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Search Results: 1 - 10 of 230802 matches for " Walter R. Cook "
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Properties of Pt Schottky Type Contacts On High-Resistivity CdZnTe Detectors
Aleksey E. Bolotnikov,Steven E. Boggs,C. M. Hubert Chen,Walter R. Cook,Fiona A. Harrison,Stephen M. Schindler
Physics , 2001, DOI: 10.1016/S0168-9002(01)01506-6
Abstract: In this paper we present studies of the I-V characteristics of CdZnTe detectors with Pt contacts fabricated from high-resistivity single crystals grown by the high-pressure Brigman process. We have analyzed the experimental I-V curves using a model that approximates the CZT detector as a system consisting of a reversed Schottky contact in series with the bulk resistance. Least square fits to the experimental data yield 0.78-0.79 eV for the Pt-CZT Schottky barrier height, and <20 V for the voltage required to deplete a 2 mm thick CZT detector. We demonstrate that at high bias the thermionic current over the Schottky barrier, the height of which is reduced due to an interfacial layer between the contact and CZT material, controls the leakage current of the detectors. In many cases the dark current is not determined by the resistivity of the bulk material, but rather the properties of the contacts; namely by the interfacial layer between the contact and CZT material.
Effects of Bulk and Surface Conductivity on the Performance of CdZnTe Pixel Detectors
Aleksey E. Bolotnikov,C. M. Hubert Chen,Walter R. Cook,Fiona A. Harrison,Irfan Kuvvetli,Stephen M. Schindler
Physics , 2001, DOI: 10.1109/TNS.2002.801673
Abstract: We studied the effects of bulk and surface conductivity on the performance of high-resistivity CdZnTe (CZT) pixel detectors with Pt contacts. We emphasize the difference in mechanisms of the bulk and surface conductivity as indicated by their different temperature behaviors. In addition, the existence of a thin (10-100 A) oxide layer on the surface of CZT, formed during the fabrication process, affects both bulk and surface leakage currents. We demonstrate that the measured I-V dependencies of bulk current can be explained by considering the CZT detector as a metal-semiconductor-metal system with two back-to-back Schottky-barrier contacts. The high surface leakage current is apparently due to the presence of a low-resistivity surface layer that has characteristics which differ considerably from those of the bulk material. This surface layer has a profound effect on the charge collection efficiency in detectors with multi-contact geometry; some fraction of the electric field lines originated on the cathode intersects the surface areas between the pixel contacts where the charge produced by an ionizing particle gets trapped. To overcome this effect we place a grid of thin electrodes between the pixel contacts; when the grid is negatively biased, the strong electric field in the gaps between the pixels forces the electrons landing on the surface to move toward the contacts, preventing the charge loss. We have investigated these effects by using CZT pixel detectors indium bump bonded to a custom-built VLSI readout chip.
A Conservative’s View from the Academic Trenches: Reply to Duarte, Crawford, Stern, Haidt, Jussim, and Tetlock (2015)  [PDF]
Walter R. Schumm
Journal of Behavioral and Brain Science (JBBS) , 2016, DOI: 10.4236/jbbs.2016.64017
Abstract: Although conservative scholars may face a variety of forms of discrimination in academia and other challenges, as elaborated in the first part of this comment, they may also have a set of unique advantages that may facilitate more careful theoretical and empirical scientific work. They may be more sensitive to flawed methodologies in some areas of controversy. In such areas, these assets of conservative scholars may be especially important.
Spectral calibration and modeling of the NuSTAR CdZnTe pixel detectors
Takao Kitaguchi,Brian W. Grefenstette,Fiona A. Harrison,Hiromasa Miyasaka,Varun B. Bhalerao,Walter R. Cook III,Peter H. Mao,Vikram R. Rana,Steven E. Boggs,Andreas C. Zoglauer
Physics , 2011, DOI: 10.1117/12.896972
Abstract: The Nuclear Spectroscopic Telescope Array (NuSTAR) will be the first space mission to focus in the hard X-ray (5-80 keV) band. The NuSTAR instrument carries two co-aligned grazing incidence hard X-ray telescopes. Each NuSTAR focal plane consists of four 2 mm CdZnTe hybrid pixel detectors, each with an active collecting area of 2 cm x 2 cm. Each hybrid consists of a 32 x 32 array of 605 micron pixels, read out with the Caltech custom low-noise NuCIT ASIC. In order to characterize the spectral response of each pixel to the degree required to meet the science calibration requirements, we have developed a model based on Geant4 together with the Shockley-Ramo theorem customized to the NuSTAR hybrid design. This model combines a Monte Carlo of the X-ray interactions with subsequent charge transport within the detector. The combination of this model and calibration data taken using radioactive sources of Co-57, Eu-155 and Am-241 enables us to determine electron and hole mobility-lifetime products for each pixel, and to compare actual to ideal performance expected for defect-free material.
The M81 Group Dwarf Irregular Galaxy DDO 165. I. High Velocity Neutral Gas in a Post-Starburst System
John M. Cannon,Hans P. Most,Evan D. Skillman,Daniel R. Weisz,David Cook,Andrew E. Dolphin,Robert C. Kennicutt, Jr.,Janice Lee,Anil Seth,Fabian Walter,Steven R. Warren
Physics , 2011, DOI: 10.1088/0004-637X/735/1/35
Abstract: We present new multi-configuration VLA HI spectral line observations of the M81 group dIrr post-starburst galaxy DDO 165. The HI morphology is complex, with multiple column density peaks surrounding a large region of very low HI surface density that is offset from the center of the stellar distribution. The bulk of the neutral gas is associated with the southern section of the galaxy; a secondary peak in the north contains ~15% of the total HI mass. These components appear to be kinematically distinct, suggesting that either tidal processes or large-scale blowout have recently shaped the ISM of DDO 165. Using spatially-resolved position-velocity maps, we find multiple localized high-velocity gas features. Cross-correlating with radius-velocity analyses, we identify eight shell/hole structures in the ISM with a range of sizes (~400-900 pc) and expansion velocities (~7-11 km/s). These structures are compared with narrow- and broad-band imaging from KPNO and HST. Using the latter data, recent works have shown that DDO 165's previous "burst" phase was extended temporally (>1 Gyr). We thus interpret the high-velocity gas features, HI holes, and kinematically distinct components of the galaxy in the context of the immediate effects of "feedback" from recent star formation. In addition to creating HI holes and shells, extended star formation events are capable of creating localized high velocity motion of the surrounding interstellar material. A companion paper connects the energetics from the HI and HST data.
The M81 Group Dwarf Irregular Galaxy DDO 165. II. Connecting Recent Star Formation with ISM Structures and Kinematics
John M. Cannon,Hans P. Most,Evan D. Skillman,Daniel R. Weisz,David Cook,Andrew E. Dolphin,Robert C. Kennicutt, Jr.,Janice Lee,Anil Seth,Fabian Walter,Steven R. Warren
Physics , 2011, DOI: 10.1088/0004-637X/735/1/36
Abstract: We compare the stellar populations and complex neutral gas dynamics of the M81 group dIrr galaxy DDO 165 using data from the HST and the VLA. Paper I identified two kinematically distinct HI components, multiple localized high velocity gas features, and eight HI holes and shells (the largest of which spans ~2.2x1.1 kpc). Using the spatial and temporal information from the stellar populations in DDO 165, we compare the patterns of star formation over the past 500 Myr with the HI dynamics. We extract localized star formation histories within 6 of the 8 HI holes identified in Paper I, as well as 23 other regions that sample a range of stellar densities and neutral gas properties. From population synthesis modeling, we derive the energy outputs (from stellar winds and supernovae) of the stellar populations within these regions over the last 100 Myr, and compare with refined estimates of the energies required to create the HI holes. In all cases, we find that "feedback" is energetically capable of creating the observed structures in the ISM. Numerous regions with significant energy inputs from feedback lack coherent HI structures but show prominent localized high velocity gas features; this feedback signature is a natural product of temporally and spatially distributed star formation. In DDO 165, the extended period of heightened star formation activity (lasting more than 1 Gyr) is energetically capable of creating the observed holes and high velocity gas features in the neutral ISM.
Preliminary Studies on the Hydracarina of Michigan: the Genus Limnesia,Subgenera Limnesiella Piersig and Limnesiella
David R. Cook
Psyche , 1954, DOI: 10.1155/1954/95982
Testing predictor contributions in sufficient dimension reduction
R. Dennis Cook
Mathematics , 2004, DOI: 10.1214/009053604000000292
Abstract: We develop tests of the hypothesis of no effect for selected predictors in regression, without assuming a model for the conditional distribution of the response given the predictors. Predictor effects need not be limited to the mean function and smoothing is not required. The general approach is based on sufficient dimension reduction, the idea being to replace the predictor vector with a lower-dimensional version without loss of information on the regression. Methodology using sliced inverse regression is developed in detail.
A smooth surface in P4 not of general type has degree at most 66
R. Braun,M. Cook
Mathematics , 1997,
Abstract: We continue the work of Braun and Floystad, and Cook bounding the degree of smooth surfaces in P4 not of general type using generic initial ideal theory.
Rejoinder: Fisher Lecture: Dimension Reduction in Regression
R. Dennis Cook
Statistics , 2007, DOI: 10.1214/088342307000000078
Abstract: Rejoinder: Fisher Lecture: Dimension Reduction in Regression [arXiv:0708.3774]
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