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Structural and magnetic studies of fcc Fe films with self-organized lateral modulation on striped Cu(110)-O(2x1) substrates.

Description: Fcc Fe wedges of 0-12 monolayer (ML) were grown by means of molecular beam epitaxy onto a novel substrate: flat Cu(110) with an oxygen-induced, long-range ordered striped phase, and studied in-situ with medium energy electron diffraction (MEED) and the surface magneto-optical Kerr effect (SMOKE). In contrast to Fe growth on either clean or oxygen-saturated Cu(110), the films on the striped substrates retain a layer-by-layer growth mode up to 6-7 ML and are fcc at least up to 12 ML. In addition, satellite peaks were observed on both sides of the MEED (0,0) streak, indicating a long-range-ordered lateral modulation of the Fe surface. We postulate that the Fe films grow conformally onto the original striped substrate. SMOKE studies show that these fcc Fe wedges are ferromagnetic with an easy axis along the original stripes for Fe thickness > 4ML and a remnant magnetization that increases linearly with thickness beyond 4 ML.
Date: September 21, 1998
Creator: Li, D.
Partner: UNT Libraries Government Documents Department


Description: The Savannah River Site (SRS) has conducted performance assessment (PA) calculations to determine the risk associated with closing liquid waste tanks. The PA estimates the risk associated with a number of scenarios, making various assumptions. Throughout all of these scenarios, it is assumed that the carbon-steel tank liners holding the liquid waste do not sorb the radionuclides. Tank liners have been shown to form corrosion products, such as Fe-oxyhydroxides (Wiersma and Subramanian 2002). Many corrosion products, including Fe-oxyhydroxides, at the high pH values of tank effluent, take on a very strong negative charge. Given that many radionuclides may have net positive charges, either as free ions or complexed species, it is expected that many radionuclides will sorb to corrosion products associated with tank liners. The objective of this report was to conduct a literature review to investigate whether Pu, U, Np, Am and Tc would sorb to corrosion products on tank liners after they were filled with reducing grout (cementitious material containing slag to promote reducing conditions). The approach was to evaluate radionuclides sorption literature with iron oxyhydroxide phases, such as hematite ({alpha}-Fe{sub 2}O{sub 3}), magnetite (Fe{sub 3}O{sub 4}), goethite ({alpha}-FeOOH) and ferrihydrite (Fe{sub 2}O{sub 3} {center_dot} 0.5H{sub 2}O). The primary interest was the sorption behavior under tank closure conditions where the tanks will be filled with reducing cementitious materials. Because there were no laboratory studies conducted using site specific experimental conditions, (e.g., high pH and HLW tank aqueous and solid phase chemical conditions), it was necessary to extend the literature review to lower pH studies and noncementitious conditions. Consequently, this report relied on existing lower pH trends, existing geochemical modeling, and experimental spectroscopic evidence conducted at lower pH levels. The scope did not include evaluating the appropriateness of K{sub d} values for the Fe-oxyhydroxides, but instead to evaluate whether ...
Date: February 29, 2012
Creator: Li, D. & Kaplan, D.
Partner: UNT Libraries Government Documents Department

Research and Development of H Ion Source and LEBT for a Kaon-neutrino Factory

Description: A baseline H{sup -} ion source and low energy beam transport system (LEBT) have been identified for Project X. The filament-discharge H{sup -} ion source has been fabricated by D-Pace, Inc. and is now in operation at LBNL. The source is capable of delivering over 10mA of H{sup -} beam in cw operation with normalized 4rms emittances less than 0.7 {pi} mm mrad. A two-solenoid magnetic lens LEBT system has been design. The design has been validated with simulations of beam transport for 5 mA 30 keV H- beams using various simulation codes.
Date: November 23, 2011
Creator: Ji, Q.; Staples, J.; Schenkel, T. & Li, D.
Partner: UNT Libraries Government Documents Department


Description: Naturally occurring mobile colloids are ubiquitous and are involved in many important processes in the subsurface zone. For example, colloid generation and subsequent mobilization represent a possible mechanism for the transport of contaminants including radionuclides in the subsurface environments. For colloid-facilitated transport to be significant, three criteria must be met: (1) colloids must be generated; (2) contaminants must associate with the colloids preferentially to the immobile solid phase (aquifer); and (3) colloids must be transported through the groundwater or in subsurface environments - once these colloids start moving they become 'mobile colloids'. Although some experimental investigations of particle release in natural porous media have been conducted, the detailed mechanisms of release and re-deposition of colloidal particles within natural porous media are poorly understood. Even though this vector of transport is known, the extent of its importance is not known yet. Colloid-facilitated transport of trace radionuclides has been observed in the field, thus demonstrating a possible radiological risk associated with the colloids. The objective of this study was to determine if cementitious leachate would promote the in situ mobilization of natural colloidal particles from a SRS sandy sediment. The intent was to determine whether cementitious surface or subsurface structure would create plumes that could produce conditions conducive to sediment dispersion and mobile colloid generation. Column studies were conducted and the cation chemistries of influents and effluents were analyzed by ICP-OES, while the mobilized colloids were characterized using XRD, SEM, EDX, PSD and Zeta potential. The mobilization mechanisms of colloids in a SRS sandy sediment by cement leachates were studied.
Date: September 20, 2011
Creator: Li, D.; Roberts, K.; Kaplan, D. & Seaman, J.
Partner: UNT Libraries Government Documents Department

RF, Thermal and Structural Analysis of the 201.25 MHz MuonIonization Cooling Cavity

Description: A finite element analysis has been carried out to characterize the RF, thermal and structural behavior of the prototype 201.25 MHz cavity for a muon ionization cooling channel. A single ANSYS model has been developed to perform all of the calculations in a multi-step process. The high-gradient closed-cell cavity is currently being fabricated for the MICE (international Muon Ionization Cooling Experiment) and MUCOOL experiments. The 1200 mm diameter cavity is constructed of 6 mm thick copper sheet and incorporates a rounded pillbox-like profile with an open beam iris terminated by 420 mm diameter, 0.38 mm thick curved beryllium foils. Tuning is accomplished through elastic deformation of the cavity, and cooling is provided by external water passages. Details of the analysis methodology will be presented including a description of the ANSYS macro that computes the heat loads from the RF solution and applies them directly to the thermal model. The process and results of a calculation to determine the resulting frequency shift due to thermal and structural distortion of the cavity will also be presented.
Date: May 10, 2005
Creator: Virostek, S. & Li, D.
Partner: UNT Libraries Government Documents Department

Recent progress on photonic band gap accelerator cavities

Description: We report on the current status of our program to apply Photonic Band Gap (PBG) concepts to produce novel high-energy, high-intensity accelerator cavities. The PBG design on which we have concentrated our initial efforts consists of a square array of metal cylinders, terminated by conducting or superconducting sheets, and surrounded by microwave absorber on the periphery of the structure. A removed cylinder from the center of the array constitutes a site defect where a localized electromagnetic mode can occur. In previous work, we have proposed that this structure could be utilized as an accelerator cavity, with advantageous properties over conventional cavity designs. In the present work, we present further studies, including MAFIA-based numerical calculations and experimental measurements, demonstrating the feasibility of using the proposed structure in a real accelerator application.
Date: February 1997
Creator: Smith, D. R.; Li, D. & Vier, D. C.
Partner: UNT Libraries Government Documents Department

Magnetic quantum well states in ultrathin film and wedge structures

Description: Magnetic quantum-well (QW) states are probed with angle- and spin-resolved photoemission to address critical issues pertaining to the origin of the giant magnetoresistance (GMR) optimization and oscillatory coupling of magnetic multilayers. Two epitaxial systems are highlighted: Cu/Co(wedge)/Cu(100) and Cr/Fe(100)-whisker. The confinement of Cu sp-QW states by a Co barrier requires a characteristic Co thickness of 2.2 {+-} 0.6 {angstrom}, which is consistent with the interfacial Co thickness reported to optimize the GMR of permalloy-Cu structures. The controversial k-space origin of the 18-{angstrom} long period oscillation in Fe/Cr multilayers is identified by the vector that spans the d-derived lens feature of the Cr Fermi surface, based on the emergence of QW states with 17 {+-} 2 {angstrom} periodicity in this region.
Date: April 1996
Creator: Li, D. & Bader, S. D.
Partner: UNT Libraries Government Documents Department

Particle dynamics design aspects for an IFMIF D{sup +} RFQ

Description: A conceptual design activity for an International Fusion Material Irradiation Facility (IFMIF) has been started to investigate the feasibility of an intense D-Li neutron source. As injector of the acceleration system, a RFQ is required to accept, bunch and accelerate a 125 mA D{sup +}-beam to 8 MeV with a very good beam quality for low losses in the following main accelerator part. To fulfill these severe requirements, extensive numerical calculations of the particle dynamics in the RFQ have been performed, with special emphasis on the equipartitioning design strategy, in which the temperatures in the transverse and longitudinal directions are balanced to prevent possible coupling resonances caused by the strong non-linear space charge forces. Design aspects and the resulting beam behaviors are presented.
Date: October 1, 1996
Creator: Li, D.; Deitinghoff, H.; Klein, H. & Jameson, R.A.
Partner: UNT Libraries Government Documents Department

Growth and magnetic properties of ultrathin Fe on Pd(110).

Description: We have investigated the growth and magnetic properties of 0-3 ML (monolayer) Fe on stepped Pd(110) with reflection high-energy electron diffraction (RHEED), low-energy electron diffraction (LEED) and the surface magneto-optic Kerr effect (SMOKE) in order to relate the morphology, structure and magnetic properties in a low-dimensional system. The Fe films, grown at 340 K, are smooth and pseudomorphic up to 1.5 ML, where three-dimensional growth and lateral lattice relaxation ensues. The in-plane row spacing along the [110] decreases by {approximately}5-6 % at 3 ML. RHEED oscillations with l-ML period are observed in the (1,0), (2,0) and the center of the (0,0) streak intensity. The tail of the (0,0) streak at low exit angle, however, has a 0.5-ML period oscillation, which suggests step decoration growth. Submonolayer Fe films remain ferromagnetic above {approximately}0.3 ML. The magnetic easy axis is initially perpendicular to the surface and is in-plane for Fe thickness >1.5 ML. Between 0.9-1.2 ML, there appear to be mixed magnetic phases as indicated by an increase in coercivity.
Date: November 1, 2000
Creator: Cuenya, B. R.; Pearson, J.; Yu, C.; Li, D. & Bader, S. D.
Partner: UNT Libraries Government Documents Department

Molecular Architectural Approach to Novel Electro-Optical Materials

Description: This is the final report of a three-year, Laboratory Directed Research and Development (LDRD) project at Los Alamos National Laboratory (LANL). The goal is to construct polar multilayers with nonlinear optical coefficients larger than classical inorganic crystals such as KDP or quartz. The strategy is to use various chemical interactions such as covalent bonds or hydrogen bonding to build polar structures. We have synthesized novel barbituric acid and melamine derivatives that will spontaneously self-assemble into a supramolecular ribbon according to their complementary H-bond motif. This supramolecular ribbon can then stack into a polar multilayer structure as verified by sum frequency generation (w{sub 1}+w{sub 2}) or second harmonic generation (when w{sub 1}=w{sub 2}). Second harmonic generation yields a value of d{sub 33}=3.2 pm/V for the self-assembled films and sum frequency generation shows a net polar orientation of the methyl groups in the multilayer along the surface normal. X-ray diffraction confirms the layered structure and produces the periodicity of {approximately}41 A, which corresponds well to the width of the supramolecular ribbons ({approximately}40 A).
Date: June 29, 1999
Creator: Li, D.; Johal, M.S.; Smilowitz, L.B. & Robinson, J.M.
Partner: UNT Libraries Government Documents Department

Cyclodextrin-based surface acoustic wave chemical microsensors

Description: Cyclodextrin thin films were fabricated using either self-assembled monolayer (SAM) or solgel techniques. The resulting host receptor thin films on the substrates of surface acoustic wave (SAW) resonators were studied as method of tracking organic toxins in vapor phase. The mass loading of surface-attached host monolayers on SAW resonators gave frequency shifts corresponding to typical monolayer surface coverages for SAM methods and ``multilayer`` coverages for sol-gel techniques. Subsequent exposure of the coated SAW resonators to organic vapors at various concentrations, typically 5,000 parts per millions (ppm) down to 100 parts per billions (ppb) by mole, gave responses indicating middle-ppb-sensitivity ({approximately}50 ppb) for those sensor-host-receptors and organic-toxin pairs with optimum mutual matching of polarity, size, and structural properties.
Date: July 1, 1996
Creator: Li, D.Q.; Shi, J.X.; Springer, K. & Swanson, B.I.
Partner: UNT Libraries Government Documents Department

Growth and characterization of epitaxial fcc Fe wedges on diamond (100).

Description: Epitaxial Fe wedges with a thickness gradation from 0--20 {angstrom} were grown on diamond(100) at room temperature, subsequently annealed, and investigated with reflection high-energy electron diffraction and the surface magneto-optical Kerr effect. The results indicate that for <5 monolayer thicknesses the Fe grows on C(100) as smooth, epitaxial fcc films, which are not ferromagnetic, but that thicker films undergo a transition to become rough and the ordinary bcc ferromagnetic phase.
Date: December 5, 1997
Creator: Bader, S. D.; Keavneu, D. J.; Keune, W.; Li, D. & Pearson, J.
Partner: UNT Libraries Government Documents Department

Incorporation of iron cations into epitaxial sapphire thin films by co-evaporation and subsequent thermal annealing

Description: Iron-doped sapphire thin films have been successfully epitaxially grown onto sapphire single crystal substrates by electron beam deposition and subsequent thermal annealing. Amorphous Al{sub 2}O{sub 3} thin films, about 280--390 nm thick, cation doped with iron have been deposited on [0001] oriented sapphire substrates. Iron doping with cation concentrations (a ratio of Fe content to total cation content) up to 5 at % can be incorporated into the octahedral sites of Al-cation sublattice during the epitaxial regrowth process at 1,000--1,400 C, as determined by Rutherford Backscattering Spectrometry and ion channeling measurements. Cross-sectional Transmission Electron Microscopy shows the presence of two distinct regions in the annealed films. One exhibits the epitaxial relationship with the sapphire substrate and the second region has amorphous type of contrast. External optical transmittance measurements in the ultra violet and visible light range have exhibited the absorption associated with Fe{sup 3+}. This study has demonstrated a simple method of incorporating dopants into single crystal sapphire, which has potential in the fabrications of thin film planar optical waveguides.
Date: May 1, 1994
Creator: Yu, N.; Kung, H.; Nastasi, M. & Li, D.
Partner: UNT Libraries Government Documents Department

Recent RF Results from the MuCool Test Area

Description: The MuCool Experiment has been continuing to take data with805 and 201 MHz cavities in the MuCool Test Area. The system uses rfpower sources from the Fermilab Linac. Although the experimental programisprimarily aimed at the Muon Ionization Cooling Experiment (MICE), wehave been studying the dependence of rf limits on frequency, cavitymaterial, high magnetic fields, gas pressure, coatings, etc. with thegeneral aim of understanding the basic mechanisms involved. The 201 MHzcavity, essentially a prototype for the MICE experiment, was made usingcleaning techniques similar to those employed for superconductingcavities and operates at its design field with very littleconditioning.
Date: June 18, 2007
Creator: Norem, J.; Bross, A.; Moretti, A.; Qian, Z.; Huang, D.; Torun,Y. et al.
Partner: UNT Libraries Government Documents Department

Progress on a Be Cavity Design

Description: Previous RF experiments with normal-conducting cavities have demonstrated that there is a significant degradation in maximum gradient when the cavity is subjected to a strong axial magnetic field. We have developed a model suggesting that a cavity with beryllium walls may perform better than copper cavities. In this paper we outline the issues that led us to propose fabricating a Be-wall cavity. We also discuss a concept for fabricating such a cavity and mention some of the manufacturing issues we expect to face.
Date: December 24, 2010
Creator: Li, D.; Palmer, R.; Stratakis, D.; Virostek, S. & Zisman, Michael S.
Partner: UNT Libraries Government Documents Department

Critical Path-Based Thread Placement for NUMA Systems

Description: Multicore multiprocessors use a Non Uniform Memory Architecture (NUMA) to improve their scalability. However, NUMA introduces performance penalties due to remote memory accesses. Without efficiently managing data layout and thread mapping to cores, scientific applications, even if they are optimized for NUMA, may suffer performance loss. In this paper, we present algorithms and a runtime system that optimize the execution of OpenMP applications on NUMA architectures. By collecting information from hardware counters, the runtime system directs thread placement and reduces performance penalties by minimizing the critical path of OpenMP parallel regions. The runtime system uses a scalable algorithm that derives placement decisions with negligible overhead. We evaluate our algorithms and runtime system with four NPB applications implemented in OpenMP. On average the algorithms achieve between 8.13% and 25.68% performance improvement compared to the default Linux thread placement scheme. The algorithms miss the optimal thread placement in only 8.9% of the cases.
Date: November 1, 2011
Creator: Su, C Y; Li, D; Nikolopoulos, D S; Grove, M; Cameron, K & de Supinski, B R
Partner: UNT Libraries Government Documents Department

Normal Conducting RF Cavity for MICE

Description: Normal conducting RF cavities must be used for the cooling section of the international Muon Ionization Cooling Experiment (MICE), currently under construction at Rutherford Appleton Laboratory (RAL) in the UK. Eight 201-MHz cavities are needed for the MICE cooling section; fabrication of the first five cavities is complete. We report the cavity fabrication status including cavity design, fabrication techniques and preliminary low power RF measurements.
Date: May 23, 2010
Creator: Li, D.; DeMello, A.; Virostek, S.; Zisman, M. & Summers, D.
Partner: UNT Libraries Government Documents Department