Publications

Abstract not provided.

Lanthanide halide alloys have recently enabled scintillating gamma ray spectrometers comparable to room-temperature semiconductors (< 3% FWHM energy resolutions at 662keV). However brittle fracture of these materials hinders the growth of large volume crystals. Efforts to improve the strength through non-lanthanide alloy substitution, while preserving scintillation, are being pursued. Isovalent alloys nominal Ce0.9Al0.1Br 3, Ce0.9Ga0.1Br3, Ce 0.9Sc0.1Br3, Ce0.9In 0.1Br3 and Ce0.8Y0.2Br3, as well as aliovalent alloys nominal (CeBr3)0.99(CdCl 2)0.01, (CeBr3)0.99(CdBr 2)0.01, (CeBr3)0.99(ZnBr 2)0.01, (CeBr3)0.99(CaBr 2)0.01, (CeBr3)0.99(SrBr 2)0.01, (CeBr3)0.99(PbBr 2)0.01, (CeBr3)0.99(ZrBr 4)0.01, (CeBr3)0.99(HfBr 4)0.01 were prepared. All of these alloys exhibit bright fluorescence under UV excitation, with varying shifts in the spectral peaks and intensities relative to pure CeBr3. Further, these alloys scintillate when coupled to a photomultiplier tube (PMT) and exposed to 137Cs gamma rays. These data and the potential for improved crystal growth will be discussed.

The extended Finite Element Method (X-FEM) is a finite-element based discretization technique developed originally to model dynamic crack propagation [1]. Since that time the method has been used for modeling physics ranging from static meso-scale material failure to dendrite growth. Here we adapt the recent advances of Vitali and Benson [2] and Song et. al. [3] to model dynamic loading of a polycry stalline material. We use demonstration problems to examine the method's efficacy for modeling the dynamic response of polycrystalline materials at the meso-scale. Specifically, we use the X-FEM to model grain boundaries. This approach allows us to i) eliminate ad-hoc mixture rules for multi-material elements and ii) avoid explicitly meshing grain boundaries. © 2007 American Institute of Physics.

An analytical model is presented for predicting the critical air-flow velocity at the onset of water-droplet detachment from the GDL/channel interfaces in PEM fuel cells. Our model is based on the force balance between pressure drag that tends to detach the droplet and surface tension that tends to hold the droplet in place. In the present work, we consider the flow regime in which pressure drag, which arises from inertia effects, dominates over viscous shear - this is the flow regime of interest in real-world PEM fuel cell applications, both automotive and stationary. Our analytical model predicts that the critical air-flow velocity varies inversely (to the 2/3 power) with water-droplet size. It further predicts that making the GDL surface more hydrophobic, decreasing contact-angle hysteresis, and shrinking channel height reduce the critical air-flow velocity. Model predictions are compared with experimental data available from the literature and reasonably good agreement is obtained. © The Electrochemical Society.

Today's world demands new ways of thinking about security solutions. The problem space is complex and ambiguous. Solutions must be multidimensional, incorporating not only technology, but the social, economic, political, and religious dynamics of a security intervention. A facilitator-led experiential training program was designed for our technical staff that leads them out of the box. The course design is based upon the theories of cognitive flexibility and situated cognition, and uses a socio-constructivist approach. Participants are led by a senior systems engineer/facilitator through a series of exercises in which they observe contextually relevant right way/wrong way videos, engage in critical thinking assessments about what they observed, and solve logic puzzles. Group interaction and problem-solving is emphasized. As in the real world, there is no one "right" solution. Outcomes can include a broader understanding of the threat space, creative solutions that enable survival in spite of an evolving enemy, and a deeper sense of the complex dynamics involved in any security decision. Training impact is being evaluated using a mixed qualitative/quantitative approach. Survey data combined with ethnographic interviewing techniques will determine whether or not participants have transferred their new understandings to the work environment. ©2007 IEEE.

Soda-lime glass (SLG) is a highly available low cost glass formulation commonly used in window applications. Over the past decade, there have been a number of studies which have examined the Hugoniot elastic limit (HEL) of this material resulting in a wide range of values from 3.1 to 6.0 GPa. The determination of the HEL is complicated by many factors including ramp loading due to the convex downward curvature of the Hugoniot at low pressures. Results of transmitted wave profile experiments up to 20 GPa are presented and analyzed to determine the loading and release characteristics of SLG near the HEL. Results indicate a response that is more complex than the elastic - plastic response typical of many materials, possibly explaining the wide range in initially reported HEL values. © 2007 American Institute of Physics.

Attenuating wave profiles from shock experiments on tungsten carbide powder are compared to calculations from the continuum P-λ model and a 2-D mesoscale model to gain insight into the suitability of the two models. When calibrated, both models accurately capture the Hugoniot response of the powder and the arrival times of unattenuated steady waves. Their amplitudes are more accurately given by the mesoscale model since its reshock states are above the Hugoniot as seen experimentally; the P-λ model, in contrast, reshocks along the Hugoniot. When the attenuating wave is in the range of the Hugoniot data, the models predict attenuation correctly. However, when attenuation falls below the Hugoniot data both models are somewhat inaccurate, and the material response seems to lie between the two models. The final aspect considered is the wave rise time, which is qualitatively correct for the mesoscale model but completely inaccurate for the P-λ model. © 2007 American Institute of Physics.

Material heterogeneity appears to give rise to variability in the yield behavior of ceramics and metals under shock loading conditions. The line-imaging VISAR provides a way to measure this variability, which may then be quantified by Weibull statistics or other methods. Weibull methods assign a 2-parameter representation of failure phenomena and variability. We have conducted experiments with tantalum (25 and 40 μm grains) and silicon carbide (SiC-N with 5 μm grains). The tantalum HEL variability did not depend systematically on peak stress, grain size or sample thickness, although the previously observed precursor attenuation was present. SiC-N HEL variability within a single shot was approximately half that of single-point variability in a large family of shots; these results are more consistent with sample-to-sample variation than with variability due to changing shot parameters. © 2007 American Institute of Physics.

Fracture of materials has a huge consequence in our daily life ranging from structural damage to loss of life. Understanding the mechanism of crack initiation and propagation in materials is very important. Great effort, both theoretically and experimentally, has been made to understand the nature of crack propagation in crystalline materials. However, crack propagation in disordered systems such as highly cross-linked polymers (e.g. epoxies) is less understood. Many composites such as carbon fibers have an epoxy matrix, and thus it is important to understand the epoxy properties by themselves. We study fracture in highly cross-linked polymer networks bonded to a solid surface using large-scale molecular dynamics simulations. An initial crack is created by forbidding bonds to occur on a fraction of the solid surface up to a crack tip. The time and length scales involved in this process dictate the use of coarse grained bead-spring model of the epoxy network. In order to avoid unwanted boundary effects, large systems of up to 300 000 particles are used. Stress-strain curves are determined for each system from tensile pull molecular dynamics simulations. We found that crack propagation and also formation of voids ahead of the crack are directly related to the network structure.

A growing number of applications involve the transmission of high-intensity laser pulses through optical fibers. Previously, our particular interests led to a series of studies on single-fiber transmission of Q-switched, 1064 nm pulses from multimode Nd:YAG lasers through step-index, multimode, fused silica fibers. The maximum pulse energy that could be transmitted through a given fiber was limited by the onset of laser-induced breakdown or damage. Breakdown at the fiber entrance face was often the first limiting process encountered, but other mechanisms were observed that could result in catastrophic damage at either fiber face, within the initial "entry" segment of the fiber, and at other internal sites along the fiber path. These studies examined system elements that can govern the relative importance of different damage mechanisms, including laser characteristics, the design and alignment of laser-to-flber injection optics, fiber end-face preparation, and fiber routing. In particular, criteria were established for injection optics in order to maximize margins between transmission requirements and thresholds for laser-induced damage. Recent interests have led us to examine laser injection into multiple fibers. Effective methods for generating multiple beams are available, but the resulting beam geometry can lead to challenges in applying the criteria for optimum injection optics. To illustrate these issues, we have examined a three-fiber injection system consisting of a beam-shaping element, a primary injection lens, and a grating beamsplitter. Damage threshold characteristics were established by testing fibers using the injection geometry imposed by this system design.

We have synthesized and tested new highly fluorescent metal organic framework (MOF) materials based on stilbene dicarboxylic acid as a linker. The crystal structure and porosity of the product are dependent on synthetic conditions and choice of solvent and a low-density cubic form has been identified by x-ray diffraction. In this work we report experiments demonstrating scintillation properties of these crystals. Bright proton-induced luminescence with large shifts relative to the fluorescence excitation spectra were recorded, peaking near 475 nm. Tolerance to fast proton radiation was evaluated by monitoring this radio-luminescence to absorbed doses of several hundred MRAD.

To provide input to numerical models for hazard and vulnerability analyses, thermal decomposition of eight polymers has been examined in both nitrogen and air atmospheres. Experiments have been done with poly(methyl methacrylate), poly(diallyl phthalate), Norwegian spruce, polyvinyl chloride), polycarbonate, poly(phenylene sulphide), and two polyurethanes. Polymers that formed a substantial amount of carbonaceous char during decomposition in a nitrogen atmosphere were completely consumed in an air atmosphere. However, in the case of polyurethanes, complete consumption did not occur until temperatures of 700° C or higher. Furthermore, to varying degrees, the presence of oxygen appeared to alter the decomposition processes in all of the materials studied.

The deployment of optical fibers in adverse radiation environments, such as those encountered in a low-Earth-orbit space setting, makes critical the development of an understanding of the effect of large accumulated ionizing-radiation doses on optical components and systems. In particular, gamma radiation is known to considerably affect the performance of optical components by inducing absorbing centers in the materials. Such radiation is present both as primary background radiation and as secondary radiation induced by proton collisions with space-craft material. This paper examines the effects of gamma radiation on erbium-, ytterbium-, and Yb/Er co-doped optical fibers by exposing a suite of such fibers to radiation from a Co-60 source over long periods of time while monitoring the temporal and spectral decrease in transmittance of a reference signal. For same total doses, results show increased photodarkening in erbium-doped fibers relative to ytterbium-doped fibers, as well as significant radiation resistance of the co-doped fibers over wavelengths of 1.0-1.6 microns. All three types of fibers were seen to exhibit dose-rate dependences.

A firing set capable of charging a 0.05 μF capacitor to 1.7 kV is constructed using a 2.5 mm diameter Series Connected Photovoltaic Array (SCPA) in lieu of a transformer as the method of high voltage generation. The source of illumination is a fiber coupled 3 W 808 nm laser diode. This paper discusses the performance and PSpice modeling of an SCPA used in a firing set application.

A methodology for decoupling cross-coupled fields in compact, integrated current sensor arrays is presented. The compactness of the current sensor array elements is made possible by using highly sensitive field detectors based upon Giant Magnetoresistive (GMR) technology, which offers galvanic isolation, small size (∼mm2) and high bandwidth (>1 MHz). By using known geometric relations between the conductor geometries and locations of the field detectors, cross-coupled magnetic field signals can be used to extract necessary current signals, as well as separate unknown disturbance fields. This methodology can also be used to simplify the magnetic biasing requirements of GMR field detectors, including decoupling of the temperature dependence of the biasing magnet. Moreover, the methodology also can be extended to estimate the temperature of the magnet to provide an extra temperature signal for thermal management algorithms. © 2007 IEEE.

This paper presents the conceptual framework that is being used to define quantification of margins and uncertainties (QMU) for application in the nuclear weapons (NW) work conducted at Sandia National Laboratories. The conceptual framework addresses the margins and uncertainties throughout the NW life cycle and includes the definition of terms related to QMU and to figures of merit. Potential applications of QMU consist of analyses based on physical data and on modeling and simulation. Appendix A provides general guidelines for addressing cases in which significant and relevant physical data are available for QMU analysis. Appendix B gives the specific guidance that was used to conduct QMU analyses in cycle 12 of the annual assessment process. Appendix C offers general guidelines for addressing cases in which appropriate models are available for use in QMU analysis. Appendix D contains an example that highlights the consequences of different treatments of uncertainty in model-based QMU analyses.

Since the first vector supercomputers in the mid-1970's, the largest scale applications have traditionally been floating point oriented numerical codes, which can be broadly characterized as the simulation of physics on a computer. Supercomputer architectures have evolved to meet the needs of those applications. Specifically, the computational work of the application tends to be floating point oriented, and the decomposition of the problem two or three dimensional. Today, an emerging class of critical applications may change those assumptions: they are combinatorial in nature, integer oriented, and irregular. The performance of both classes of applications is dominated by the performance of the memory system. This paper compares the memory performance sensitivity of both traditional and emerging HPC applications, and shows that the new codes are significantly more sensitive to memory latency and bandwidth than their traditional counterparts. Additionally, these codes exhibit lower base-line performance, which only exacerbates the problem. As a result, the construction of future supercomputer architectures to support these applications will most likely be different from those used to support traditional codes. Quantitatively understanding the difference between the two workloads will form the basis for future design choices. ©2007 IEEE.

Abstract not provided.

Split grating-gate field effect transistors (FETs) detectors made from high mobility quantum well two-dimensional electron gas material have been shown to exhibit greatly improved tunable resonant photoresponse compared to single grating-gate detectors due to the formation of a 'diode-like' element by the split-gate structure. These detectors are relatively large for FETs (1mm × 1mm area or larger) to match typical focused THz beam spot sizes. In the case where the focused THz spot size is smaller than the detector area, we have found evidence, through positional scanning of the detector element, that only a small portion of the detector is active. To further investigate this situation, detectors with the same channel width (1mm), but various channel lengths, were fabricated and tested. The results indicate that indeed, only a small portion of the split grating gated FET is active. This finding opens up the possibility for further enhancement of detector sensitivity by increasing the active area.

Sandia National Laboratories has developed a means of manufacturing high precision aspheric lenslet arrays turned on-center. An innovative chucking and indexing mechanism was designed and implemented which allows the part to be indexed in two orthogonal directions parallel to the spindle face. This system was designed to meet a need for center to center positioning of 2μm and form error of λ/10. The part utilizes scribed orthogonal sets of grooves that locate the part on the chuck. The averaging of the grooves increases the repeatability of the system. The part is moved an integral number of grooves across the chuck by means of a vacuum chuck on a tool post that is mated to the part and holds the part while the chuck repositions to receive the part. The current setup is designed to create as many as 169 lenslets distributed over a 3mm square area while holding a true position tolerance of 1μm for all lenslets.

Abstract not provided.

The rapid autonomous detection of pathogenic microorganisms and bioagents by field deployable platforms is critical to human health and safety. To achieve a high level of sensitivity for fluidic detection applications, we have developed a 330 MHz Love wave acoustic biosensor on 36° YX Lithium Tantalate (LTO). Each die has four delay-line detection channels, permitting simultaneous measurement of multiple analytes or for parallel detection of single analyte containing samples. Crucial to our biosensor was the development of a transducer that excites the shear horizontal (SH) mode, through optimization of the transducer, minimizing propagation losses and reducing undesirable modes. Detection was achieved by comparing the reference phase of an input signal to the phase shift from the biosensor using an integrated electronic multi-readout system connected to a laptop computer or PDA The Love wave acoustic arrays were centered at 330 MHz, shifting to 325-328 MHz after application of the silicon dioxide waveguides. The insertion loss was -6 dB with an out-of-band rejection of 35 dB. The amplitude and phase ripple were 2.5 dB p-p and 2-3° pp, respectively. Time-domain gating confirmed propagation of the SH mode while showing suppression of the triple transit. Antigen capture and mass detection experiments demonstrate a sensitivity of 7.19 ± 0.74° mm2/ ng with a detection limit of 6.7 ± 0.40 pg / mm2 for each channel.

An innovative helium3 high pressure gas detection system, made possible by utilizing Sandia's expertise in Micro-electrical Mechanical fluidic systems, is proposed which appears to have many beneficial performance characteristics with regards to making these neutron measurements in the high bremsstrahlung and electrical noise environments found in High Energy Density Physics experiments and especially on the very high noise environment generated on the fast pulsed power experiments performed here at Sandia. This same system may dramatically improve active WMD and contraband detection as well when employed with ultrafast (10-50 ns) pulsed neutron sources.

When searching for SNM simply designing a better detector to optimize the signal S from the source is not enough. It is important to know the background B to maximize S/N, where N is the noise in B. Cosmic rays are a dominant source of neutron background. It is therefore important to know their flux, angular and energy distribution. Over the last 50 years work has been done to study cosmic ray neutrons and their variation. The full hemispherical neutron flux is usually quoted at a certain altitude (e.g. Altitude = 0 meters above sea level, pressure = 1033 g/cm2) and geomagnetic rigidity (e.g. GMR = 1.2GV). Neutron fluxes at other locations are scaled from the sea level data using a well determined prescription. However, there is a lack in knowledge of the angular dependence of the neutron flux at sea level. The angular dependence is important for two reasons; first many detectors have an efficiency that changes with the direction of the incident neutron. Second none of the measurements to date have determined how the flux changes with angle, their data must be modeled to estimate the full hemispherical flux. In this paper we present the cosmic neutron background flux measured by a neutron scatter camera in the energy range 0.2-10MeV. Our measurements are in agreement with the best fit to past data. We present for the 1st time the neutron zenith angle dependence at fission energies which is observed to be a function of the form cos 2.7⊖. ©2007 IEEE.

Parallel adaptive mesh refinement methods potentially lead to realistic modeling of complex three-dimensional physical phenomena. However, they also present significant challenges in data partitioning and load balancing. As the mesh adapts to the solution, the partitioning requirements change. By explicitly considering these dynamic conditions, the scalability for large, realistic simulations could possibly be significantly improved. Our hypothesis is that adaptive partitioning, meaning dynamic and automatic switching of partitioning techniques, based on the current run-time state, can be beneficial for these simulations. However, switching partitioners can be expensive due to differences in the algorithms' native mapping of data onto processors. We suggest forcing a uniform starting point for all included partitioners. We present a penalty-based method for determining whether switching is beneficial. We study the effects on data migration, as well as on overall cost, of using the uniform starting point and the switching-penalties to select the best partitioning algorithm, among a set of graph-based and geometric partitioning algorithms, for each adaptive time-step for four different adaptive scientific applications. The results show that data migration can be significantly reduced and that adaptive partitioning indeed can be effective for unstructured adaptive applications.