Access
Partner
UNT Libraries Government Documents Department
862
UNT Libraries
178
UNT Libraries Special Collections
142
Oklahoma Historical Society
121
Sterling Municipal Library
31
Carl Albert Congressional Research and Studies Center
21
San Antonio Public Library
20
National Museum of the Pacific War/Admiral Nimitz Foundation
19
Gaines County Library
9
Livingston Municipal Library
9
59 More
Collection
Office of Scientific & Technical Information Technical Reports
547
Texas Digital Newspaper Program
344
Congressional Research Service Reports
239
Oklahoma Digital Newspaper Program
147
Tocker Foundation Grant
118
Photographing Texas
86
The Greensheet
69
Multicultural Center Records
53
The Baytown Sun
31
Government Accountability Office Reports
29
111 More
Degree Department
Degree Discipline
Degree Level
Country
Year
Month
Language
1,631 Matching Results
Results open in a new window/tab.
CRD Report, October 2007
A review of various projects undertaken by Lawrence Berkeley National Laboratory's Computational Research Division.
Date:
October 30, 2006
Creator:
Wang, Ucilia
Object Type:
Report
System:
The UNT Digital Library
PASSIVITY BREAKDOWN AND EVOLUTION OF LOCALIZED CORROSION ON TYPE 316L STAINLESS STEEL
Passivity breakdown of 316L SS in the presence of aggressive Cl{sup -} and inhibitive NO{sub 3}{sup -} anions has been experimentally studied and the results have been interpreted in terms of the Point Defect Model (PDM). By introducing the competitive adsorption of Cl{sup -} and NO{sub 3}{sup -} into surface oxygen vacancies at the passive film/solution interface, the PDM yields a critical breakdown potential (V{sub c}) that is predicted to vary linearly with log[Cl{sup -}], or with log ([Cl{sup -}]/[NO{sub 3}{sup -}]) [1] when nitrate ions are present, which is shown in Fig. 1. The Point Defect Model also explains the fact that the slope of V{sub c} vs. log[Cl{sup -}] does not change in the presence of NO{sub 3}{sup -}, which is attributed to the quasi-equilibrium ejection of a cation from the barrier layer to form the vacancy pair V{sub M}V{sub O}{sup (2-{chi})} at the barrier layer/solution interface. The Point Defect Model predicts that measured V{sub c} increases linearly with the square root of voltage scan rate {nu}{sup 1/2} [1]. From this correlation, the critical, areal concentration of cation vacancies at the metal/barrier layer interface, {zeta}, has been estimated and found to be comparable to that calculated from the …
Date:
October 23, 2006
Creator:
S. Yang, G. Engelhardt, and D. D. Macdonald
Object Type:
Article
System:
The UNT Digital Library
Renewable Energy Development on Tribal Lands
Brochure describes the Tribal Energy Program, which provides American Indian tribes with financial and technical assistance for developing renewable energy projects on tribal land.
Date:
October 1, 2006
Creator:
unknown
Object Type:
Report
System:
The UNT Digital Library
Linear Scaling 3D Fragment Method for Large-Scale ElectronicStructure Calculations
We present a linear scaling 3 dimensional fragment (LS3DF)method that uses a novel decomposition and patching scheme to do abinitio density functional theory (DFT) calculations for large systems.This method cancels out the artificial boundary effects that arise fromthe spatial decomposition. As a result, the LS3DF results are essentiallythe same as the original full-system DFT results with errors smaller thanthe errors introduced by other sources of numerical approximations. Inaddition, the resulting computational times are thousands of timessmaller than conventional DFT methods, making calculations with 100,000atom systems possible. The LS3DF method is applicable to insulator andsemiconductor systems, which covers a current gap in the DOE's materialsscience code portfolio for large-scale ab initio simulations.
Date:
October 16, 2006
Creator:
Wang, Lin-Wang; Zhao, Zhengji & Meza, Juan
Object Type:
Report
System:
The UNT Digital Library
Computing Criticality of Lines in Power Systems
We propose a computationally efficient method based onnonlinear optimization to identify critical lines, failure of which cancause severe blackouts. Our method computes criticality measure for alllines at a time, as opposed to detecting a single vulnerability,providing a global view of the system. This information on criticality oflines can be used to identify multiple contingencies by selectivelyexploring multiple combinations of broken lines. The effectiveness of ourmethod is demonstrated on the IEEE 30 and 118 bus systems, where we canvery quickly detect the most critical lines in the system and identifysevere multiple contingencies.
Date:
October 13, 2006
Creator:
Pinar, Ali; Reichert, Adam & Lesieutre, Bernard
Object Type:
Article
System:
The UNT Digital Library
Design and Application of an Electronic Logbook for Space System Integration and Test Operations
In the highly technological aerospace world paper is still widely used to document space system integration and test (I&T) operations. E-Logbook is a new technology designed to substitute the most commonly used paper logbooks in space system I&T, such as the connector mate/demate logbook, the flight hardware and flight software component installation logbook, the material mix record logbook and the electronic ground support equipment validation logbook. It also includes new logbook concepts, such as the shift logbook, which optimizes management oversight and the shift hand-over process, and the configuration logbook, which instantly reports on the global I&T state of the space system before major test events or project reviews. The design of E-Logbook focuses not only on a reliable and efficient relational database, but also on an ergonomic human-computer interactive (HCI) system that can help reduce human error and improve I&T management and oversight overall. E-Logbook has been used for the I&T operation of the Gamma-ray Large Area Space Telescope (GLAST) Large Area Telescope (LAT) at the Stanford Linear Accelerator Center (SLAC). More than 41,000 records have been created for the different I&T logbooks, with no data having been corrupted or critically lost. 94% of the operators and 100% of …
Date:
October 10, 2006
Creator:
Kavelaars, Alicia T. & /SLAC /Stanford U., Dept. Aeronaut. Astronaut.
Object Type:
Report
System:
The UNT Digital Library
Final Report for OJI grant.
This document is a final report for DOE grant DE-FG02-00ER41147. The research described herein was funded in large part by this grant with additional support from the National Science Foundation. The primary focus of Averett's research effort is centered around the polarized {sup 3}He target in Hall A at Jefferson Lab. The close proximity of the College of William and Mary to Jefferson Lab has provided an outstanding opportunity to maintain a very active research program which still satisfying the demands of the college. Our research group includes four faculty, two post-doctoral fellows and eight graduate students. Averett also maintains a fully functional polarized {sup 3}e target lab at William and Mary which allows him to support the research program at Jefferson Lab while also doing research on polarized targets themselves. Since 1998, seven experiments using polarized {sup 3}He have been completed by the Jefferson Lab Hall A Polarized {sup 3}He Collaboration. Ten publications have been produced on this research and analysis of the two most recently completed experiments is underway. A description of the recent experiments and results is given below. In addition to target expertise, Averett has remained one of the most active collaborators in the data analysis …
Date:
October 13, 2006
Creator:
Averett, Todd
Object Type:
Report
System:
The UNT Digital Library
Effects of Biodiesel Blends on Vehicle Emissions: Fiscal Year 2006 Annual Operating Plan Milestone 10.4
The objective was to determine if testing entire vehicles, vs. just the engines, on a heavy-duty chassis dynamometer provides a better, measurement of the impact of B20 on emissions.
Date:
October 1, 2006
Creator:
McCormick, R. L.; Williams, A.; Ireland, J. & Hayes, R. R.
Object Type:
Report
System:
The UNT Digital Library
Exploring nanomagnetism with soft x-ray microscopy
Magnetic soft X-ray microscopy images magnetism in nanoscale systems with a spatial resolution down to 15nm provided by state-of-the-art Fresnel zone plate optics. X-ray magnetic circular dichroism (X-MCD) is used as element-specific magnetic contrast mechanism similar to photoemission electron microscopy (PEEM), however, with volume sensitivity and the ability to record the images in varying applied magnetic fields which allows to study magnetization reversal processes at fundamental length scales. Utilizing a stroboscopic pump-probe scheme one can investigate fast spin dynamics with a time resolution down to 70 ps which gives access to precessional and relaxation phenomena as well as spin torque driven domain wall dynamics in nanoscale systems. Current developments in zone plate optics aim for a spatial resolution towards 10nm and at next generation X-ray sources a time resolution in the fsec regime can be envisioned.
Date:
October 30, 2006
Creator:
Fischer, P.; Kim, D. H.; Mesler, B. L.; Chao, W.; Sakdinawat,A. E. & Anderson, E. H.
Object Type:
Article
System:
The UNT Digital Library
Microbial Impacts on the Migration of Actinides -Effects of Exudates on Adsorption-
The interaction of actinides with microorganisms has been extensively studied to elucidate migration behavior of actinides in the environments. However, the mechanisms of interaction of microorganisms and actinides are poorly understood. They have been conducting basic science on microbial accumulation of actinides in order to elucidate the environmental behavior of actinides under relevant conditions. The effect of exudates from bacteria cells on the sorption of Eu(III) and Cm(III) by Chlorella vulgaris was studied by a batch method. The pH dependence of log K{sub d} of Eu(III) and Cm(III) for cellulose, major component of C. vulgaris cell, differed from that for C. vulgaris. On the contrary, log K{sub d} of Eu(III) and Cm(III) for cellulose in the solution containing exudates from C. vulgaris cells in a 0.5% NaCl solution showed a similar pH dependence to that by C. vulgaris. These results strongly suggested that exudates affect on the sorption of Eu(III) and Cm(III) on C. vulgaris. Effect of desferrioxamine B (DFO), one of exudates to chelate the insoluble Fe(III), on the sorption of Pu(IV), Th(IV) and Eu(III) by Pseudomonas fluorescens was studied. In the presence of DFO the sorption of Pu(IV), Th(IV) and Eu(III) on the cells increased with a decrease …
Date:
October 18, 2006
Creator:
Ohnuki, T.; Ozaki, T.; Yoshida, T.; Nankawa, T.; Kozai, N.; Sakamoto, F. et al.
Object Type:
Article
System:
The UNT Digital Library
A Penalty Method to Model Particle Interactions in DNA-laden Flows
We present a hybrid fluid-particle algorithm to simulate flow and transport of DNA-laden fluids in microdevices. Relevant length scales in microfluidic systems range from characteristic channel sizes of millimeters to micron scale geometric variation (e.g., post arrays) to 10 nanometers for the length of a single rod in a bead-rod polymer representation of a biological material such as DNA. The method is based on a previous fluid-particle algorithm in which long molecules are represented as a chain of connected rods, but in which the physically unrealistic behavior of rod crossing occurred. We have extended this algorithm to include screened Coulombic forces between particles by implementing a Debye-Hueckel potential acting between rods. In the method an unsteady incompressible Newtonian fluid is discretized with a second-order finite difference method in the interior of the Cartesian grid domain; an embedded boundary volume-of-fluid formulation is used near boundaries. The bead-rod polymer model is fully coupled to the solvent through body forces representing hydrodynamic drag and stochastic thermal fluctuations. While intrapolymer interactions are modeled by a soft potential, polymer-structure interactions are treated as perfectly elastic collisions. We demonstrate this method on flow and transport of a polymer through a post array microchannel in 2D where …
Date:
October 6, 2006
Creator:
Trebotich, D; Miller, G H & Bybee, M D
Object Type:
Article
System:
The UNT Digital Library
Development Of An Acoustice Sensor For On-Line Gas Temperature Measurement In Gasifiers
This project was awarded under U.S. Department of Energy (DOE) National Energy Technology Laboratory (NETL) Program Solicitation DE-PS26-02NT41422 and specifically addresses Technical Topical Area 2 - Gasification Technologies. The project team includes Enertechnix, Inc. as the main contractor and ConocoPhillips Company as a technical partner, who also provides access to the SG Solutions Gasification Facility (formerly Wabash River Energy Limited), host for the field-testing portion of the research. The objective of this project was to adapt acoustic pyrometer technology to make it suitable for measuring gas temperature inside a coal gasifier, to develop a prototype sensor based on this technology, and to demonstrate its performance through testing on a commercial gasifier. The project was organized in three phases, each of approximately one year duration. The first phase consisted of researching a variety of sound generation and coupling approaches suitable for use with a high pressure process, evaluation of the impact of gas composition variability on the acoustic temperature measurement approach, evaluation of the impact of suspended particles and gas properties on sound attenuation, evaluation of slagging issues and development of concepts to deal with this issue, development and testing of key prototype components to allow selection of the best approaches, …
Date:
October 1, 2006
Creator:
Ariessohn, Peter & Hornung, Hans
Object Type:
Report
System:
The UNT Digital Library
Oak Ridge National Laboratory Annual Progress Report for the Power Electronics and Electric Machinery Program
The U.S. Department of Energy (DOE) and the U.S. Council for Automotive Research (composed of automakers Ford, General Motors, and DaimlerChrysler) announced in January 2002 a new cooperative research effort. Known as FreedomCAR (derived from 'Freedom' and 'Cooperative Automotive Research'), it represents DOE's commitment to developing public/private partnerships to fund high-risk, high-payoff research into advanced automotive technologies. Efficient fuel cell technology, which uses hydrogen to power automobiles without air pollution, is a very promising pathway to achieve the ultimate vision. The new partnership replaces and builds upon the Partnership for a New Generation of Vehicles initiative that ran from 1993 through 2001. The Vehicle Systems subprogram within the FreedomCAR and Vehicle Technologies Program provides support and guidance for many cutting-edge automotive and heavy truck technologies now under development. Research is focused on understanding and improving the way the various new components of tomorrow's automobiles and heavy trucks will function as a unified system to improve fuel efficiency. This work also supports the development of advanced automotive accessories and the reduction of parasitic losses (e.g., aerodynamic drag, thermal management, friction and wear, and rolling resistance). In supporting the development of hybrid propulsion systems, the Vehicle Systems subprogram has enabled the development …
Date:
October 31, 2006
Creator:
Olszewski, M.
Object Type:
Report
System:
The UNT Digital Library
Remaining Sites Verification Package for the 1607-F7, 141-M Building Septic Tank, Waste Site Reclassification Form 2006-040
The 1607-F7, 141-M Building Septic Tank waste site was a septic tank and drain field that received sanitary sewage from the former 141-M Building. Remedial action was performed in August and November 2005. The results of verification sampling demonstrate that residual contaminant concentrations support future unrestricted land uses that can be represented by a rural-residential scenario. These results also show that residual concentrations support unrestricted future use of shallow zone soil and that contaminant levels remaining in the soil are protective of groundwater and the Columbia River.
Date:
October 19, 2006
Creator:
Dittmer, L. M.
Object Type:
Report
System:
The UNT Digital Library
Designing PV Incentive Programs to Promote Performance: A Reviewof Current Practice in the U.S.
In the U.S., the increasing financial support for customer-sited photovoltaic (PV) systems provided through publicly-funded incentive programs has heightened concerns about the long-term performance of these systems. Given the barriers that customers face to ensuring that their PV systems perform well, and the responsibility that PV incentive programs bear to ensure that public funds are prudently spent, these programs should, and often do, play a critical role in addressing PV system performance. To provide a point of reference for assessing the current state of the art, and to inform program design efforts going forward, we examine the approaches to encouraging PV system performance used by 32 prominent PV incentive programs in the U.S. We identify eight general strategies or groups of related strategies that these programs have used to address factors that affect performance, and describe key implementation details. Based on this review, we then offer recommendations for how PV incentive programs can be effectively designed to mitigate potential performance issues.
Date:
October 6, 2006
Creator:
Barbose, Galen; Wiser, Ryan & Bolinger, Mark
Object Type:
Article
System:
The UNT Digital Library
Supernova Recognition using Support Vector Machines
We introduce a novel application of Support Vector Machines(SVMs) to the problem of identifying potential supernovae usingphotometric and geometric features computed from astronomical imagery.The challenges of this supervised learning application are significant:1) noisy and corrupt imagery resulting in high levels of featureuncertainty,2) features with heavy-tailed, peaked distributions,3)extremely imbalanced and overlapping positiveand negative data sets, and4) the need to reach high positive classification rates, i.e. to find allpotential supernovae, while reducing the burdensome workload of manuallyexamining false positives. High accuracy is achieved viaasign-preserving, shifted log transform applied to features with peaked,heavy-tailed distributions. The imbalanced data problem is handled byoversampling positive examples,selectively sampling misclassifiednegative examples,and iteratively training multiple SVMs for improvedsupernovarecognition on unseen test data. We present crossvalidationresults and demonstrate the impact on a largescale supernova survey thatcurrently uses the SVM decision value to rank-order 600,000 potentialsupernovae each night.
Date:
October 1, 2006
Creator:
Romano, Raquel A.; Aragon, Cecilia R. & Ding, Chris
Object Type:
Article
System:
The UNT Digital Library
Screening Program Reduced Melanoma Mortality at the Lawrence Livermore National Laboratory, 1984-1996
Worldwide incidence of cutaneous malignant melanoma has increased substantially, and no screening program has yet demonstrated reduction in mortality. We evaluated the education, self examination and targeted screening campaign at the Lawrence Livermore National Laboratory (LLNL) from its beginning in July 1984 through 1996. The thickness and crude incidence of melanoma from the years before the campaign were compared to those obtained during the 13 years of screening. Melanoma mortality during the 13-year period was based on a National Death Index search. Expected yearly deaths from melanoma among LLNL employees were calculated by using California mortality data matched by age, sex, and race/ethnicity and adjusted to exclude deaths from melanoma diagnosed before the program began or before employment at LLNL. After the program began, crude incidence of melanoma thicker than 0.75 mm decreased from 18 to 4 cases per 100,000 person-years (p = 0.02), while melanoma less than 0.75mm remained stable and in situ melanoma increased substantially. No eligible melanoma deaths occurred among LLNL employees during the screening period compared with a calculated 3.39 expected deaths (p = 0.034). Education, self examination and selective screening for melanoma at LLNL significantly decreased incidence of melanoma thicker than 0.75 mm and reduced …
Date:
October 12, 2006
Creator:
Schneider, MD, J S; II, PhD, D & MD, PhD, M
Object Type:
Article
System:
The UNT Digital Library
A micromechanical basis for partitioning the evolution of grainbridging in brittle materials
A micromechanical model is developed for grain bridging inmonolithic ceramics. Specifically, bridge formation of a single,non-equiaxed grain spanning adjacent grains is addressed. A cohesive zoneframework enables crack initiation and propagation along grainboundaries. The evolution of the bridge is investigated through avariance in both grain angle and aspect ratio. We propose that thebridging process can be partitioned into five distinct regimes ofresistance: propagate, kink, arrest, stall, and bridge. Although crackpropagation and kinking are well understood, crack arrest and subsequent"stall" have been largely overlooked. Resistance during the stall regimeexposes large volumes of microstructure to stresses well in excess of thegrain boundary strength. Bridging can occur through continued propagationor reinitiation ahead of the stalled crack tip. The driving forcerequired to reinitiate is substantially greater than the driving forcerequired to kink. In addition, the critical driving force to reinitiateis sensitive to grain aspect ratio but relatively insensitive to grainangle. The marked increase in crack resistance occurs prior to bridgeformation and provides an interpretation for the rapidly risingresistance curves which govern the strength of many brittle materials atrealistically small flaw sizes.
Date:
October 9, 2006
Creator:
Foulk, J. W., III; Cannon, R. M.; Johnson, G. C.; Klein, P. A. & Ritchie, R. O.
Object Type:
Article
System:
The UNT Digital Library
Prototype Weigh-In-Motion Performance
Oak Ridge National Laboratory (ORNL) has developed and patented methods to weigh slowly moving vehicles. We have used this technology to produce a portable weigh-in-motion system that is robust and accurate. This report documents the performance of the second-generation portable weigh-in-motion prototype (WIM Gen II). The results of three modes of weight determination are compared in this report: WIM Gen II dynamic mode, WIM Gen II stop-and-go mode, and static (parked) mode on in-ground, static scales. The WIM dynamic mode measures axle weights as the vehicle passes over the system at speeds of 3 to 7 miles per hour (1.3 to 3.1 meters/second). The WIM stop-and-go mode measures the weight of each axle of the vehicle as the axles are successively positioned on a side-by-side pair of WIM measurement pads. In both measurement modes the center of balance (CB) and the total weight are obtained by a straight-forward calculation from axle weights and axle spacings. The performance metric is measurement error (in percent), which is defined as 100 x (sample standard deviation)/(average); see Appendix A for details. We have insufficient data to show that this metric is predictive. This report details the results of weight measurements performed in May 2005 …
Date:
October 1, 2006
Creator:
Abercrombie, Robert K; Beshears, David L; Hively, Lee M; Scudiere, Matthew B & Sheldon, Frederick T
Object Type:
Report
System:
The UNT Digital Library
Accelerating DSMC data extraction.
In many direct simulation Monte Carlo (DSMC) simulations, the majority of computation time is consumed after the flowfield reaches a steady state. This situation occurs when the desired output quantities are small compared to the background fluctuations. For example, gas flows in many microelectromechanical systems (MEMS) have mean speeds more than two orders of magnitude smaller than the thermal speeds of the molecules themselves. The current solution to this problem is to collect sufficient samples to achieve the desired resolution. This can be an arduous process because the error is inversely proportional to the square root of the number of samples so we must, for example, quadruple the samples to cut the error in half. This work is intended to improve this situation by employing more advanced techniques, from fields other than solely statistics, for determining the output quantities. Our strategy centers on exploiting information neglected by current techniques, which collect moments in each cell without regard to one another, values in neighboring cells, nor their evolution in time. Unlike many previous acceleration techniques that modify the method itself, the techniques examined in this work strictly post-process so they may be applied to any DSMC code without affecting its fidelity …
Date:
October 1, 2006
Creator:
Gallis, Michail A. & Piekos, Edward Stanley
Object Type:
Report
System:
The UNT Digital Library
Study of collisons of supersymmetric top Quark in the channel stop anti-stop -> e+- mu-+ sneutrino anti-sneutrino b anti-b with the experience of D0 at the Tevatron. Callibration of the electromagnetic calorimeter at D0.
Supersymmetry is one of the most natural extensions of the Standard Model. At low energy it may consist in the Minimal Supersymmetric Standard Model which is the framework chosen to perform the search of the stop with 350 pb{sup -1} of data collected by D0 during the RunIIa period of the TeVatron. They selected the events with an electron, a muon, missing transverse energy and non-isolated tracks, signature for the stop decay in 3-body ({bar t} {yields} bl{bar {nu}}). Since no significant excess of signal is seen, the results are interpreted in terms of limit on the stop production cross-sections, in such a way that they extend the existing exclusion region in the parameter space (m{sub {bar t}},m{sub {bar {nu}}}) up to stop masses of 168 (140) GeV for sneutrino masses of 50 (94) GeV. Finally because of the crucial role of the electromagnetic calorimeter, a fine calibration was performed using Z {yields} e{sup +}e{sup -} events, which improved significantly the energy resolution.
Date:
October 1, 2006
Creator:
Mendes, Aurelien & /Marseille U., Luminy
Object Type:
Thesis or Dissertation
System:
The UNT Digital Library
Conservation assessment for the Siskiyou Mountains salamander and Scott Bar salamander in northern California.
The purpose of this conservation assessment is to summarize existing knowledge regarding the biology and ecology of the Siskiyou Mountains salamander and Scott Bar salamander, identify threats to the two species, and identify conservation considerations to aid federal management for persistence of the species. The conservation assessment will serve as the basis for a conservation strategy for the species.
Date:
October 20, 2006
Creator:
Vinikour, W. S.; LaGory, K. E.; Adduci, J. J. & Division, Environmental Science
Object Type:
Report
System:
The UNT Digital Library
Chemical Effects Head-Loss Research in Support of Generic Safety Issue 191.
This summary report describes studies conducted at Argonne National Laboratory on the potential for chemical effects on head loss across sump screens. Three different buffering solutions were used for these tests: trisodium phosphate (TSP), sodium hydroxide, and sodium tetraborate. These pH control agents used following a LOCA at a nuclear power plant show various degrees of interaction with the insulating materials Cal-Sil and NUKON. Results for Cal-Sil dissolution tests in TSP solutions, settling rate tests of calcium phosphate precipitates, and benchmark tests in chemically inactive environments are also presented. The dissolution tests were intended to identify important environmental variables governing both calcium dissolution and subsequent calcium phosphate formation over a range of simulated sump pool conditions. The results from the dissolution testing were used to inform both the head loss and settling test series. The objective of the head loss tests was to assess the head loss produced by debris beds created by Cal-Sil, fibrous debris, and calcium phosphate precipitates. The effects of both the relative arrival time of the precipitates and insulation debris and the calcium phosphate formation process were specifically evaluated. The debris loadings, test loop flow rates, and test temperature were chosen to be reasonably representative of …
Date:
October 31, 2006
Creator:
Park, J. H.; Kasza, K.; Fisher, B.; Oras, J.; Natesan, K.; Shack, W. J. et al.
Object Type:
Report
System:
The UNT Digital Library
Methods for high precision 14C AMS measurement of atmospheric CO2 at LLNL
Development of {sup 14}C analysis with precision better than 2{per_thousand} has the potential to expand the utility of {sup 14}CO{sub 2} measurements for carbon cycle investigations as atmospheric gradients currently approach traditional measurement precision of 2-5{per_thousand}. The AMS facility at the Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, produces high and stable beam currents that enable efficient acquisition times for large numbers of {sup 14}C counts. One million {sup 14}C atoms can be detected in approximately 25 minutes, suggesting that near 1{per_thousand} counting precision is economically feasible at LLNL. The overall uncertainty in measured values is ultimately determined by the variation between measured ratios in several sputtering periods of the same sample and by the reproducibility of replicate samples. Experiments on the collection of one million counts on replicate samples of CO{sub 2} extracted from a whole air cylinder show a standard deviation of 1.7{per_thousand} in 36 samples measured over several wheels. This precision may be limited by the reproducibility of Oxalic Acid I standard samples, which is considerably poorer. We outline the procedures for high-precision sample handling and analysis that have enabled reproducibility in the cylinder extraction samples at the <2{per_thousand} level and describe future directions to …
Date:
October 18, 2006
Creator:
Graven, H D; Guilderson, T P & Keeling, R F
Object Type:
Article
System:
The UNT Digital Library