Bioresource and Agricultural Engineering Commons^™

The Mineralogic Transformation Of Ferrihydrite Induced By Heterogeneous Reaction With Bioreduced Anthraquinone Disulfonate (Aqds) And The Role Of Phosphate, John M. Zachara, Ravi K. Kukkadapu, Tanya Peretyazhko, Mark Bowden, Chongmin Wang, David Kennedy, Dean Moore, Bruce Arey

United States Department of Energy: Publications

Bioreduced anthraquinone-2,6-disulfonate (AH2DS; dihydro-anthraquinone) was reacted with a 2-line, Si-substituted ferrihydrite under anoxic conditions at neutral pH in PIPES buffer. Phosphate (P) and bicarbonate (C); common adsorptive oxyanions and media/buffer components known to effect ferrihydrite mineralization; and Fe(II)_aq (as a catalytic mineralization agent) were used in comparative experiments. Heterogeneous AH2DS oxidation coupled with Fe(III) reduction occurred within 0.13–1 day, with mineralogic transformation occurring thereafter. The product suite included lepidocrocite, goethite, and/or magnetite, with proportions varing with reductant:oxidant ratio (r:o) and the presence of P or C. Lepidocrocite was the primary product at low r:o in the absence of P …

Interfacial Reactivity Of Radionuclides: Emerging Paradigms From Molecular-Level Observations, A.R. Felmy, E.S. Ilton, K.M. Rosso, John M. Zachara

United States Department of Energy: Publications

Over the past few decades an increasing array of molecular-level analytical probes has provided new detailed insight into mineral and radionuclide interfacial reactivity in subsurface environments. This capability has not only helped change the way mineral surface reactivity is studied but also how field scale contaminant migration problems are addressed and ultimately resolved. Here we review relatively new interfacial reactivity paradigms and assess their implications for future research directions. Specific examples include understanding the following: the role of site-to-site electron conduction at mineral surfaces and through bulk mineral phases and the effects of local chemical environment on the stability of …

Structure Of A Bacterial Cell Surface Decaheme Electron Conduit, Thomas A. Clarke, Marcus Edwards, Andrew J. Gates, Andrea Hall, Gaye F. White, Justin Bradley, Catherine L. Reardon, Liang Shi, Alexander Beliaev, Matthew Marshall, Zheming Wang, Nicholas Watmough, James K. Fredrickson, John M. Zachara, Julea Butt, David J. Richardson

United States Department of Energy: Publications

Some bacterial species are able to utilize extracellular mineral forms of iron and manganese as respiratory electron acceptors. In Shewanella oneidensis this involves decaheme cytochromes that are located on the bacterial cell surface at the termini of transouter- membrane electron transfer conduits. The cell surface cytochromes can potentially play multiple roles in mediating electron transfer directly to insoluble electron sinks, catalyzing electron exchange with flavin electron shuttles or participating in extracellular intercytochrome electron exchange along “nanowire” appendages. We present a 3.2-Å crystal structure of one of these decaheme cytochromes, MtrF, that allows the spatial organization of the 10 hemes to …

Sop For Pathway Inference In Integrated Microbial Genomes (Img), Iain Anderson, Amy Chen, Victor Markowitz, Nikos Kyrpides, Natalia Ivanova

United States Department of Energy: Publications

One of the most important aspects of genomic analysis is the prediction of which pathways, both metabolic and non-metabolic, are present in an organism. In IMG, this is carried out by the assignment of IMG terms, which are organized into IMG pathways. Based on manual and automatic assignment of IMG terms, the presence or absence of IMG pathways is automatically inferred. The three categories of pathway assertion are asserted (likely present), not asserted (likely absent), and unknown. In the unknown category, at least one term necessary for the pathway is missing, but an ortholog in another organism has the corresponding …

Complete Genome Sequence Of The Cellulolytic Thermophile Clostridium Thermocellum Dsm1313, Lawrence Feinberg, Justine Foden, Trisha Barrett, Karen Walston Davenport, David Bruce, Chris Detter, Roxanne Tapia, Cliff Han, Alla Lapidus, Susan Lucas, Jan-Fang Cheng, Samuel Pitluck, Tanja Woyke, Natalia Ivanova, Natalia Mikhailova, Miriam Land, Loren Hauser, D. Aaron Argyros, Lynne Goodwin, David Hogsett, Nicky Caiazza

United States Department of Energy: Publications

Clostridium thermocellum DSM1313 is a thermophilic, anaerobic bacterium with some of the highest rates of cellulose hydrolysis reported. The complete genome sequence reveals a suite of carbohydrate-active enzymes and demonstrates a level of diversity at the species level distinguishing it from the type strain ATCC 27405.

Img/M: The Integrated Metagenome Data Management And Comparative Analysis System, Victor M. Markowitz, I-Min A. Chen, Ken Chu, Ernest Szeto, Krishna Palaniappan, Yuri Grechkin, Anna Ratner, Biju Jacob, Amrita Pati, Marcel Huntemann, Konstantinos Liolios, Ioanna Pagani, Iain Anderson, Konstantinos Mavromatis, Natalia N. Ivanova, Nikos C. Kyrpides

United States Department of Energy: Publications

The integrated microbial genomes and metagenomes (IMG/M) system provides support for comparative analysis of microbial community aggregate genomes (metagenomes) in a comprehensive integrated context. IMG/M integrates metagenome data sets with isolate microbial genomes from the IMG system. IMG/M’s data content and analytical capabilities have been extended through regular updates since its first release in 2007. IMG/M is available at http://img.jgi.doe.gov/m. A companion IMG/M systems provide support for annotation and expert review of unpublished metagenomic data sets (IMG/M ER: http://img.jgi.doe.gov/mer).

Complete Genome Sequence Of Lactobacillus Buchneri Nrrl B-30929, A Novel Strain From A Commercial Ethanol Plant, Siqing Liu, Timothy D. Leathers, Alex Copeland, Olga Chertkov, Lynne Goodwin, David A. Mills

United States Department of Energy: Publications

Lactobacillus buchneri strain NRRL B-30929 was a contaminant obtained from a commercial ethanol fermentation. This facultative anaerobe is unique because of its rapid growth on xylose and simultaneous fermentation of xylose and glucose. The strain utilizes a broad range of carbohydrate substrates and possesses a high tolerance to ethanol and other stresses, making it an attractive candidate for bioconversion of biomass substrates to various bioproducts. The genome sequence of NRRL B-30929 will provide insight into the unique properties of this lactic acid bacterium.

Vista Region Viewer (Rviewer)—A Computational System For Prioritizing Genomic Intervals For Biomedical Studies, Igor Lukashin, Pavel Novichkov, Dario Boffelli, Alex R. Paciorkowski, Simon Minovitsky, Song Yang, Inna Dubchak

United States Department of Energy: Publications

Summary:
Current genome browsers are designed for linear browsing of individual genomic regions, but the high-throughput nature of experiments aiming to elucidate the genetic component of human disease makes it very important to develop user-friendly tools for comparing several genomic regions in parallel and prioritizing them based on their functional content. We introduce VISTA Region Viewer (RViewer), an interactive online tool that allows for efficient screening and prioritization of regions of the human genome for follow-up studies. The tool takes as input genetic variation data from different biomedical studies, determines a number of various functional parameters for both coding …

Complete Genome Sequence Of The Plant Growth-Promoting Endophyte Burkholderia Phytofirmans Strain Psjn, Alexandra Weilharter, Birgit Mitter, Maria V. Shin, Patrick S. G. Chain, Jerzy Nowak, Angela Sessitsch

United States Department of Energy: Publications

Burkholderia phytofirmans PsJN^T is able to efficiently colonize the rhizosphere, root, and above-ground plant tissues of a wide variety of genetically unrelated plants, such as potatoes, canola, maize, and grapevines. Strain PsJN shows strong plant growth-promoting effects and was reported to enhance plant vigor and resistance to biotic and abiotic stresses. Here, we report the genome sequence of this strain, which indicates the presence of multiple traits relevant for endophytic colonization and plant growth promotion.

Characterization Of The Intragranular Water Regime Within Subsurface Sediments: Pore Volume, Surface Area, And Mass Transfer Limitations, Michael B. Hay, Deborah L. Stoliker, James A. Davis, John M. Zachara

United States Department of Energy: Publications

Although ‘‘intragranular’’ pore space within grain aggregates, grain fractures, and mineral surface coatings may contain a relatively small fraction of the total porosity within a porous medium, it often contains a significant fraction of the reactive surface area, and can thus strongly affect the transport of sorbing solutes. In this work, we demonstrate a batch experiment procedure using tritiated water as a high-resolution diffusive tracer to characterize the intragranular pore space. The method was tested using uranium contaminated sediments from the vadose and capillary fringe zones beneath the former 300A process ponds at the Hanford site (Washington). Sediments were contacted …

Multispecies Diffusion Models: A Study Of Uranyl Species Diffusion, Chongxuan Liu, Jianying Shang, John M. Zachara

United States Department of Energy: Publications

Rigorous numerical description of multispecies diffusion requires coupling of species, charge, and aqueous and surface complexation reactions that collectively affect diffusive fluxes. The applicability of a fully coupled diffusion model is, however, often constrained by the availability of species self-diffusion coefficients, as well as by computational complication in imposing charge conservation. In this study, several diffusion models with variable complexity in charge and species coupling were formulated and compared to describe reactive multispecies diffusion in groundwater. Diffusion of uranyl [U(VI)] species was used as an example in demonstrating the effectiveness of the models in describing multispecies diffusion. Numerical simulations found …

Transient Groundwater Chemistry Near A River: Effects On U(Vi) Transport In Laboratory Column Experiments, Jun Yin, Roy Haggerty, Deborah L. Stoliker, Douglas B. Kent, Jonathan D. Istok, Janek Greskowiak, John M. Zachara

United States Department of Energy: Publications

In the 300 Area of a U(VI)-contaminated aquifer at Hanford, Washington, USA, inorganic carbon and major cations, which have large impacts on U(VI) transport, change on an hourly and seasonal basis near the Columbia River. Batch and column experiments were conducted to investigate the factors controlling U(VI) adsorption/desorption by changing chemical conditions over time. Low alkalinity and low Ca concentrations (Columbia River water) enhanced adsorption and reduced aqueous concentrations. Conversely, high alkalinity and high Ca concentrations (Hanford groundwater) reduced adsorption and increased aqueous concentrations of U(VI). An equilibrium surface complexation model calibrated using laboratory batch experiments accounted for the decrease …

Simulating Adsorption Of U(Vi) Under Transient Groundwater Flow And Hydrochemistry: Physical Versus Chemical Nonequilibrium Model, Janek Greskowiak, Michael Hay, Henning Prommer, Chongxuan Liu, Vincent Post, Rui Ma, James Davis, Chunmiao Zheng, John M. Zachara

United States Department of Energy: Publications

Coupled intragrain diffusional mass transfer and nonlinear surface complexation processes play an important role in the transport behavior of U(VI) in contaminated aquifers. Two alternative model approaches for simulating these coupled processes were analyzed and compared: (1) the physical nonequilibrium approach that explicitly accounts for aqueous speciation and instantaneous surface complexation reactions in the intragrain regions and approximates the diffusive mass exchange between the immobile intragrain pore water and the advective pore water as multirate first-order mass transfer and (2) the chemical nonequilibrium approach that approximates the diffusion-limited intragrain surface complexation reactions by a set of multiple first-order surface complexation …

Transition Of Cellulose Crystalline Structure And Surface Morphology Of Biomass As A Function Of Ionic Liquid Pretreatment And Its Relation To Enzymatic Hydrolysis, Gang Cheng, Patanjali Varanasi, Chenlin Li, Hanbin Liu, Yuri B. Melnichenko, Blake A. Simmons, Michael S. Kent, Seema Singh

United States Department of Energy: Publications

Cellulose is inherently resistant to breakdown, and the native crystalline structure (cellulose I) of cellulose is considered to be one of themajor factors limiting its potential in terms of cost-competitive lignocellulosic biofuel production. Here we report the impact of ionic liquid pretreatment on the cellulose crystalline structure in different feedstocks, including microcrystalline cellulose (Avicel), switchgrass (Panicum virgatum), pine (Pinus radiata), and eucalyptus (Eucalyptus globulus), and its influence on cellulose hydrolysis kinetics of the resultant biomass. These feedstocks were pretreated using 1-ethyl-3-methyl imidazolium acetate ([C2mim][OAc]) at 120 and 160°C for 1, 3, 6, and 12 …

Absolute High-Resolution Se+ Photoionization Cross-Section Measurements With Rydberg-Series Analysis, D. A. Esteves, R. C. Bilodeau, N. C. Sterling, R. A. Phaneuf, A. L. D. Kilcoyne, E. C. Red, A. Aguilar

United States Department of Energy: Publications

Absolute single photoionization cross-section measurements for Se⁺ ions were performed at the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory using the photo-ion merged-beams technique. Measurements were made at a photon energy resolution of 5.5 meV from 17.75 to 21.85 eV spanning the 4s²4p³4S^o_3/2 ground-state ionization threshold and the ²P^o_3/2,²P^o_1/2,²D^o_5/2, and²D^o_3/2 metastable state thresholds. Extensive analysis of the complex resonant structure in this region identified numerous Rydberg series of resonances and …

Photoionization Measurements For The Iron Isonuclear Sequence Fe3+, Fe5+, And Fe7+, M. F. Gharaibeh, A. Aguilar, A. M. Covington, E. D. Emmons, S. W. J. Scully, R. A. Phaneuf, A. Muller, J. D. Bozek, A. L. D. Kilcoyne, A. S. Schlachter, I. Alvarez, C. Cisneros, G. Hinojosa

United States Department of Energy: Publications

Cross sections for single photoionization of Fe³⁺, Fe⁵⁺, and Fe⁷⁺ ions were measured at spectral resolutions of 0.04, 0.15, and 0.13 eV, respectively, by merging mass- and charge-selected ion beams with a beam of monochromatized synchrotron undulator radiation. The measurements span photon energy ranges beginning at the ionization thresholds and extending several tens of electron volts to include the most important resonant contributions due to 3p–nd transitions to autoionizing states. The photoion yield spectra are characterized by narrow resonances and also broad features in Fe³⁺ and Fe⁵⁺ that are believed to result …

An Approach To Crystallizing Proteins By Metal-Mediated Synthetic Symmetrization, Arthur Laganowsky, Minglei Zhao, Angela B. Soriaga, Michael R. Sawaya, Duilio Cascio, Todd O. Yeates

United States Department of Energy: Publications

Combining the concepts of synthetic symmetrization with the approach of engineering metal-binding sites, we have developed a new crystallization methodology termed metal-mediated synthetic symmetrization. In this method, pairs of histidine or cysteine mutations are introduced on the surface of target proteins, generating crystal lattice contacts or oligomeric assemblies upon coordination with metal. Metal-mediated synthetic symmetrization greatly expands the packing and oligomeric assembly possibilities of target proteins, thereby increasing the chances of growing diffraction-quality crystals. To demonstrate this method, we designed various T4 lysozyme (T4L) and maltose-binding protein (MBP) mutants and cocrystallized them with one of three metal ions: copper (Cu …

Modification Of Native Grasses For Biofuel Production May Increase Virus Susceptibility, Abbie C. Schrotenboer, Michael S. Allen, Carolyn M. Malmstrom

United States Department of Energy: Publications

Bioenergy production is driving modifications to native plant species for use as novel biofuel crops. Key aims are to increase crop growth rates and to enhance conversion efficiency by reducing biomass recalcitrance to digestion. However, selection for these biofuel-valuable traits has potential to compromise plant defenses and alter interactions with pests and pathogens. Insect-vectored plant viruses are of particular concern because perennial crops have potential to serve as virus reservoirs that influence regional disease dynamics. In this study, we examined relationships between growth rates and biomass recalcitrance in five switchgrass (Panicum virgatum) populations, ranging from near wild type …

Charge And Fluence Lifetime Measurements Of A Dc High Voltage Gaas Photogun At High Average Current, J. Grames, R. Suleiman, P. A. Adderley, J. Clark, J. Hansknecht, D. Machie, M. Poelker, M. L. Stutzman

United States Department of Energy: Publications

GaAs-based dc high voltage photoguns used at accelerators with extensive user programs must exhibit long photocathode operating lifetime. Achieving this goal represents a significant challenge for proposed high average current facilities that must operate at tens of milliamperes or more. This paper describes techniques to maintain good vacuum while delivering beam, and techniques that minimize the ill effects of ion bombardment, the dominant mechanism that reduces photocathode yield of a GaAs-based dc high voltage photogun. Experimental results presented here demonstrate enhanced lifetime at high beam currents by: (a) operating with the drive laser beam positioned away from the electrostatic center …

Focused Ion Beam Patterned Fe Thin Films: A Study By Selective Area Stokes Polarimetry And Soft X-Ray Microscopy, P. J. Cook, T. H. Shen, P. J. Grundy, M.-Y. Im, P. Fischer, S. A. Morton, A. L. D. Kilcoyne

United States Department of Energy: Publications

A combination of analytical techniques, with special emphasis on selective area Stokes polarimetry, has been used to explore the structural properties and magnetic behavior of focused ion beam patterned Fe thin films under controlled Ga⁺ ion irradiation. Ion irradiation at doses ranging from 7.7×10¹⁵ to 5.2×10¹⁶ Ga ions cm^-2 did not noticeably alter the chemical properties of the Fe, but changes to the film structure and increased coercivity were observed even after the lowest doses. Magnetic transmission x-ray microscopy provided detailed information about the magnetization reversal process occurring within a patterned area of film comprising both …

Error Analysis Of Large-Eddy Simulation Of The Turbulent Non-Premixed Sydney Bluff-Body Flame, A. M. Kempf, B. J. Geurts, J. C, Oefelein

United States Department of Energy: Publications

A computational error analysis is applied to the large-eddy simulation of the turbulent non-premixed Sydney bluff-body flame, where the error is defined with respect to experimental data. The errorlandscape approach is extended to heterogeneous compressible turbulence, which is coupled to combustion as described by a flamelet model. The Smagorinsky model formulation is used to model the unknown turbulent stresses. We introduce several measures to quantify the total simulation error and observe a striking ‘valley-structure’ in the error that arises as function of the spatial resolution and the Smagorinsky length parameter. The optimal refinement strategy that can be extracted from this …

Combining Power Plant Water Needs And Carbon Dioxide Storage Using Saline Formations: Implications For Carbon Dioxide And Water Management Policies, Peter Kobos, Malynda Cappelle, Jim Krumhansl, Thomas Dewers, Andrea Mcnemar, David Borns

United States Department of Energy: Publications

Research involving management of carbon dioxide has increased markedly over the last decade as it relates to concerns over climate change. Capturing and storing carbon dioxide (CO₂) in geological formations is one of many proposed methods to manage, and likely reduce, CO₂ emissions from burning fossil fuels in the electricity sector. Saline formations represent a vast storage resource, and the waters they contain could be managed for beneficial use. To address this issue, a methodology was developed to test the feasibility of linking coal-fired power plants, deep saline formations for CO₂ storage, and extracting and treating …

Solubility Of Fe2(Oh)3cl (Pure-Iron End-Member Of Hibbingite) In Nacl And Na2so4 Brines, Martin Nemer, Yongliang Xiong, Ahmed Ismail, Je-Hun Jang

United States Department of Energy: Publications

Pure-iron end-member hibbingite, Fe₂(OH)₃Cl(s), may be important to geological repositories in salt formations, as it may be a dominant corrosion product of steel waste canisters in an anoxic environment in Na–Cl- and Na–Mg–Cl-dominated brines. In this study, the solubility of Fe₂(OH)₃Cl(s), the pure-iron endmember of hibbingite (Fe^II, Mg)₂(OH)₃Cl(s), and Fe(OH)₂(s) in 0.04 m to 6 m NaCl brines has been determined. For the reaction

Fe₂ (OH)₃Cl(s) + 3H⁺↔ 3H₂O + 2Fe²⁺ + Cl⁻,

the …

Nonlocal Instability Analysis Of Fcc Bulk And (100) Surfaces Under Uniaxial Stretching, Geng Yun, Penghui Cao, Jonathan Zimmerman, Terry Delph, Harold Park

United States Department of Energy: Publications

The objective of this paper is to examine the instability characteristics of both a bulk FCC crystal and a (100) surface of an FCC crystal under uniaxial stretching along a h100i direction using an atomisticbased nonlocal instability criterion. By comparison to benchmark atomistic simulations, we demonstrate that for both the FCC bulk and (100) surface, about 5000–10,000 atoms are required in order to obtain an accurate converged value for the instability strain and a converged instability mode. The instability modes are fundamentally different at the surface as compared to the bulk, but in both cases a strong dependence of the …

Evidence For Multiple Modes Of Uranium Immobilization By An Anaerobic Bacterium, Allison E. Ray, John R. Bargar, Vaideeswaran Sivaswamy, Alice C. Dohnalkova, Yoshiko Fujita, Brent M. Peyton, Timothy S. Magnuson

United States Department of Energy: Publications

Microbial reduction of hexavalent uranium has been studied widely for its potential role in bioremediation and immobilization of soluble U(VI) in contaminated groundwater. More recently, some microorganisms have been examined for their role in immobilization of U(VI) via precipitation of uranyl phosphate minerals mediated by microbial phosphate release, alleviating the requirement for long-term redox control. Here, we investigated the mechanism of U(VI) removal mediated by an environmental isolate, strain UFO1, that is indigenous to the Field Research Center (FRC) in Oak Ridge, TN and has been detected in U(VI)- contaminated sediments. Changes in U(VI) speciation were examined in the presence …

Quantifying Differences In The Impact Of Variable Chemistry On Equilibrium Uranium(Vi) Adsorption Properties Of Aquifer Sediments, Deborah Stoliker, Douglas Kent, John M. Zachara

United States Department of Energy: Publications

Uranium adsorption − desorption on sediment samples collected from the Hanford 300-Area, Richland, WA varied extensively over a range of field-relevant chemical conditions, complicating assessment of possible differences in equilibrium adsorption properties. Adsorption equilibrium was achieved in 500−1000 h although dissolved uranium concentrations increased over thousands of hours owing to changes in aqueous chemical composition driven by sediment-water reactions. A nonelectrostatic surface complexation reaction, >SOH + UO₂²⁺ + 2CO₃^2- = >SOUO₂(CO₃HCO₃)^2- , provided the best fit to experimental data for each sediment sample resulting in a range of conditional …

River-Induced Flow Dynamics In Long-Screen Wells And Impact On Aqueous Samples, Vince R. Vermeul, James Mckinley, Darrell R. Newcomer, Robert D. Mackley, John M. Zachara

United States Department of Energy: Publications

Previously published field investigations and modeling studies have demonstrated the potential for sample bias associated with vertical wellbore flow in conventional monitoring wells constructed with long-screened intervals. This article builds on the existing body of literature by (1) demonstrating the utility of continuous (i.e., hourly measurements for ∼1 month) ambient wellbore flow monitoring and (2) presenting results from a field experiment where relatively large wellbore flows (up to 4 L/min) were induced by aquifer hydrodynamics associated with a fluctuating river boundary located approximately 250 m from the test well. The observed vertical wellbore flows were strongly correlated with fluctuations in …

Genome Sequence Of Victivallis Vadensis Atcc Baa-548, An Anaerobic Bacterium From The Phylum Lentisphaerae, Isolated From The Human Gastrointestinal Tract, Mark W. J. Van Passel, Ravi Kant, Airi Palva, Susan Lucas, Alex Copeland, Alla Lapidus, Tijana Glavina Del Rio, Eileen Dalin, Hope Tice, David Bruce, Lynne Goodwin, Sam Pitluck, Karen Walston Davenport, David Sims, Thomas Brettin, John Detter, Shunsheng Han, Frank Larimer, Miriam Land, Loren Hauser, Nikos C. Kyrpides, Galina Ovchinnikova, P. Paul Richardson, Willem M. De Vos, Hauke Smidt, Erwin G. Zoetendal

United States Department of Energy: Publications

Victivallis vadensis ATCC BAA-548 represents the first cultured representative from the novel phylum Lentisphaerae, a deep-branching bacterial lineage. Few cultured bacteria from this phylum are known, and V. vadensis therefore represents an important organism for evolutionary studies. V. vadensis is a strictly anaerobic sugar-fermenting isolate from the human gastrointestinal tract.

Ellipsometric Study Of Single-Crystal Γ-Inse From 1.5 To 9.2 Ev, S. G. Choi, D. E. Aspnes, A. L. Fuchser, C. Martinez-Tomas, V. Muñoz Sanjosé, D. H. Levi

United States Department of Energy: Publications

We report the component E ┴ c of the pseudodielectric-function tensor <ε(E)> = <ε₁(E)> + i< ε₂(E)> of γ-phase single-crystal InSe, obtained from 1.5 to 9.2 eV by vacuum-ultraviolet spectroscopic ellipsometry with the sample at room temperature. Overlayer artifacts were reduced as far as possible by measuring fresh surfaces prepared by cleavage. Accurate critical-point energies of observed structures were obtained by a combined method of spectral analysis.

Load-Locked Dc High Voltage Gaas Photogun With An Inverted-Geometry Ceramic Insulator, P. A. Adderley, J. Clark, J. Grames, J. Hansknecht, K. Surles-Law, D. Machie, M. Poelker, M. L. Stutzman, R. Suleiman

United States Department of Energy: Publications

A new dc high voltage spin-polarized photoelectron gun has been constructed that employs a compact inverted-geometry ceramic insulator. Photogun performance at 100 kV bias voltage is summarized.

I. INTRODUCTION

All of the nuclear physics experiments conducted at the Continuous Electron Beam Accelerator Facility (CEBAF) at the Thomas Jefferson National Accelerator Facility (Jefferson Lab) receive electron beams from a dc high voltage spin-polarized GaAs photoemission gun. Since 1995, the year the first polarized electron source was installed at CEBAF [1], there have been four different photogun designs with each new gun an improvement over its predecessor. The most recent design employs …