23 Matching Results

Search Results

Advanced search parameters have been applied.

Instability localized at the inner surface of an imploding spherical shell

Description: It is shown that in an imploding spherical shell the surface instabilities are of two different types. The first, which occurs at the outer surfaces, is the Rayleigh-Taylor instability. The second instability occurs at the inner surface. This latter instability is not as disruptive as R-T modes, but it has three basic properties which differ considerably from those of the R-T instability: (1) it is oscillatory at early times; (2) it grows faster in the long wavelength modes; (3) it depends on the equation of state. It is further shown that this new instability is driven by amplified sound waves in the shell.
Date: January 1, 1983
Creator: Han, S.J.
Partner: UNT Libraries Government Documents Department

The ARTT motif and a unified structural understanding of substraterecognition in ADP ribosylating bacterial toxins and eukaryotic ADPribosyltransferases

Description: ADP-ribosylation is a widely occurring and biologically critical covalent chemical modification process in pathogenic mechanisms, intracellular signaling systems, DNA repair, and cell division. The reaction is catalyzed by ADP-ribosyltransferases, which transfer the ADP-ribose moiety of NAD to a target protein with nicotinamide release. A family of bacterial toxins and eukaryotic enzymes has been termed the mono-ADP-ribosyltransferases, in distinction to the poly-ADP-ribosyltransferases, which catalyze the addition of multiple ADP-ribose groups to the carboxyl terminus of eukaryotic nucleoproteins. Despite the limited primary sequence homology among the different ADP-ribosyltransferases, a central cleft bearing NAD-binding pocket formed by the two perpendicular b-sheet core has been remarkably conserved between bacterial toxins and eukaryotic mono- and poly-ADP-ribosyltransferases. The majority of bacterial toxins and eukaryotic mono-ADP-ribosyltransferases are characterized by conserved His and catalytic Glu residues. In contrast, Diphtheria toxin, Pseudomonas exotoxin A, and eukaryotic poly-ADP-ribosyltransferases are characterized by conserved Arg and catalytic Glu residues. The NAD-binding core of a binary toxin and a C3-like toxin family identified an ARTT motif (ADP-ribosylating turn-turn motif) that is implicated in substrate specificity and recognition by structural and mutagenic studies. Here we apply structure-based sequence alignment and comparative structural analyses of all known structures of ADP-ribosyltransfeases to suggest that this ARTT motif is functionally important in many ADP-ribosylating enzymes that bear a NAD binding cleft as characterized by conserved Arg and catalytic Glu residues. Overall, structure-based sequence analysis reveals common core structures and conserved active sites of ADP-ribosyltransferases to support similar NAD binding mechanisms but differing mechanisms of target protein binding via sequence variations within the ARTT motif structural framework. Thus, we propose here that the ARTT motif represents an experimentally testable general recognition motif region for many ADP-ribosyltransferases and thereby potentially provides a unified structural understanding of substrate recognition in ADP-ribosylation processes.
Date: August 1, 2001
Creator: Han, S. & Tainer, J.A.
Partner: UNT Libraries Government Documents Department

Mixed field dosimetry using focused and unfocused laser heating of thermoluminescent materials

Description: The incidents at the Three Mile Island and Chernobyl have triggered the need for better personnel dosimetry methods in mixed radiation fields. This thesis presents a detailed computational study of a new method for mixed radiation field dosimetry using single-element TL dosimeters with pulsed laser heating schemes. The main objective of this study was to obtain an optimum heating scheme so that the depth-dose distribution in a thick TL dosimeter could be accurately determined. The major parts of the study include: (a) heat conduction calculations for TL dosimeters with various heating schemes, (b) glow curve calculations for TL dosimeters based on a first-order kinetic model, (c) unfolding of the depth-dose distribution based on the glow curve data, and (d) estimation of shallow and deep doses from the unfolded depth-dose distribution. Two optimum heating schemes were obtained in this study. The first one was obtained for a focused laser beam, and the second one was obtained for a uniform laser beam. Both heating schemes consist of two processes: top surface heating and bottom surface heating, and each process in turn consists of a sequence of laser pulses with various heating durations and power levels. Compared to the ``true`` depth-dose distribution obtained using Monte Carlo transport code EGS4, relative errors associated with the shallow and deep doses obtained from the unfolded depth-dose distributions are 5% and 25%, respectively, for the focused laser beam, and 15% in both doses for the uniform laser beam. 74 refs., 148 figs.
Date: March 1, 1994
Creator: Han, S.
Partner: UNT Libraries Government Documents Department

Stability of eutectic interface during directional solidification

Description: Directional solidification of eutectic alloys shows different types of eutectic morphologies. These include lamellar, rod, oscillating and tilting modes. The growth of these morphologies occurs with a macroscopically planar interface. However, under certain conditions, the planar eutectic front becomes unstable and gives rise to a cellular or a dendritic structure. This instability leads to the cellular/dendritic structure of either a primary phase or a two-phase structure. The objective of this work is to develop a fundamental understanding of the instability of eutectic structure into cellular/dendritic structures of a single phase and of two-phases. Experimental studies have been carried out to examine the transition from a planar to two-phase cellular and dendritic structures in a ceramic system of Alumina-Zirconia (Al{sub 2}O{sub 3}-ZrO{sub 2}) and in a transparent organic system of carbon tetrabromide and hexachloroethane (CBr{sub 4}-C{sub 2}Cl{sub 6}). Several aspects of eutectic interface stability have been examined.
Date: April 23, 1996
Creator: Han, S.H.
Partner: UNT Libraries Government Documents Department

New developments in photoconductive detectors

Description: Nearly ideal for detecting ionizing radiation, wide bandgap semiconductors present a possibility of having outstanding radiation hardness, fast charge collection and low leakage current that will allow them to be used in high radiation, high temperature, and chemically aggressive environments. Over the past few years, the improvements in the electrical quality of wide bandgap semiconductors have progressed enormously. One particular wide bandgap semiconductor, diamond, has properties which may be ideal for radiation detection. Since the discovery of low pressure and low temperature deposition of diamond, the possibility of large area diamond films have become a reality. Over the past few years, great progress has been made in advancing the electrical quality of chemical-vapor-deposited (CVD) diamond. Presently, unprecedented diamond wafer size of 7 in. diameter is possible. Due to both the present electrical quality and the available size, the utilization of diamond in radiation detection applications is not just a dream but a reality. The progression of CVD diamond`s electrical properties in the last few years will be presented along with what is currently possible. Applications of CVD diamond for the National Ignition Facility (NIF) diagnostics will be reviewed. In addition, a brief review concerning other possible wide bandgap semiconductors for ICF diagnostics will be presented.
Date: July 1, 1996
Creator: Han, S.
Partner: UNT Libraries Government Documents Department

Mean carrier transport properties and charge collection dynamics of single-crystal, natural type IIa diamonds from ion-induced conductivity measurements

Description: Ion-induced conductivity has been used to investigate the detector characteristics of diamond detectors. Both integrated-charge, and time-resolved current measurements were performed to examine the mean carrier transport properties of diamond and the dynamics of charge collection under highly-localized and high-density excitation conditions. The integrated-charge measurements were conducted with a standard pulse-counting system with {sup 241}Am radioactivity as the excitation source for the detectors. The time-resolved current measurements were performed using a 70 GHz random sampling oscilloscope with the detectors incorporated into high-speed microstrip transmission lines and the excitation source for these measurements was an ion beam of either 5-MeV He{sup +} or 10-MeV Si{sup 3+}. The detectors used in both experiments can be described as metal-semiconductor-metal (MSM) devices where a volume of the detector material is sandwiched between two metal plates. A charge collection model was developed to interpret the integrated-charge measurements which enabled estimation of the energy required to produce an electron-hole pair ({epsilon}{sub di}) and the mean carrier transport properties in diamond, such as carrier mobility and lifetime, and the behavior of the electrical contacts to diamond.
Date: September 1, 1993
Creator: Han, S. S.
Partner: UNT Libraries Government Documents Department

Correlation of electrical properties with defects in a homoepitaxial chemical-vapor-deposited diamond

Description: A high-quality, low-stress 200 {mu}m epitaxial diamond film has been grown on a 400 {mu}m thick high-temperature-high-pressure Ha diamond. X-ray diffraction images of the film indicate that a large region of the film is fairly defect free and individual dislocations have been imaged in this region. Depth-resolved Raman results indicate that the region of the film with a low density of defects also has lower stress than in the higher defect density region. Transient photoconductivity measurements were performed on the high and low line defect density regions of the homoepitaxial diamond film to determine the effects of the stress and defect density on the combined electron-hole mobility and carrier lifetime. The correlation between the electrical properties and the x-ray diffraction imaging suggests that line defects may not be the limiting factor in the carrier transport at the present film quality.
Date: December 31, 1995
Creator: Han, S.; Rodriguez, G. & Taylor, A.
Partner: UNT Libraries Government Documents Department

The utility of diamond sensors for space flight

Description: Los Alamos National Laboratory has developed diamond sensors with interdigitated electrodes that operate in a photoconducting mode. The specific application for this work was for the Department of Energy`s instruments flown on the Global Positioning System satellites. Sensors have been fabricated and tested for their response to low-energy x-rays. These sensors can be operated to extremely high volumetric radiation doses. We find that the sensors are extremely useful for situations where the surface radiation dose is not excessive, but that this limit is exceeded for the GPS orbit. It is possible that further studies and special detector arrangements or auxiliary heating of the sensor may push this limit to higher values.
Date: March 1996
Creator: Higbie, P. R.; Han, S. S. & Wagner, R. S.
Partner: UNT Libraries Government Documents Department

Response of diamond photoconductors to soft x-ray in the spectral range 125 {angstrom} to 240 {angstrom}

Description: Due to the large bandgap of diamond, it is transparent to the visible spectrum, making it an attractive material for soft x-ray detection. Response of diamond photoconductors fabricated using Polycrystalline chemical-vapor-deposited (CVD) diamond to soft x-rays has been measured using x-rays emitted from a laser-produced plasma source in the spectral range 125 {Angstrom} to 240 {Angstrom}. These photoconductors have interdigitated electrode structure in order to increase the active area as well as detector sensitivity. Contributions to the detector sensitivity by the photoelectrons is discussed.
Date: December 1, 1995
Creator: Han, S.; Wagner, R.S. & Gullikson, E.
Partner: UNT Libraries Government Documents Department

Radiation hardness of 3HF-tile/O2-WLS-fiber calorimeter

Description: The radiation hardness of a 3HF-tile/O2-WLS-fiber calorimeter with two different tile/fiber patterns has been studied. Two calorimeter modules were irradiated up to 10 Mrad with the BEPC 1.3 GeV electron beam. The radiation damage of these modules is compared with our previous measurements from SCSN81-tile/BCF91A-WLS-fiber modules. The longitudinal damage profiles are fitted as a function of depth.
Date: November 1, 1993
Creator: Han, S. W.; Hu, L. D. & Liu, N. Z.
Partner: UNT Libraries Government Documents Department

Temperature-dependent x-ray diffraction study of Pd/Cu site interchange in non-Fermi liquid UCu(4)Pd

Description: A pair distribution function (PDF) analysis of temperature-dependent x-ray diffraction measurements from UCu{sub 4}Pd is presented. Fits to the displacement parameters (u{sup 2}'s) with a Debye model show better agreement with a model that includes 25% of the Pd atoms on 16$e$ (Cu) sites. In addition, significant non-thermal disorder is observed in the Cu environment, in contrast to previous measurements of local order in the U-Cu pairs.
Date: June 1, 2002
Creator: Han, S.-W.; Booth, C.H.; Bauer, E.D. & Maple, M.B.
Partner: UNT Libraries Government Documents Department

Modeling high-density-plasma deposition of SiO{sub 2} in SiH{sub 4}/O{sub 2}/Ar

Description: The authors have compiled sets of gas-phase and surface reactions for use in modeling plasma-enhanced chemical vapor deposition of silicon dioxide from silane, oxygen and argon gas mixtures in high-density-plasma reactors. They have applied the reaction mechanisms to modeling three different kinds of high-density plasma deposition chambers, and tested them by comparing model predictions to a variety of experimental measurements. The model simulates a well mixed reactor by solving global conservation equations averaged across the reactor volume. The gas-phase reaction mechanism builds from fundamental electron-impact cross section data available in the literature, and also includes neutral-molecule, ion-ion, and ion-molecule reaction paths. The surface reaction mechanism is based on insight from attenuated total-reflection Fourier-transform infrared spectroscopy experiments. This mechanism describes the adsorption of radical species on an oxide surface, ion-enhanced reactions leading to species desorption from the surface layer, radical abstractions competing for surface sites, and direct energy-dependent ion sputtering of the oxide material. Experimental measurements of total ion densities, relative radical densities as functions of plasma operating conditions, and net deposition-rate have been compared to model predictions to test and modify the chemical kinetics mechanisms. Results show good quantitative agreement between model predictions and experimental measurements.
Date: March 1, 1997
Creator: Meeks, E.; Larson, R.S.; Ho, P.; Apblett, C.; Han, S.M.; Edelberg, E. et al.
Partner: UNT Libraries Government Documents Department

Development of More Effective Biosurfactants for Enhanced Oil Recovery

Description: The objectives of this were two fold. First, core displacement studies were done to determine whether microbial processes could recover residual oil at elevated pressures. Second, the importance of biosurfactant production for the recovery of residual oil was studies. In these studies, a biosurfactant-producing, microorganisms called Bacillus licheniformis strain JF-2 was used. This bacterium produces a cyclic peptide biosurfactant that significantly reduces the interfacial tension between oil and brine (7). The use of a mutant deficient in surfactant production and a mathematical MEOR simulator were used to determine the major mechanisms of oil recovery by these two strains.
Date: January 16, 2003
Creator: McInerney, J.J.; Han, S.O.; Maudgalya, S.; Mouttaki, H.; Folmsbee, M.; Knapp, R. et al.
Partner: UNT Libraries Government Documents Department

Lattice disorder and size-induced Kondo behavior in CeAl2 andCePt2+x

Description: When the particle size of CeAl{sub 2} and CePt{sub 2+x} samples is reduced to the nanometer scale, antiferromagnetism is suppressed and Kondo behavior dominates. We find that the Kondo temperature T{sub K} can either decrease (CeAl{sub 2}) or increase (CePt{sub 2+x}) in the nanoparticles relative to the bulk. Extended x-ray absorption fine-structure data show that the Ce-Al and Ce-Pt environments are significantly distorted in the nanoparticles. While such distortions should strongly affect magnetic and electronic properties, we find they cannot explain the observed changes in T{sub K}. Changes in the conduction density of states or other parameters must, therefore, play a significant role.
Date: March 14, 2006
Creator: Han, S.-W.; Booth, C.H.; Bauer, E.D.; P.H., Huang; Chen, Y.Y. & Lawrence, J.M.
Partner: UNT Libraries Government Documents Department

A study of defects on EUV mask using blank inspection, patterned mask inspection, and wafer inspection

Description: The availability of defect-free masks remains one of the key challenges for inserting extreme ultraviolet lithography (EUVL) into high volume manufacturing. yet link data is available for understanding native defects on real masks. In this paper, a full-field EUV mask is fabricated to investigate the printability of various defects on the mask. The printability of defects and identification of their source from mask fabrication to handling were studied using wafer inspection. The printable blank defect density excluding particles and patterns is 0.63 cm{sup 2}. Mask inspection is shown to have better sensitivity than wafer inspection. The sensitivity of wafer inspection must be improved using through-focus analysis and a different wafer stack.
Date: March 12, 2010
Creator: Huh, S.; Ren, L.; Chan, D.; Wurm, S.; Goldberg, K. A.; Mochi, I. et al.
Partner: UNT Libraries Government Documents Department

Lattice disorder and magnetism in f-electron intermetallics

Description: Real materials can have real differences compared to ideal systems. For instance, non-Fermi liquid (NFL) behavior was initially thought to be due to chemical disorder, since the first such materials were all substituted. Although several nominally well-ordered NFL's have been discovered and extensively studied, the effect of disorder on the magnetic properties of f-electron intermetallic systems remains poorly understood. Disorder in NFL systems is reviewed from an experimental, local structure point of view, including a discussion of results on the nominally ordered U{sub 3}Ni{sub 3}Sn{sub 4} and CeCoIn{sub 5} systems, and the chemically disordered UCu{sub 4}Pd and CeRhRuSi{sub 2} systems.
Date: July 29, 2004
Creator: Booth, C.H.; Han, S.-W.; Skanthakumar, S. & Sarrao, J.L.
Partner: UNT Libraries Government Documents Department

Neutron and x-ray scattering studies of (FeF{sub 2}){sub m}(CoF{sub 2}){sub n} multilayers

Description: We have performed elastic neutron scattering measurements of the staggered magnetization in (FeF{sub 2}){sub m}(CoF{sub 2}){sub n} superlattices. Thermal expansion measurements, which are proportional to the magnetic contribution to the specific heat, wwre also carried out using high resolution x-ray diffraction. One of the two measured samples has thickness of m = n = 4.5 and the other m = 26 and n = 28 monolayers, as determined from high angle x-ray {theta} - 2{theta} scans. In the m = n = 4.5 sample, only one transition is observed T{sub N} = 62.9K. Analysis of the neutron data, including the rounding effects, indicates an effective {beta} {approx} 0.42. This does not compare well with the 3D Ising exponent {beta} = 0.325. The X-ray data also shows the existence of only one specific heat anomaly at T = 62.8 K. For the m = 26, n = 28 sample, dips in the staggered magnetization, and peaks in the thermal expansion were observed at T {approx} 40 K and 74 K. The higher temperature anomaly, associated primarily with the FeF{sub 2} layers, is sharper than the lower one, which is presumably rounded by the staggered ordering field improsed by the long range order in the FeF{sub 2} regions on the CoF{sub 2} regions.
Date: June 1, 1993
Creator: Lederman, D.; Belanger, D. P.; Wang, J.; Han, S. J.; Paduani, C. P.; Ramos, C. A. et al.
Partner: UNT Libraries Government Documents Department

Perturbing the superconducting planes in CeCoIn5 by Snsubstitution

Description: In contrast to substitution on the Co or Ce site, Sn substitution has a remarkably strong effect on superconductivity in CeCoIn{sub 5-x}Sn{sub x}, with T{sub c} {yields} 0 beyond only {approx}3.6% Sn. Instead of being randomly distributed on in-plane and out-of-plane In sites, extended x-ray absorption fine structure measurements show the Sn atoms preferentially substitute within the Ce-In plane. This result highlights the importance of the In(1) site to impurity scattering and clearly demonstrates the two-dimensional nature of superconductivity in CeCoIn{sub 5}.
Date: January 11, 2005
Creator: Daniel, M.; Bauer, E.D.; Han, S.-W.; Booth, C.H.; Cornelius,A.L.; Pagliuso, P.G. et al.
Partner: UNT Libraries Government Documents Department

Local structure around Sn in CeCoIn{sub 5-x}Sn{sub x}

Description: The local structure around Sn dopants in CeCoIn{sub 5-x}Sn{sub z} has been probed by extended x-ray absorption fine structure (EXAFS) technique. The fit results for both x = 0.12 and x = 0.18 clearly indicate the dopant Sn atoms predominantly occupying the planar In(1) site. These results are consistent with the quasi-two-dimensional electronic properties of CeCoIn{sub 5} and is discussed in relation to the observed bulk properties.
Date: June 16, 2004
Creator: Daniel, M.; Han, S.-W.; Booth, C.H.; Cornelius, A.L.; Bauer, E.D. & Sarrao, J.L.
Partner: UNT Libraries Government Documents Department

X-ray absorption studies of the local structure and f-level occupancy in CeIr(1-x)Rh(x)In(5)

Description: The CeIr{sub 1-x}Rh{sub x}In{sub 5} series exhibits a range of interesting phenomena, including heavy-fermion superconductivity, non-Fermi liquid behavior, and concomitant antiferromagnetism (AF) and superconductivity (SC). In the low-Rh concentration range (0.1 {ge} x {ge} 0.5), specific heat measurements show a broad anomaly, suggestive of gross phase separation. We have performed x-ray absorption experiments at the Ce L{sub III}, Ir L{sub III}, and Rh K-edges as a function of Rh concentration and temperature. X-ray absorption near-edge structure (XANES) measurements indicate that cerium is close to trivalent in this system, with no measurable change with temperature from 20-300 K, consistent with a heavy-fermion material. Extended x-ray absorption fine structure (EXAFS) measurements as a function of temperature from all measured edges indicate the local crystal structure of all samples is well ordered, with no gross phase separation observed, even for samples with x = 0.125 and x = 0.25. These results therefore suggest that the anomalous specific heat behavior in the 0.1 {ge} x {ge} 0.5 range have some other explanation, and some possibilities are discussed.
Date: April 15, 2004
Creator: Daniel, M.; Han, S.-W.; Booth, C.H.; Cornelius, A.L.; Pagliuso, P.G.; Sarrao, J.L. et al.
Partner: UNT Libraries Government Documents Department

Radiation damage of tile/fiber scintillator modules for the SDC calorimeter

Description: The measurements of radiation damage of tile/fiber scintillator modules to be used for the SDC calorimeter are described. Four tile/fiber scintillator modules were irradiated up to 6 Mrad with the BEPC 1.1 GeV electron beam. We have studied the light output at different depths in the modules and at different integrated doses, the recovery process and the dependence on the ambient atmosphere.
Date: February 1, 1992
Creator: Hu, L.; Liu, N.; Mao, H.; Tan, Y.; Wang, G.; Zhang, C. et al.
Partner: UNT Libraries Government Documents Department

Radiation damage of tile/fiber scintillator modules for the SDC calorimeter

Description: The measurements of radiation damage of tile/fiber scintillator modules to be used for the SDC calorimeter are described. Four tile/fiber scintillator modules were irradiated up to 6 Mrad with the BEPC 1.1 GeV electron beam. We have studied the light output at different depths in the modules and at different integrated doses, the recovery process and the dependence on the ambient atmosphere.
Date: February 1, 1992
Creator: Hu, L.; Liu, N.; Mao, H.; Tan, Y.; Wang, G.; Zhang, C. et al.
Partner: UNT Libraries Government Documents Department