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Caltech Authors (162.509 recursos)

Repository of works by Caltech published authors.

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Mostrando recursos 1 - 20 de 1.876

  1. Lean Crowdsourcing: Combining Humans and Machines in an Online System

    Branson, Steve; Van Horn, Grant; Perona, Pietro
    We introduce a method to greatly reduce the amount of redundant annotations required when crowdsourcing annotations such as bounding boxes, parts, and class labels. For example, if two Mechanical Turkers happen to click on the same pixel location when annotating a part in a given image-an event that is very unlikely to occur by random chance-, it is a strong indication that the location is correct. A similar type of confidence can be obtained if a single Turker happened to agree with a computer vision estimate. We thus incrementally collect a variable number of worker annotations per image based on...

  2. Seeing into Darkness: Scotopic Visual Recognition

    Chen, Bo; Perona, Pietro
    Images are formed by counting how many photons traveling from a given set of directions hit an image sensor during a given time interval. When photons are few and far in between, the concept of image breaks down and it is best to consider directly the flow of photons. Computer vision in this regime, which we call scotopic, is radically different from the classical image-based paradigm in that visual computations (classification, control, search) have to take place while the stream of photons is captured and decisions may be taken as soon as enough information is available. The scotopic regime is...

  3. Batch and Recursive Bayesian Estimation Methods for Nonlinear Structural System Identification

    Astroza, Rodrigo; Ebrahimian, Hamed; Conte, Joel P.
    This chapter presents a framework for the identification of nonlinear finite element (FE) structural models using Bayesian inference methods. Using the input-output dynamic data recorded during an earthquake event, batch and recursive Bayesian estimation methods are employed to update a mechanics-based nonlinear FE model of the structure of interest (building, bridge, dam, etc.). Unknown parameters of the nonlinear FE model characterizing material constitutive models, inertia, geometric, and/or constraint properties of the structure can be estimated using limited response data recorded through accelerometers or heterogeneous sensor arrays. The updated nonlinear FE model can be used to identify the damage in the...

  4. Imaging carrier dynamics on the surface of the N-type silicon

    Najafi, Ebrahim
    The nonequilibrium dynamics of carriers in semiconductors plays a major role in the performance and efficiency of the electronic and photovoltaic devices. In this study, we use the scanning ultrafast electron microscopy (SUEM) technique to study the surface photovoltage dynamics in doped silicon samples. We observe that the optical excitation of lightly doped n-type and p-type silicon as well as heavily doped n-type silicon increases the electron density on the surface. In contrast, the optical excitation of heavily doped p-type silicon increases the hole density on the surface. Furthermore, we show that the rise and the decay timescales of these...

  5. Development and production of hard X-ray multilayer optics for HEFT

    Koglin, Jason E.; Christensen, Finn E.; Chonko, Jim; Craig, William W.; Decker, Todd R.; Jimenez-Garate, Mario A.; Gunderson, Kurt; Hailey, Charles J.; Harrison, Fiona A.; Jensen, Carsten P.; Sileo, Mike; Windt, David L.; Yu, Haitao
    The High Energy Focusing Telescope (HEFT) will observe a wide range of objects including young supernova remnants, active galactic nuclei, and galaxy clusters at energies between 20 and 70 keV. Large collecting areas are achieved by tightly nesting layers of grazing incidence mirrors in a conic approximation Wolter-I design. The segmented mirrors that form these layers are made of thermally formed glass substrates coated with depth-graded multilayer films for enhanced reflectivity. The mirrors are assembled using an over-constraint method that forces the overall shape of the nominally cylindrical substrates to the appropriate conic form. We will present performance data on...

  6. W/SiC X-ray multilayers optimized for use above 100 keV

    Windt, David L.; Donguy, Soizik; Hailey, Charles J.; Koglin, Jason; Honkimaki, Veijo; Ziegler, Eric; Christensen, Finn E.; Chen, C. M. Hubert; Harrison, Fiona A.; Craig, William W.
    We have developed a new depth-graded multilayer system comprising W and SiC layers, suitable for use as hard X-ray reflective coatings operating in the energy range 100 - 200 keV. Grazing incidence X-ray reflectance at E=8 keV was used to characterize the interface widths, as well as the temporal and thermal stability in both periodic and depth-graded W/SiC structures, while synchrotron radiation was used to measure the hard X-ray reflectance of a depth-graded multilayer designed specifically for use in the range E~150 - 170 keV. We have modeled the hard X-ray reflectance using newly-derived optical constants, which we determined from...

  7. Effects of carbon coat thickness and contamination on quantitative analysis: a new look at an old problem

    Armstrong, John T.
    Due to considerable improvements in electronic stability and detector sensitivity, modern electron microprobes and SEMs have the capability of routinely making x-ray measurements much more precisely than previously possible (often with a precision of as low as a few parts per thousand). The improvements in measurement precision, however, have not necessarily resulted in significant improvements in analytical accuracy. Factors that were once considered secondary now have may have significant importance in the accuracy of high-precision analyses. Among these is the variability in thickness of the carbon (or other conducting element) film evaporated or sputtered on insulating samples and standards. In this paper,...

  8. X-ray multilayer coatings for use at energies above 100 keV

    Windt, D.; Christensen, F.; Craig, W.; Hailey, C.; Harrison, F.; Jimenez-Garate, M.; Kalyanaraman, R.; Mao, P.
    We discuss the development of X-ray multilayer coatings for use as broad-band reflectors operating at energies above 100 keV. Such coatings can be used to produce hard X-ray telescopes that will make possible a variety of entirely new astronomical observations. We summarize our recent investigation into the growth, structure and hard X-ray performance of depth-graded W/Si multilayers, present follow-up information on Cu/Si multilayers, and discuss preliminary results obtained with Ni/Si, Ni._8Cr._2/Si, and Ni._(93)V._(07)/Si multilayers.

  9. High-resolution spectroscopic imaging (HSI) mission

    Harrison, F. A.; Boggs, S. E.; Christensen, F. E.; Gehrels, N.; Grindlay, J. E.; Chen, C. M. H.; Craig, W. W.; Hailey, C. J.; Pinto, P.; Thorsett, S. E.; Tueller, J.; Windt, D.; Woosley, S. E.
    The High-Resolution Spectroscopic Imaging Mission is designed to be the first instrument to make true images of the hard X-ray/soft gamma-ray (2 - 600 keV) sky. By focusing energetic X-rays and low-energy gamma-rays, HSI will observe the cosmos with an unprecedented combination of sensitivity, spectral resolution, and angular resolving power. HSI is based on an array of multilayer grazing-incidence optics focusing onto high-resolution solid-state germanium pixel detectors with a focal length of 30-50 m. This paper describes the primary scientific objectives, technical approach to the instrumentation, and mission design.

  10. Investigation optimal contact geometry for CdZnTe pixel detectors

    Bolotnikov, Aleksey E.; Boggs, Steven E.; Chen, C. M. Hubert; Cook, Walter R.; Harrison, Fiona A.; Schindler, Steven M.
    We are developing CdZnTe pixel detectors for use as focal plane sensors in astronomical hard X-ray telescopes. To optimize the spectral response and imaging performance, we are investigating the effect of contact geometry on charge collection. Specifically, we have studied contact designs with orthogonal thin strips placed between pixel contacts. We apply a negative bias on the grid with respect to the pixel potential to force charge to drift toward the contacts. The grid bias is selected to be just sufficient to avoid charge collection on the grid, while increasing the transverse electric field on the surface between contacts. In...

  11. The cosmic gamma-ray bursts and their host galaxies in a cosmological context

    Djorgovski, Stan G.; Kulkarni, Shrinivas R.; Frail, Dale A.; Harrison, Fiona A.; Bloom, Joshua S.; Berger, Edo; Price, P. A.; Fox, D.; Soderberg, A. M.; Galama, Titus J.; Reichart, D. E.; Sari, Re'em; Yost, S.; Mahabal, Ashish; Castro, S. M.; Goodrich, Robert; Chaffee, F.
    Studies of the cosmic gamma-ray bursts (GRBs) and their host galaxies are now starting to provide interesting or even unique new insights in observational cosmology. Observed GRB host galaxies have a median magnitude R~25 mag, and show a range of luminosities, morphologies, and star formation rates, with a median redshift z~1. They represent a new way of identifying a population of star-forming galaxies at cosmological redshifts, which is mostly independent of the traditional selection methods. They seem to be broadly similar to the normal field galaxy populations at comparable redshifts and magnitudes, and indicate at most a mild luminosity evolution...

  12. Development of uniform CdTe pixel detectors based on Caltech ASIC

    Oonuki, Kousuke; Inoue, Hokuto; Nakazawa, Kazuhiro; Mitani, Takefumi; Tanaka, Takaaki; Takahashi, Tadayuki; Chen, C. M. H.; Cook, Walter R.; Harrison, Fiona A.
    We have developed a large CdTe pixel detector with dimensions of 23.7 x 13.0 mm^2 and a pixel size of 448 x 448 μm^2. The detector is based on recent technologies of an uniform CdTe single crystal, a two-dimensional ASIC, and stud bump-bonding to connect pixel electrodes on the CdTe surface to the ASIC. Good spectra are obtained from 1051 pixels out of total 1056 pixels. When we operate the detector at -50°C, the energy resolution is 0.67 keV and 0.99 keV at 14 keV and 60 keV, respectively. Week-long stability of the detector is confirmed at operating temperatures of...

  13. Development of CdZnTe pixel detectors for astrophysical applications

    Harrison, F. A.; Boggs, S. E.; Bolotnikov, A.; Chen, C. M. H.; Cook, W. R.; Schindler, S. M.
    Over the last four years we have been developing imaging Cadmium Zinc Telluride pixel detectors optimized for astrophysical focusing hard X-ray telescopes. This application requires sensors with modest area (~2cm X 2 cm), relatively small (≾ 500µm) pixels and sub-keV energy resolution. For experiments operating in satellite orbits, low energy thresholds of ~1 - 2 keV are also desirable. In this paper we describe the desired detector performance characteristics, and report on the status of our development effort. In particular, we present results from a prototype sensor with a custom low- noise VLSI readout designed to achieve excellent spectral resolution...

  14. First test results from a high-resolution CdZnTe pixel detector with VLSI readout

    Cook, Walter R.; Boggs, Steven E.; Bolotnikov, Aleksey E.; Burnham, Jill A.; Harrison, Fiona A.; Kecman, Branislav; Matthews, Brian W.; Schindler, Steven M.; Fitzsimmons, Michael
    We are developing a CdZnTe pixel detector with a custom low- noise analog VLSI readout for use in the High-Energy Focusing Telescope balloon experiment, as well as for future space astronomy applications. The goal of the program is to achieve good energy resolution (< 1 keV FWHM at 60 keV) and low threshold in a sensor with approximately 500 micrometers pixels. We have fabricated several prototype detector assemblies with 2 mm thick, 680 by 650 micrometers pitch CdZnTe pixel sensors indium bump bonded a VLSI readout chip developed at Caltech. Each readout circuit in the 8 X 8 prototype is...

  15. X-ray reflectivity and mechanical stress in W/Si multilayers deposited on thin substrates of glass, epoxy-replicated aluminum foil, and Si wafer

    Platonov, Yuriy Ya.; Broadway, David; DeGroot, Brian; Mao, Peter; Harrison, Fiona; Gutman, George; Rodriguez, Jim
    Reflectivity at λ = 0.154 nm and mechanical stress in the bulk thin films of tungsten and silicon and single d- spacing multilayers on their basis with d approximately equals 2.8 nm deposited by the magnetron sputtering technique on flat thin substrates of Si wafer (~ 0.2 mm), glass (~ 0.3 mm), and epoxy gold replicated aluminum foil (~ 0.3 mm) have been studied. The interfacial roughness of the multilayers has been calculated from the x- ray reflectivity curves as the following: on Si wafer σ ≃ 0.31 nm, on glass σ ≃ 0.32 nm, and on foil σ ≃...

  16. CdZnTe background measurement at balloon altitudes with an active BGO shield

    Bloser, P.; Grindlay, J.; Narita, T.; Harrison, F.
    We report results of an experiment conducted in May 1997 to measure CdZnTe background and background reduction schemes in space flight conditions similar to those of proposed hard x-ray astrophysics missions. A 1 cm^2 CdZnTe detector was placed adjacent to a thick BGO anticoincidence shield and flown piggy backed onto the EXITE2 scientific balloon payload. The planar shield was designed to veto background counts produced by local gamma-ray production in passive material and neutron interactions in the detector. The CdZnTe and BGO were partially surrounded by a Pb-Sn-Cu shield to approximate the grammage of an x-ray collimator, although the field...

  17. Hard x-ray characterization of a HEFT single-reflection prototype

    Christensen, Finn E.; Craig, William W.; Hailey, Charles J.; Jimenez-Garate, Mario A.; Windt, David L.; Harrison, Fiona A.; Mao, Peter H.; Ziegler, Eric; Honkimaki, Veijo; S. del Rio, Manuel; Freund, Andreas K.; Ohler, Michael
    We have measured the hard X-ray reflectivity and imaging performance from depth graded W/Si multilayer coated mirror segments mounted in a single reflection cylindrical prototype for the hard X-ray telescopes to be flown on the High Energy Focusing Telescope (HEFT) balloon mission. Data have been obtained in the energy range from 18 - 170 keV at the European Synchrotron Radiation Facility and at the Danish Space Research Institute at 8 keV. The modeling of the reflectivity data demonstrate that the multilayer structure can be well described by the intended power law distribution of the bilayer thicknesses optimized for the telescope...

  18. X-ray scatter measurements from thermally slumped thin glass substrates for the HEFT hard x-ray telescopes

    Hussain, Ahsen M.; Christensen, Finn E.; Jimenez-Garate, Mario A.; Craig, William W.; Hailey, Charles J.; Decker, Todd R.; Stern, Marcela; Windt, David L.; Mao, Peter H.; Harrison, Fiona A.; Pareschi, Giovanni; Sanchez del Rio, Manuel; Souvorov, Alexei; Freund, Andreas K.; Tucoulou, Remi; Madsen, Anders; Mammen, Christian
    We have performed x-ray specular reflectivity and scattering measurements of thermally slumped glass substrates on x-ray diffractometers utilizing a rotating anode x-ray source at the Danish Space Research Institute (DSRI) and synchrotron radiation at the European Synchrotron Radiation Facility (ESRF) optics Bending Magnet beamline. In addition, we tested depth graded W/Si multilayer-coated slumped glass using x-ray specular reflectivity measurements at 8.048 keV and 28 keV and energy-dispersive measurements in the 20-50 keV rang at a double-axis diffractometer at the Orsted Laboratory, University of Copenhagen. The thermally slumped glass substrates will be used to fabricate the hard x-ray grazing incidence optics...

  19. Use of a pulsed laser to study properties of CdZnTe pixel detectors

    Bolotnikov, A. E.; Boggs, S. E.; Cook, W. R.; Harrison, F. A.; Schindler, S. M.
    We have investigated the utility of employing a short (<4 ns) pulsed laser with wavelength tunable between 600 - 950 nm as a tool for studying and characterizing CdZnTe detectors. By using a single mode optical fiber and simple optics, we can focus the beam to a spot size of less than 10 micrometers and generate the number of the excess carriers equivalent to a several MeV gamma-ray either at the surface or deep inside the sample. The advantages of this technique over use of a collimated X-ray or alpha particle source are strong induced signal, precise pointing, and triggering...

  20. Development of precision hard x-ray multilayer optics with sub-arcminute performance

    Koglin, Jason E.; Chen, Hubert C.; Christensen, Finn E.; Chonko, Jim; Craig, William W.; Decker, Todd R.; Jimenez-Garate, Mario A.; Hailey, Charles J.; Harrison, Fiona A.; Jensen, Carsten P.; Sileo, Mike; Windt, David L.; Yu, Haitao
    A new generation of hard X-ray telescopes using focusing optics are poised to dramatically improve the sensitivity and angular resolution at energies above 10 keV to levels that were previously unachievable by the past generation of background-limited collimated and coded-aperture instruments. Active balloon programs (HEFT, InFocus), possible Explorer-class satellites, and major X-ray observatories (Constellation-X, XEUS) using focusing optics will play a major role in future observations of a wide range of objects including young supernova remnants, active galactic nuclei, and galaxy clusters. These instruments call for grazing incidence optics coated with depth-graded multilayer films to achieve large collecting areas. To...

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