Modeling Temporal Behavior in Large Networks: A Dynamic Mixed-Membership Model (open access)

Modeling Temporal Behavior in Large Networks: A Dynamic Mixed-Membership Model

Given a large time-evolving network, how can we model and characterize the temporal behaviors of individual nodes (and network states)? How can we model the behavioral transition patterns of nodes? We propose a temporal behavior model that captures the 'roles' of nodes in the graph and how they evolve over time. The proposed dynamic behavioral mixed-membership model (DBMM) is scalable, fully automatic (no user-defined parameters), non-parametric/data-driven (no specific functional form or parameterization), interpretable (identifies explainable patterns), and flexible (applicable to dynamic and streaming networks). Moreover, the interpretable behavioral roles are generalizable, computationally efficient, and natively supports attributes. We applied our model for (a) identifying patterns and trends of nodes and network states based on the temporal behavior, (b) predicting future structural changes, and (c) detecting unusual temporal behavior transitions. We use eight large real-world datasets from different time-evolving settings (dynamic and streaming). In particular, we model the evolving mixed-memberships and the corresponding behavioral transitions of Twitter, Facebook, IP-Traces, Email (University), Internet AS, Enron, Reality, and IMDB. The experiments demonstrate the scalability, flexibility, and effectiveness of our model for identifying interesting patterns, detecting unusual structural transitions, and predicting the future structural changes of the network and individual nodes.
Date: November 11, 2011
Creator: Rossi, R.; Gallagher, B.; Neville, J. & Henderson, K.
Object Type: Report
System: The UNT Digital Library
TARGET: A multi-channel digitizer chip for very-high-energy gamma-ray telescopes (open access)

TARGET: A multi-channel digitizer chip for very-high-energy gamma-ray telescopes

The next-generation very-high-energy (VHE) gamma-ray observatory, the Cherenkov Telescope Array, will feature dozens of imaging atmospheric Cherenkov telescopes (IACTs), each with thousands of pixels of photosensors. To be affordable and reliable, reading out such a mega-channel array requires event recording technology that is highly integrated and modular, with a low cost per channel. We present the design and performance of a chip targeted to this application: the TeV Array Readout with GSa/s sampling and Event Trigger (TARGET). This application-specific integrated circuit (ASIC) has 16 parallel input channels, a 4096-sample buffer for each channel, adjustable input termination, self-trigger functionality, and tight window-selected readout. We report the performance of TARGET in terms of sampling frequency, power consumption, dynamic range, current-mode gain, analog bandwidth, and cross talk. The large number of channels per chip allows a low cost per channel ($10 to $20 including front-end and back-end electronics but not including photosensors) to be achieved with a TARGET-based IACT readout system. In addition to basic performance parameters of the TARGET chip itself, we present a camera module prototype as well as a second-generation chip (TARGET 2), both of which have been produced.
Date: August 11, 2011
Creator: Bechtol, K.; Funk, S.; Okumura, A.; Ruckman, L.; Simons, A.; Tajima, H. et al.
Object Type: Article
System: The UNT Digital Library
ASSESSMENT OF THE ABILITY OF STANDARD SLURRY PUMPS TO MIX SOLIDS WITH LIQUIDS IN TANK 50H (open access)

ASSESSMENT OF THE ABILITY OF STANDARD SLURRY PUMPS TO MIX SOLIDS WITH LIQUIDS IN TANK 50H

Tank 50H is the feed tank for the Saltstone Production Facility (SPF). In the summer of 2011, Tank 50H contained two standard slurry pumps and two quad volute slurry pumps. Current requirements for mixing operation is to run three pumps for one hour prior to initiating a feed transfer to SPF. Savannah River Site (SRS) Liquid Waste moved both of the Quad Volute pumps from Tank 50H to Tank 51H to replace pumps in Tank 51H that were failing. In addition, one of the standard pumps in Tank 50H exhibits high seal leakage and vibration. SRS Liquid Waste requested Savannah River National Laboratory (SRNL) to conduct a study to evaluate the feasibility of mixing the contents of Tank 50H with one to three standard slurry pumps. To determine the pump requirements to mix solids with liquids in Tank 50H, the author reviewed the pilot-scale blending work performed for the Small Column Ion Exchange Process (SCIX), SRNL computational fluid dynamics (CFD) modeling, Tank 50H operating experience, and the technical literature, and applied the results to Tank 50H to determine the number, size, and operating parameters of pumps needed to mix the solid particles with the liquid in Tank 50H. The analysis …
Date: November 11, 2011
Creator: Poirier, M.
Object Type: Report
System: The UNT Digital Library
Definitive Alkene Identification Needed for In-Vitro Studies with Ole (Olefin Synthesis) Proteins (open access)

Definitive Alkene Identification Needed for In-Vitro Studies with Ole (Olefin Synthesis) Proteins

None
Date: July 11, 2011
Creator: Beller, Harry; Goh, Ee-Been & Keasling, Jay
Object Type: Article
System: The UNT Digital Library
Direct probe of Mott-Hubbard to charge-transfer insulator transition and electronic structure evolution in transition-metal systems (open access)

Direct probe of Mott-Hubbard to charge-transfer insulator transition and electronic structure evolution in transition-metal systems

We report the most direct experimental verification of Mott-Hubbard and charge-transfer insulators through x-ray emission spectroscopy in transition-metal (TM) fluorides. The p-d hybridization features in the spectra allow a straightforward energy alignment of the anion-2p and metal-3d valence states, which visually shows the difference between the two types of insulators. Furthermore, in parallel with the theoretical Zaanen-Sawatzky-Allen diagram, a complete experimental systematics of the 3d Coulomb interaction and the 2p-3d charge-transfer energy is reported and could serve as a universal experimental trend for other TM systems including oxides.
Date: July 11, 2011
Creator: Olalde-Velasco, P.; Jimenez-Mier, J.; Denlinger, J. D.; Hussain, Z. & Yang, W. L.
Object Type: Article
System: The UNT Digital Library
Present Status And First Results of the Final Focus Beam Line at the KEK Accelerator Test Facility (open access)

Present Status And First Results of the Final Focus Beam Line at the KEK Accelerator Test Facility

ATF2 is a final-focus test beam line which aims to focus the low emittance beam from the ATF damping ring to a vertical size of about 37 nm and to demonstrate nanometer level beam stability. Several advanced beam diagnostics and feedback tools are used. In December 2008, construction and installation were completed and beam commissioning started, supported by an international team of Asian, European, and U.S. scientists. The present status and first results are described.
Date: November 11, 2011
Creator: Bambade, P.; Alabau Pons, M.; Amann, J.; Angal-Kalinin, D.; Apsimon, R.; Araki, S. et al.
Object Type: Article
System: The UNT Digital Library
Assessment of the Group 5-6 (LB C2, LB S2, LV S1) Stack Sampling Probe Locations for Compliance with ANSI/HPS N13.1 1999 (open access)

Assessment of the Group 5-6 (LB C2, LB S2, LV S1) Stack Sampling Probe Locations for Compliance with ANSI/HPS N13.1 1999

This document reports on a series of tests to assess the proposed air sampling locations for the Hanford Tank Waste Treatment and Immobilization Plant (WTP) Group 5-6 exhaust stacks with respect to the applicable criteria regarding the placement of an air sampling probe. The LB-C2, LV-S1, and LB S2 exhaust stacks were tested together as a group (Test Group 5-6) because the common factor in their design is that the last significant flow disturbance upstream of the air sampling probe is a reduction in duct diameter. Federal regulations( ) require that a sampling probe be located in the exhaust stack according to the criteria of the American National Standards Institute/Health Physics Society (ANSI/HPS) N13.1-1999, Sampling and Monitoring Releases of Airborne Radioactive Substances from the Stack and Ducts of Nuclear Facilities. These criteria address the capability of the sampling probe to extract a sample that represents the effluent stream. The testing on scale models of the stacks conducted for this project was part of the River Protection Project—Waste Treatment Plant Support Program under Contract No. DE-AC05-76RL01830 according to the statement of work issued by Bechtel National Inc. (BNI, 24590-QL-SRA-W000-00101, N13.1-1999 Stack Monitor Scale Model Testing and Qualification, Revision 1, 9/12/2007) and …
Date: March 11, 2011
Creator: Glissmeyer, John A.; Flaherty, Julia E. & Piepel, Gregory F.
Object Type: Report
System: The UNT Digital Library
Coil Creep and Skew-Quadrupole Field Components in the Tevatron (open access)

Coil Creep and Skew-Quadrupole Field Components in the Tevatron

During the start-up of Run II of the Tevatron Collider program, several issues surfaced which were not present, or not seen as detrimental, during Run I. These included the repeated deterioration of the closed orbit requiring orbit smoothing every two weeks or so, the inability to correct the closed orbit to desired positions due to various correctors running at maximum limits, regions of systematically strong vertical dipole corrections, and the identification of very strong coupling between the two transverse degrees-of-freedom. It became apparent that many of the problems being experienced operationally were connected to a deterioration of the main dipole magnet alignment, and remedial actions were undertaken. However, the alignment alone was not enough to explain the corrector strengths required to handle transverse coupling. With one exception, strong coupling had generally not been an issue in the Tevatron during Run I. Based on experience with the Main Ring, the Tevatron was designed with a very strong skew quadrupole circuit to compensate any quadrupole alignment and skew quadrupole field errors that might present themselves. The circuit was composed of 48 correctors placed evenly throughout the arcs, 8 per sector, evenly placed in every other cell. Other smaller circuits were installed but …
Date: July 11, 2011
Creator: Annala, G.; Harding, D. J.; Syphers, M. J. & /Fermilab
Object Type: Article
System: The UNT Digital Library
Independent Confirmatory Survey Report for the University of Arizona Nuclear Reactor Laboratory, Tucson, Arizona (open access)

Independent Confirmatory Survey Report for the University of Arizona Nuclear Reactor Laboratory, Tucson, Arizona

The University of Arizona (University) research reactor is a TRIGA swimming pool type reactor designed by General Atomics and constructed at the University in 1958. The reactor first went into operation in December of 1958 under U.S. Nuclear Regulatory Commission (NRC) license R-52 until final shut down on May 18, 2010. Initial site characterization activities were conducted in February 2009 during ongoing reactor operations to assess the radiological status of the Nuclear Reactor Laboratory (NRL) excluding the reactor tank, associated components, and operating systems. Additional post-shutdown characterization activities were performed to complete characterization activities as well as verify assumptions made in the Decommissioning Plan (DP) that were based on a separate activation analysis (ESI 2009 and WMG 2009). Final status survey (FSS) activities began shortly after the issuance of the FSS plan in May 2011. The contractor completed measurement and sampling activities during the week of August 29, 2011.
Date: November 11, 2011
Creator: Altic, Nick A.
Object Type: Report
System: The UNT Digital Library
A Heteroepitaxial Perovskite Metal-Base Transistor (open access)

A Heteroepitaxial Perovskite Metal-Base Transistor

'More than Moore' captures a concept for overcoming limitations in silicon electronics by incorporating new functionalities in the constituent materials. Perovskite oxides are candidates because of their vast array of physical properties in a common structure. They also enable new electronic devices based on strongly-correlated electrons. The field effect transistor and its derivatives have been the principal oxide devices investigated thus far, but another option is available in a different geometry: if the current is perpendicular to the interface, the strong internal electric fields generated at back-to-back heterojunctions can be used for oxide electronics, analogous to bipolar transistors. Here we demonstrate a perovskite heteroepitaxial metal-base transistor operating at room temperature, enabled by interface dipole engineering. Analysis of many devices quantifies the evolution from hot-electron to permeable-base behaviour. This device provides a platform for incorporating the exotic ground states of perovskite oxides, as well as novel electronic phases at their interfaces.
Date: August 11, 2011
Creator: Yajima, T.; Hikita, Y. & Hwang, H. Y.
Object Type: Article
System: The UNT Digital Library
Reco level Smin and subsystem Smin: improved global inclusive variables for measuring the new physics mass scale in MET events at hadron colliders (open access)

Reco level Smin and subsystem Smin: improved global inclusive variables for measuring the new physics mass scale in MET events at hadron colliders

The variable {radical}s{sub min} was originally proposed in [1] as a model-independent, global and fully inclusive measure of the new physics mass scale in missing energy events at hadron colliders. In the original incarnation of {radical}s{sub min}, however, the connection to the new physics mass scale was blurred by the effects of the underlying event, most notably initial state radiation and multiple parton interactions. In this paper we advertize two improved variants of the {radical}s{sub min} variable, which overcome this problem. First we show that by evaluating the {radical}s{sub min} variable at the RECO level, in terms of the reconstructed objects in the event, the effects from the underlying event are significantly diminished and the nice correlation between the peak in the {radical}s{sub min}{sup (reco)} distribution and the new physics mass scale is restored. Secondly, the underlying event problem can be avoided altogether when the {radical}s{sub min} concept is applied to a subsystem of the event which does not involve any QCD jets. We supply an analytic formula for the resulting subsystem {radical}s{sub min}{sup (sub)} variable and show that its peak exhibits the usual correlation with the mass scale of the particles produced in the subsystem. Finally, we contrast {radical}s{sub …
Date: August 11, 2011
Creator: Konar, Partha; Kong, Kyoungchul; Matchev, Konstantin T. & Park, Myeonghun
Object Type: Article
System: The UNT Digital Library
FIFTH INTERIM STATUS REPORT: MODEL 9975 PCV O-RING FIXTURE LONG-TERM LEAK PERFORMANCE (open access)

FIFTH INTERIM STATUS REPORT: MODEL 9975 PCV O-RING FIXTURE LONG-TERM LEAK PERFORMANCE

A series of experiments to monitor the aging performance of Viton{reg_sign} GLT O-rings used in the Model 9975 package has been ongoing for six years at the Savannah River National Laboratory. Sixty-seven mock-ups of 9975 Primary Containment Vessels (PCVs) were assembled and heated to temperatures ranging from 200 to 450 F. They were leak-tested initially and have been tested at nominal six month intervals to determine if they meet the criterion of leaktightness defined in ANSI standard N14.5-97. Fourteen additional tests were initiated in 2008 with GLT-S O-rings heated to temperatures ranging from 200 to 400 F. High temperature aging continues for 36 GLT O-ring fixtures at 200-350 F. Room temperature leak test failures have been experienced in 6 of the GLT O-ring fixtures aging at 300 and 350 F, and in all 3 of the GLT O-ring fixtures aging at higher temperatures. No failures have yet been observed in GLT O-ring fixtures aging at 200 F for 30-48 months, which is still bounding to O-ring temperatures during storage in KAMS. High temperature aging continues for 6 GLT-S O-ring fixtures at 200-300 F. Room temperature leak test failures have been experienced in all 8 of the GLT-S O-ring fixtures aging …
Date: April 11, 2011
Creator: Daugherty, W. & Hoffman, E.
Object Type: Report
System: The UNT Digital Library
Searching for optimal mitigation geometries for laser resistant multilayer high reflector coatings (open access)

Searching for optimal mitigation geometries for laser resistant multilayer high reflector coatings

Growing laser damage sites on multilayer high reflector coatings can limit mirror performance. One of the strategies to improve laser damage resistance is to replace the growing damage sites with pre-designed benign mitigation structures. By mitigating the weakest site on the optic, the large aperture mirror will have a laser resistance comparable to the intrinsic value of the multilayer coating. To determine the optimal mitigation geometry, the finite difference time domain method (FDTD) was used to quantify the electric-field intensification within the multilayer, at the presence of different conical pits. We find that the field intensification induced by the mitigation pit is strongly dependent on the polarization and the angle of incidence (AOI) of the incoming wave. Therefore the optimal mitigation conical pit geometry is application specific. Furthermore, our simulation also illustrates an alternative means to achieve an optimal mitigation structure by matching the cone angle of the structure with the AOI of the incoming wave, except for the p-polarization wave at a range of incident angles between 30{sup o} and 45{sup o}.
Date: February 11, 2011
Creator: Qiu, S R; Wolfe, J E; Monterrosa, A M; Feit, M D; Pistor, T V & STolz, C J
Object Type: Article
System: The UNT Digital Library
The U. S. Transportation Sector in the Year 2030: Results of a Two-Part Delphi Survey (open access)

The U. S. Transportation Sector in the Year 2030: Results of a Two-Part Delphi Survey

A two-part Delphi Survey was given to transportation experts attending the Asilomar Conference on Transportation and Energy in August, 2011. The survey asked respondents about trends in the US transportation sector in 2030. Topics included: alternative vehicles, high speed rail construction, rail freight transportation, average vehicle miles traveled, truck versus passenger car shares, vehicle fuel economy, and biofuels in different modes. The survey consisted of two rounds -- both asked the same set of seven questions. In the first round, respondents were given a short introductory paragraph about the topic and asked to use their own judgment in their responses. In the second round, the respondents were asked the same questions, but were also given results from the first round as guidance. The survey was sponsored by Argonne National Lab (ANL), the National Renewable Energy Lab (NREL), and implemented by University of California at Davis, Institute of Transportation Studies. The survey was part of the larger Transportation Energy Futures (TEF) project run by the Department of Energy, Office of Energy Efficiency and Renewable Energy. Of the 206 invitation letters sent, 94 answered all questions in the first round (105 answered at least one question), and 23 of those answered all …
Date: October 11, 2011
Creator: Morrison, G.; Stephens, T. S.; Johnson, L.; Brown, A.; Patterson, P. & Ward, J.
Object Type: Report
System: The UNT Digital Library
Discovering New Light States at Neutrino Experiments (open access)

Discovering New Light States at Neutrino Experiments

Experiments designed to measure neutrino oscillations also provide major opportunities for discovering very weakly coupled states. In order to produce neutrinos, experiments such as LSND collide thousands of Coulombs of protons into fixed targets, while MINOS and MiniBooNE also focus and then dump beams of muons. The neutrino detectors beyond these beam dumps are therefore an excellent arena in which to look for long-lived pseudoscalars or for vector bosons that kinetically mix with the photon. We show that these experiments have significant sensitivity beyond previous beam dumps, and are able to partially close the gap between laboratory experiments and supernovae constraints on pseudoscalars. Future upgrades to the NuMI beamline and Project X will lead to even greater opportunities for discovery. We also discuss thin target experiments with muon beams, such as those available in COMPASS, and show that they constitute a powerful probe for leptophilic PNGBs.
Date: August 11, 2011
Creator: Essig, Rouven; Harnik, Roni; Kaplan, Jared & Toro, Natalia
Object Type: Article
System: The UNT Digital Library
DOE 2010 Occupational Radiation Exposure November 2011 (open access)

DOE 2010 Occupational Radiation Exposure November 2011

This report discusses radiation protection and dose reporting requirements, presents the 2010 occupational radiation dose data trended over the past 5 years, and includes instructions to submit successful ALARA projects.
Date: November 11, 2011
Creator: U.S. Department of Energy, Office of Health, Safety and Security, Office of Analysis
Object Type: Report
System: The UNT Digital Library
Supercritical carbon dioxide cycle control analysis. (open access)

Supercritical carbon dioxide cycle control analysis.

This report documents work carried out during FY 2008 on further investigation of control strategies for supercritical carbon dioxide (S-CO{sub 2}) Brayton cycle energy converters. The main focus of the present work has been on investigation of the S-CO{sub 2} cycle control and behavior under conditions not covered by previous work. An important scenario which has not been previously calculated involves cycle operation for a Sodium-Cooled Fast Reactor (SFR) following a reactor scram event and the transition to the primary coolant natural circulation and decay heat removal. The Argonne National Laboratory (ANL) Plant Dynamics Code has been applied to investigate the dynamic behavior of the 96 MWe (250 MWt) Advanced Burner Test Reactor (ABTR) S-CO{sub 2} Brayton cycle following scram. The timescale for the primary sodium flowrate to coast down and the transition to natural circulation to occur was calculated with the SAS4A/SASSYS-1 computer code and found to be about 400 seconds. It is assumed that after this time, decay heat is removed by the normal ABTR shutdown heat removal system incorporating a dedicated shutdown heat removal S-CO{sub 2} pump and cooler. The ANL Plant Dynamics Code configured for the Small Secure Transportable Autonomous Reactor (SSTAR) Lead-Cooled Fast Reactor (LFR) …
Date: April 11, 2011
Creator: Moisseytsev, A. & Sienicki, J. J. (Nuclear Engineering Division)
Object Type: Report
System: The UNT Digital Library
U.S. Department of Energy (DOE)--Surface Biogeochemical Research (SBR) 6th Annual PI Meeting: Abstracts (open access)

U.S. Department of Energy (DOE)--Surface Biogeochemical Research (SBR) 6th Annual PI Meeting: Abstracts

On behalf of the Subsurface Biogeochemical Research (SBR) program managers in the Climate and Environmental Sciences Division (CESD), Office of Biological and Environmental Research (BER), welcome to the 2011 SBR Principal Investigators meeting. Thank you in advance for your attendance and your presentations at this year's meeting. As the events in Japan continue to unfold, we are all reminded that the research we perform on radionuclide behavior in the environment has implications beyond legacy waste cleanup and in fact has its place in the discussion on the expanded use of nuclear power. As in the past, there are three broad objectives to the Principal Investigators meeting: (1) to provide opportunities to share research results and promote interactions among the SBR scientists and other invited guests; (2) to evaluate the progress of each project within the program; and (3) to showcase the scientific expertise and research progress over the past year to senior managers within the DOE Office of Science, the technology offices within DOE, and other invited attendees from other Federal Agencies. This past year has seen a few significant changes within BER and within the SBR program. In November, our Associate Director for BER, Anna Palmisano, retired from Federal …
Date: April 11, 2011
Creator: Hazen Ed., T.C.
Object Type: Article
System: The UNT Digital Library
Cost-Effective Solar Thermal Energy Storage (open access)

Cost-Effective Solar Thermal Energy Storage

Advanced Research Projects Agency-Energy project sheet summarizing general information about a new program to develop a cost-effective thermal storage system utilizing a supercritical fluid to replace molten salt systems (project title "Thermal Energy Storage With Supercritical Fluids") including critical needs, innovation and advantages, impact, and contact information. This sheet is the first open solicitation, announcing funding opportunities for involvement in the project.
Date: May 11, 2011
Creator: University of California at Los Angeles
Object Type: Text
System: The UNT Digital Library
Reduction of Glass Surface Reflectance by Ion Beam Surface Modification (open access)

Reduction of Glass Surface Reflectance by Ion Beam Surface Modification

This is the final report for DOE contract DE-EE0000590. The purpose of this work was to determine the feasibility of the reduction of the reflection from the front of solar photovoltaic modules. Reflection accounts for a power loss of approximately 4%. A solar module having an area of one square meter with an energy conversion efficiency of 18% generates approximately 180 watts. If reflection loss can be eliminated, the power output can be increased to 187 watts. Since conventional thin-film anti-reflection coatings do not have sufficient environmental stability, we investigated the feasibility of ion beam modification of the glass surface to obtain reduction of reflectance. Our findings are generally applicable to all solar modules that use glass encapsulation, as well as commercial float glass used in windows and other applications. Ion implantation of argon, fluorine, and xenon into commercial low-iron soda lime float glass, standard float glass, and borosilicate glass was studied by implantation, annealing, and measurement of reflectance. The three ions all affected reflectance. The most significant change was obtained by argon implantation into both low-iron and standard soda-lime glass. In this way samples were formed with reflectance lower than can be obtained with a single-layer coatings of magnesium …
Date: March 11, 2011
Creator: Spitzer, Mark
Object Type: Report
System: The UNT Digital Library
Computational Modeling and Assessment Of Nanocoatings for Ultra Supercritical Boilers (open access)

Computational Modeling and Assessment Of Nanocoatings for Ultra Supercritical Boilers

Coal-fired power plants are a significant part of the nation's power generating capacity, currently accounting for more than 55% of the country's total electricity production. Extending the reliable lifetimes of fossil fired boiler components and reducing the maintenance costs are essential for economic operation of power plants. Corrosion and erosion are leading causes of superheater and reheater boiler tube failures leading to unscheduled costly outages. Several types of coatings and weld overlays have been used to extend the service life of boiler tubes; however, the protection afforded by such materials was limited approximately one to eight years. Power companies are more recently focused in achieving greater plant efficiency by increasing steam temperature and pressure into the advanced-ultrasupercritical (A-USC) condition with steam temperatures approaching 760°C (1400°F) and operating pressures in excess of 35MPa (5075 psig). Unfortunately, laboratory and field testing suggests that the resultant fireside environment when operating under A-USC conditions can potentially cause significant corrosion to conventional and advanced boiler materials1-2. In order to improve reliability and availability of fossil fired A-USC boilers, it is essential to develop advanced nanostructured coatings that provide excellent corrosion and erosion resistance without adversely affecting the other properties such as toughness and thermal fatigue …
Date: May 11, 2011
Creator: Gandy, David W. & Shingledecker, John P.
Object Type: Text
System: The UNT Digital Library
Order and disorder in the local and long-range structure of the spin-glass pyrochlore, Tb{sub 2}Mo{sub 2}O{sub 7} (open access)

Order and disorder in the local and long-range structure of the spin-glass pyrochlore, Tb{sub 2}Mo{sub 2}O{sub 7}

To understand the origin of the spin-glass state in molybdate pyrochlores, the structure of Tb{sub 2}Mo{sub 2}O{sub 7} is investigated using two techniques: the long-range lattice structure was measured using neutron powder diffraction (NPD), and local structure information was obtained from the extended x-ray absorption fine structure (EXAFS) technique. While the long-range structure appears generally well ordered, enhanced mean-squared site displacements on the O(1) site and the lack of temperature dependence of the strongly anisotropic displacement parameters for both the Mo and O(1) sites indicate some disorder exists. Likewise, the local structure measurements indicate some Mo-Mo and Tb-O(1) nearest-neighbor disorder exists, similar to that found in the related spin-glass pyrochlore, Y{sub 2}Mo{sub 2}O{sub 7}. Although the freezing temperature in Tb{sub 2}Mo{sub 2}O{sub 7}, 25 K, is slightly higher than in Y{sub 2}Mo{sub 2}O{sub 7}, 22 K, the degree of local pair distance disorder is actually less in Tb{sub 2}Mo{sub 2}O{sub 7}. This apparent contradiction is considered in light of the interactions involved in the freezing process.
Date: February 11, 2011
Creator: Jiang, Yu; Huq, Ashfia; Booth, Corwin H.; Ehlers, Georg; Greedan, John E. & Gardner, Jason S.
Object Type: Article
System: The UNT Digital Library
Summary of “Future of DIS” Working Group Session (open access)

Summary of “Future of DIS” Working Group Session

Despite the closure of the HERA accelerator in the past few years, much physics still remains to be understood, from the quark and gluon content of the nucleon/nucleus across all x to the still unknown spin structure of the proton. The 'Future of DIS' working group was dedicated to discussions on these and many other subjects. This paper represents a brief overview of the discussions. For further details, please refer to individual contributions.
Date: April 11, 2011
Creator: M., Lamont; Guzey, V. & Polini, A.
Object Type: Article
System: The UNT Digital Library
Gas Electron Multiplier (GEM) Chamber Characteristics Test (open access)

Gas Electron Multiplier (GEM) Chamber Characteristics Test

Gas Electron Multipliers (GEMs) have been used in many HEP experiments as tracking detectors. They are sensitive to X-rays which allows use beyond that of HEP. The UTA High Energy group has been working on using GEMs as the sensitive gap detector in a DHCAL for the ILC. The physics goals at the ILC put a stringent requirement on detector performance. Especially the precision required for jet mass and positions demands an unprecedented jet energy resolution to hadronic calorimeters. A solution to meet this requirement is using the Particle Flow Algorithm (PFA). In order for PFA to work well, high calorimeter granularity is necessary. Previous studies based on GEANT simulations using GEM DHCAL gave confidence on the performance of GEM in the sensitive gap in a sampling calorimeter and its use as a DHCAL in PFA. The UTA HEP team has built several GEM prototype chambers, including the current 30cm x 30cm chamber integrated with the SLAC-developed 64 channel kPiX analog readout chip. This chamber has been tested on the bench using radioactive sources and cosmic ray muons. In order to have fuller understanding of various chamber characteristics, the experiments plan to expose 1-3 GEM chambers of dimension 35cm x …
Date: January 11, 2011
Creator: Yu, Jaehoon; White, Andy; Park, Seongtae; Hahn, Changhie; Baldeloma, Edwin; Tran, Nam et al.
Object Type: Report
System: The UNT Digital Library