyapatite cement systems.

Support Vector Machines Applied to Face Recognition

Abstract: Face recognition is a K class problem. where K is the number of known individuals; and support vector machines (SVMs) are a binary classification method. By reformulating the face recognition problem and reinterpreting the output of the SVM classifier. we developed a SVM -based face recognition algorithm. The face recognition problem is formulated as a problem in difference space. which models dissimilarities between two facial images. In difference space we formulate face recognition as a two class problem. The classes are: dissimilarities between faces of the same person. and dissimilarities between faces of different people. By modifying the interpretation of the decision surface generated by SVM. we generated a similarity metric between faces that is learned from examples of differences between faces. The SVM-based algorithm is compared with a principal component analysis (PCA) based algorithm on a difficult set of images from the FERET database. Performance was measured for both verification and identification scenarios. The identification performance for SVM is 77-78% versus 54% for PCA. For verification. the equal error rate is 7% for SVM and 13% for PCA.

Ultrasensitive detections in atomic and molecular physics: demonstration in molecular overtone spectroscopy

Abstract: We consider several highly sensitive techniques commonly used in detection of atomic and molecular absorptions. Their basic operating principles and corresponding performances are summarized and compared. We then present our latest results on the ultrasensitive detection of molecular overtone transitions to illustrate the principle and application of the cavity-enhanced frequency-modulation (FM) spectroscopy. An external cavity is used to enhance the molecular response to the light field, and an FM technique is applied for shotnoise-limited signal recovery. A perfect match between the FM sideband frequency and the cavity free spectral range makes the detection process insensitive to the laser-frequency noise relative to the cavity, and, at the same time, overcomes the cavity bandwidth limit. Working with a 1.064-mm Nd:YAG laser, we obtained sub-Doppler overtone resonances of C2HD, C2H2, and CO2 molecules. A detection sensitivity of 5 3 10 213 of

Frequency-stabilized diode laser with the Zeeman shift in an atomic vapor

Abstract: We demonstrate a robust method of stabilizing a diode laser frequency to an atomic transition. This technique employs the Zeeman shift to generate an antisymmetric signal about a Doppler-broadened atomic resonance, and therefore offers a large recapture range as well as high stability. The frequency of a 780-nm diode laser, stabilized to such a signal in Rb, drifted less than 0.5 MHz peak–peak (1 part in 109) in 38 h. This tunable frequency lock can be constructed inexpensively, requires little laser power, rarely loses lock, and can be extended to other wavelengths by use of different atomic species.

A dielectric resonator for measurements of complex permittivity of low loss dielectric materials as a function of temperature

Abstract: An application of a mode dielectric resonator is described for precise measurements of complex permittivity and the thermal effects on permittivity for isotropic dielectric materials. The Rayleigh-Ritz technique was employed to find a rigorous relationship between permittivity, resonant frequency, and the dimensions of the resonant structure, with relative computational accuracy of less than . The influence of conductor loss and its temperature dependence was taken into account in the dielectric loss tangent evaluation. Complex permittivities of several materials, including cross-linked polystyrene, polytetrafluoroethylene, and alumina, were measured in the temperature range of 300-400 K. Absolute uncertainties of relative permittivity measurements were estimated to be smaller than 0.2%, limited mainly by uncertainty in the sample dimensions. For properly chosen sample dimensions, materials with dielectric loss tangents in the range of to can be measured using the mode dielectric resonator.

Instability of Toroidal Magnetic Field in Jets and Plerions

Abstract: Astrophysical jets and pulsar-fed supernova remnants (plerions) are expected to develop highly organized magnetic structures dominated by concentric loops of toroidal field, B. It has been argued that such structures could explain the polarization properties of some jets and contribute to their lateral confinement through magnetic tension forces. A concentric toroidal field geometry is also central to the Rees-Gunn model for the Crab Nebula, the archetypal plerion, and leads to the deduction that the Crab pulsar's wind must have a weak magnetic field. Yet this kind of equilibrium between magnetic and gas pressure forces, the "equilibrium Z-pinch" of the controlled fusion literature, is well known to be susceptible to disruptive localized instabilities, even when the magnetic field is weak and/or boundary conditions (e.g., a dense external medium) slow or suppress global modes. Thus, the magnetic field structures imputed to the interiors of jets and plerions are unlikely to persist for very long. To determine the growth rates of Z-pinch instabilities under astrophysical conditions, I derive a dispersion relation that is valid for the relativistic fluids of which jets and plerions may be composed, in the ideal magnetohydrodynamics (MHD) limit. The dominant instabilities are kink (m = 1) and pinch (m = 0) modes. The former generally dominate, destroying the concentric field structure and probably driving the system toward a more chaotic state in which the mean field strength is independent of radius (and in which resistive dissipation of the field may be enhanced). I estimate the timescales over which the field structure is likely to be rearranged and relate these to distances along relativistic jets and radii from the central pulsar in a plerion. I conclude that the central tenet of the Rees-Gunn model for the Crab Nebula, the existence of a concentric toroidal field well outside the pulsar wind's termination shock, is physically unrealistic. With this assumption gone, there is no dynamical reason to conclude that the magnetic energy flux carried by the pulsar wind is much weaker than the kinetic energy flux. Abandoning the principal conclusion of Rees & Gunn would resolve a long-standing puzzle in pulsar wind theory.

Least-Squares Fitting Algorithms of the NIST Algorithm Testing System.

Abstract: This report describes algorithms for fitting certain curves and surfaces to points in three dimensions. All fits are based on orthogonal distance regression. The algorithms were developed as reference software for the National Institute of Standards and Technology's Algorithm Testing System, which has been used for 5 years by NIST and by members of the American Society of Mechanical Engineers' B89.4.10 standards committee. The Algorithm Testing System itself is described only briefly; the main part of this paper covers the general linear algebra, numerical analysis, and optimization methods it employs. Most of the fitting routines rely on the Levenberg-Marquardt optimization routine.

Optical Absorption of Insulators and the Electron-Hole Interaction: An Ab Initio Calculation

Abstract: We present a computationally efficient scheme to calculate the optical absorption of insulators from first principles, including the electron-hole interaction. Excited states of the solid are chosen to consist of single electron-hole pairs. The electron-hole interaction is statically screened using a model dielectric function. Only two pieces of input are required, the crystal structure of the material and the macroscopic dielectric constant. We apply this scheme to two wide-gap insulators, LiF and MgO, and obtain excellent agreement with experimental measurements of their UV reflectance spectra.

Measurements of Relative Phase in Two-Component Bose-Einstein Condensates

Abstract: We have measured the relative phase of two Bose-Einstein condensates (BEC) using a time-domain separated-oscillatory-field condensate interferometer. A single two-photon coupling pulse prepares the double condensate system with a well-defined relative phase; at a later time, a second pulse reads out the phase difference accumulated between the two condensates. We find that the accumulated phase difference reproduces from realization to realization of the experiment, even after the individual components have spatially separated and their relative center-of-mass motion has damped.

Growth kinetics of nanocrystalline zno particles from colloidal suspensions

Abstract: Colloidal chemistry techniques were used to synthesize ZnO particles in the nanometer size regime. The particle aging kinetics were determined by monitoring the optical band edge absorption and using the effective mass model to approximate the particle size as a function of time. We show that the growth kinetics of the ZnO particles follow the Lifshitz−Slyozov−Wagner theory for Ostwald ripening. In this model, the higher curvature and hence chemical potential of smaller particles provides a driving force for dissolution. The larger particles continue to grow by diffusion-limited transport of species dissolved in solution.

Phase-Separation-Induced Surface Patterns in Thin Polymer Blend Films

Abstract: Atomic force microscopy (AFM), neutron reflection (NR) and secondary ion mass spectroscopy (SIMS) are used to examine phase separation in symmetrically segregating thin polymer blend films (≤1000 A). Phase separation in the film leads to undulations of the liquid−air interface, provided the film is sufficiently thin to suppress surface-directed spinodal decomposition waves. Flattened droplets are formed at a very late stage of phase separation, and the aspect ratio of these droplets can be rationalized by an interfacial free energy minimization argument.

Vibrationally resolved sum-frequency generation with broad-bandwidth infrared pulses.

Abstract: We present a novel procedure for vibrationally resolved sum-frequency generation (SFG) in which a broad-bandwidth IR pulse is mixed with a narrow-bandwidth visible pulse. The resultant SFG spectrum is dispersed with a spectrograph and detected in parallel with a scientific-grade CCD detector, permitting rapid and high signal-to-noise ratio data acquisition over a 400?cm-1 spectral region without scanning the IR frequency. Application to the study of a self-assembled monolayer of octadecanethiol is discussed.

Fresh concrete: A Herschel-Bulkley material

Abstract: Some preliminary results of an experimental program on the rheological behavior of fresh concrete are presented. In the rheological tests, performed with a plane-to-plane rheometer, it appears tha the relationship between torque and rotation speed is not exactly linear. The fresh concrete behavior is better described by the Herschel-Bulkley model: $$\tau = \tau \prime _0 + a \dot \gamma ^b $$ ; τ and $$\dot \gamma $$ are the shear stress and the strain gradient applied to the specimen, respectively. τ′0,a andb are three material parameters describing the concrete behavior. Among other advantages, this new description avoids the problem of negative yield stress encountered with the Bingham model.