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Nuclear Science, IEEE Transactions on

Issue 4 • Date Aug. 2009

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Displaying Results 1 - 25 of 36
  • [Front cover]

    Page(s): C1
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    Freely Available from IEEE
  • IEEE Transactions on Nuclear Science publication information

    Page(s): C2
    Save to Project icon | Request Permissions | PDF file iconPDF (39 KB)  
    Freely Available from IEEE
  • Table of contents

    Page(s): 1697 - 1699
    Save to Project icon | Request Permissions | PDF file iconPDF (88 KB)  
    Freely Available from IEEE
  • Fast Neutron Damage of a Pixelated CdZnTe Gamma Ray Spectrometer

    Page(s): 1700 - 1705
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (627 KB) |  | HTML iconHTML  

    This study describes the damage to a pixelated 1 cm times 1 cm times 1 cm CdZnTe detector caused by fast neutrons in the energy range 1-7 MeV. Measurements of electron mutau product were performed before and after irradiation and also following thermal annealing. Spectroscopic information was acquired with a 133Ba gamma source. Before neutron irradiation, sharp peaks were observed for the 133Ba lines, with combined pixel energy resolution for the 356 keV line of 2.1% FWHM. After irradiation, clear deterioration of the spectral response is observed, with combined energy resolution of the 356 keV line of about 6% FWHM. After thermal annealing, the spectral response shows significant recovery, with combined energy resolution of 2.3% FWHM. We successfully simulate these results with a theoretical model. It is concluded that the fast neutrons cause displacement damage in the lattice of the bulk CdZnTe material. View full abstract»

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  • Evaluation of the Concentration of Deep Levels in Semi-Insulating CdTe by Photoconductivity and TEES

    Page(s): 1706 - 1711
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    The concentrations of near-midgap levels in high-resistivity CdTe were estimated based on a combined evaluation of room temperature lux-ampere characteristics and thermoelectric effect spectroscopy measurements (77-400 K). A simulation of experimental data was performed by a numerical solution of drift-diffusion and Poisson equations using a model with two deep levels. A comparison of crystals doped with shallow and deep dopants is given. View full abstract»

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  • Evaluation of Surface Recombination Velocity on CdTe Radiation Detectors by Time-of-Flight Measurements

    Page(s): 1712 - 1716
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (232 KB) |  | HTML iconHTML  

    The surface recombination velocity on high-resistivity CdTe with several different crystallographic orientations-(111), 5deg off from (111), 8deg off from (311), and (511)-has been investigated by using a ldquomutau-modelrdquo spectral fitting method in combination with time-of-flight drift mobility measurement. In the samples orientated parallel to (111) and 5deg off from (111), the Cd face exhibits a higher surface recombination velocity ( ~ 6 times 105 cm/s) than the Te face ( ~ 3 times 105 cm/s). Away from the polar face and toward the (511) face, the difference is less pronounced, although not completely absent. View full abstract»

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  • THM Growth and Characterization of 100 mm Diameter CdTe Single Crystals

    Page(s): 1717 - 1723
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (2037 KB) |  | HTML iconHTML  

    The THM growth technology for 100 mm diameter CdTe single crystals has been intensively developed. In consequence of the optimization of growth conditions, we have succeeded in controlling growth interface shape and growing 100 mm diameter CdTe single crystals with 300 mm in length. Concerning the behavior of Te inclusions in the grown crystal, their size and density were investigated by IR transmission microscopy. The size distribution of Te inclusions was divided into two groups, and the density of them was less than 1 times 105 cm-3. Charge transport properties of the grown crystal were investigated by using the ldquomutau-modelrdquo spectral fitting method, and were found to be quite uniform all over the wafer. To investigate the homogeneity of radiation detector performances, Ohmic type and Schottky type detectors with 4 mm times 4 mm times 1 mm were fabricated from the left half and the right half of the 100 mm diameter wafer, respectively. Standard deviations of their energy resolutions for the 122 keV line from 57Co were less than 6%. This excellent uniformity is essential for the room temperature semiconductor detectors in the major application areas, such as medical imaging, non-destructive inspection and homeland security. View full abstract»

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  • Compensation and Photosensitivity in CdTe Doped With Indium

    Page(s): 1724 - 1730
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (452 KB) |  | HTML iconHTML  

    To better our knowledge of the characteristics of semi-insulated cadmium telluride (CdTe) doped with indium (In), we explored the role of deep levels in compensation and trapping. We assessed the defects and their distribution across a wafer in several ways; by measuring dark resistivity and photosensitivity maps, photoluminescence, Photo-Induced Current Transient Spectroscopy (PICTS), and Thermoelectric Effect Spectroscopy (TEES). We determined that electron trapping to a near midgap level in CdTe:In begins when the Fermi-level lies above this level. We demonstrated first that a small movement (ap 1divide2 kT) of the Fermi-level downward significantly increases electron trapping. PICTS and TEES measurements confirmed the presence of a positively charge electron trap at E C-0.65 eV (plusmn 0.05 eV) with a high capture cross-section. This level transforms into a neutral one when the Fermi-level moves above it. Photoluminescence measurements detected this energy level that, when positively charged, was responsible for a 0.68-eV emission, while in a neutral state, it was accountable for an emission peak located at 0.87 eV. We discuss the nature of the deep donors, considering the latest ldquoab initiordquo calculations: also, the Te anti-site is compared to complex defects, such as H-OCd. View full abstract»

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  • Electrical Properties of Iodine-Doped CdTe Epitaxial Films on Si Substrates Grown by MOVPE

    Page(s): 1731 - 1735
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (430 KB) |  | HTML iconHTML  

    We studied the effect of iodine doping of CdTe films on Si substrates grown by MOVPE at different growth conditions. A high resistivity film was obtained by adjusting the growth temperature, Te/Cd precursor flow ratio and the dopant flow-rate. Our results show the film resistivity does not change linearly with the dopant flow-rate. The resistivity remains low and similar to that of undoped value for low dopant flow-rate, but increases abruptly when the dopant flow-rate is increased beyond certain value. Photoluminescence measurements of the films confirmed the dopant incorporation in the crystal. We explained the result of resistivity change due to the compensation of excess shallow acceptors by deep intrinsic donors. View full abstract»

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  • Study on Instability Phenomena in CdTe Diode-Like Detectors

    Page(s): 1736 - 1742
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (597 KB) |  | HTML iconHTML  

    Diode-like In/CdTe/Pt detectors are widely used thanks to their excellent spectroscopic performance. However, when operated at room temperature they are not stable, and their performance degrades with time. The aim of this paper is to investigate in detail the physical mechanisms underlying this effect, by studying the evolution of the space charge inside the detector. Our approach makes use of the Pockels effect, by looking at the evolution of the electric field distribution inside the detector at different temperatures. The results show that a negative space charge accumulation occurs at the anode and that the process is thermally activated. The effect is attributed to a midgap acceptor which, under reverse bias, increases its ionization because of hole detrapping. Moreover, we compare the results with those obtained on stable detectors realized from nominally same crystals but having ldquostandard,rdquo i.e., not rectifying contacts. Consistently with the detection performance stability of this kind of detectors, the electric field is relatively stable. Furthermore, it decreases from cathode and it shows peculiar features which will be discussed in conjunction with charge transport properties. View full abstract»

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  • Boron Oxide Encapsulated Vertical Bridgman Grown CdZnTe Crystals as X-Ray Detector Material

    Page(s): 1743 - 1746
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (863 KB) |  | HTML iconHTML  

    Two-inch-diameter CdZnTe crystals doped with indium were grown by the boron oxide encapsulated vertical Bridgman technique. The crystals showed large single crystalline yield and low etch pit density. The background impurity content was dominated by boron in concentration lower than 1 ppm. High resistivity was obtained and a procedure for contact preparation was developed. The mobility-lifetime product of the material was determined by both X-ray irradiation and photocurrent spectroscopy. The X-ray detector prepared with this material showed good spectroscopic performance. View full abstract»

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  • Dewetting During the Crystal Growth of (Cd,Zn)Te:In Under Microgravity

    Page(s): 1747 - 1751
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1423 KB) |  | HTML iconHTML  

    The phenomenon of ldquodewettingrdquo associated with the Vertical Bridgman (VB) crystal growth technique leads to the growth of a crystal without contact with the crucible. One dramatic consequence of this modified VB process is the reduction of structural defects within the crystal. It has been observed in several microgravity experiments for different semiconductor crystals. This work is concentrated on the growth of high resistivity (Cd,Zn)Te:In (CZT) crystals by achieving the phenomenon of dewetting under microgravity condition and its application in the processing of CZT detectors. Two Cd0.9Zn0.1Te:In crystals were grown in space on the Russian FOTON satellite in the POLIZON-M facility in September 2007 (mission M3). At the end of the preliminary melting phase of one crystal, a Rotating Magnetic Field (RMF) was applied in order to reduce the typical tellurium clusters within the melt before the pulling. The other crystal was superheated with 20 K above the melting point before the pulling. A third reference crystal has been grown on the ground in similar thermal conditions. Profiles measurements of the space grown crystals surface gave the evidence of a successful dewetting during the crystal growth. Characterization methods such as IR microscopy and CoReMa have been performed on the three crystals. CZT detectors have been processed from the grown part of the different crystals. The influence of the dewetting on the material quality and the detector properties completes the study. View full abstract»

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  • Electromigration of Mobile Defects in CdTe

    Page(s): 1752 - 1757
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (225 KB) |  | HTML iconHTML  

    Electromigration of mobile charged defects in external electric field is investigated at various temperatures and biases in conductive undoped and semiinsulating In-doped CdTe, respectively. A set of electric contacts as potential probes arranged linearly along the sample was used for the detection of the drift of the local resistance modulation. The observed modulation drifting along the sample always from the positive toward negative contact after step-like bias polarity reversion points to the migration of positively charged point defects. Mobility and diffusion coefficient of mobile defects at 100degC and 600degC, respectively are determined. Electromigration of point defects is also tested by low temperature photoluminescence and a model explaining migration of charged defects is suggested. View full abstract»

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  • Preparation of Inclusion and Precipitate Free Semi-Insulating CdTe

    Page(s): 1758 - 1762
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (279 KB) |  | HTML iconHTML  

    Annealing conditions used for the preparation of semi-insulating CdTe after elimination of inclusions and precipitates are discussed. Second phase defects are eliminated by post-growth stoichiometric annealing and the semi-insulating CdTe is prepared by re-annealing under Te or Cd overpressure. In case of In-doped samples with high In concentration, semi-insulating CdTe with resistivity approximately 109 Omegacm was prepared by annealing under Te overpressure. In-situ high temperature measurement of the electrical conductivity and Hall coefficient is used to find annealing conditions for undoped and slightly In-doped CdTe. The dominant extrinsic acceptor level with a concentration of NA = 4 times 1015 cm-3 was determined and Cd-rich annealing was used for the preparation of semi-insulating CdTe. View full abstract»

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  • Multi-Species Diffusion in CdTe

    Page(s): 1763 - 1767
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (181 KB) |  | HTML iconHTML  

    We studied theoretically chemical self-diffusion and the diffusion of extrinsic atoms in CdTe. We compiled a general model describing the multi-species diffusion of arbitrary amounts of elements in a form optimized for numerical calculations and applied it to a model system of CdTe doped with slow- or fast-diffusing elements. The diffusion of slowly diffusing atoms was analyzed and compared with experimental findings. We uncovered possible drawbacks in the experimental data that might affect the ensuing analysis by researchers, so generating incorrect conclusions. We suggest a method of purifying CdTe from fast-diffusing impurities based on a proper annealing/etching process. View full abstract»

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  • Investigation of Growth Conditions of CdTe Thick Films on Properties and Demands for X-Ray Detector Applications

    Page(s): 1768 - 1774
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (5471 KB) |  | HTML iconHTML  

    CdTe thick films were prepared by vacuum deposition on amorphous substrates using MBE technique. The growth was performed at different temperatures to investigate the development of the growth rate, surface morphology, structure and optical properties. Properties of films deposited with a single CdTe source are compared with films grown with an additional Cd source. The growth experiments are discussed with regard to demands for X-ray detector applications. View full abstract»

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  • Extended Defects in CdZnTe Radiation Detectors

    Page(s): 1775 - 1783
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (5156 KB) |  | HTML iconHTML  

    Large-volume CdZnTe (CZT) single crystals with electron lifetime exceeding 10 mus have recently become commercially available. This opened the opportunity for making room temperature CZT gamma-ray detectors with extended thicknesses and larger effective areas. However, the extended defects that are present even in the highest-quality material remain a major drawback which affects the availability and cost of large CZT detectors. In contrast to the point defects that control electron lifetime and whose effects on the charge collection can be electronically corrected, the extended defects introduce significant fluctuations in the collected charge, which increase with a crystal's thickness. The extended defects limit the uniformity in the electrons' drift distance in CZT crystals, above which electron trapping cannot effectively be corrected. In this paper, we illustrate the roles of the extended defects in CZT detectors with different geometries. We emphasize that the crystallinity of commercial CZT materials remains a major obstacle on the path to developing thick, large-volume CZT detectors for gamma-ray imaging and spectroscopy. View full abstract»

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  • Dopant Content and Thermal Treatment of {\rm Cd} _{1-{\rm x}} {\rm Zn} _{\rm x} {\rm Te} \langle {\rm In}\rangle : Effects on Point-Defect Structures

    Page(s): 1784 - 1790
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (535 KB) |  | HTML iconHTML  

    We measured, in the 873-1173 K temperature range, the temperature- and Cd vapor-pressure-dependences of the free electron density in single CdTe(In) crystals with different In contents. Increasing the cooling rate of the crystals and/or decreasing the well-defined Cd vapor pressure reduced the free-electron density. We interpreted and modelled these phenomena and the crystal's high-temperature electrical properties within the framework of Kroger's point-defect theory. Our experiments demonstrated the possibility of controlling the free-electron density in CdTe(In) crystals by changing the cooling rate. We supplemented a point-defect structural study of CZT(In) crystals by low-temperature (80-420 K) electrical measurements. These findings allowed us to identify the nature of the point defects responsible for free-carrier scattering, which is an important parameter influencing the mutau-product value in detector-grade material. View full abstract»

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  • Development of 4-Sides Buttable CdTe-ASIC Hybrid Module for X-ray Flat Panel Detector

    Page(s): 1791 - 1794
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1456 KB) |  | HTML iconHTML  

    A 4-sides buttable CdTe-ASIC hybrid module suitable for use in an X-ray flat panel detector (FPD) has been developed by applying through silicon via (TSV) technology to the readout ASIC. The ASIC has 128 times 256 channels of charge integration type readout circuitry and an area of 12.9 mm times 25.7 mm. The CdTe sensor of 1 mm thickness, having the same area and pixel of 100 mum pitch, was fabricated from the Cl-doped CdTe single crystal grown by traveling heater method (THM). Then the CdTe pixel sensor was hybridized with the ASIC using the bump-bonding technology. The basic performance of this 4-sides buttable module was evaluated by taking X-ray images, and it was compared with that of a commercially available indirect type CsI(Tl) FPD. A prototype CdTe FPD was made by assembling 9 pieces of the 4-sides buttable modules into 3 times 3 arrays in which the neighboring modules were mounted on the interface board. The FPD covers an active area of 77 mm times 39 mm. The results showed the great potential of this 4-sides buttable module for the new real time X-ray FPD with high spatial resolution. View full abstract»

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  • Optimization of Medipix-2 Threshold Masks for Spectroscopic X-Ray Imaging

    Page(s): 1795 - 1799
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1075 KB) |  | HTML iconHTML  

    Spectroscopic X-ray imaging enhances image contrast and provides advanced object information due to energy resolution. The Medipix-2 chip is a photon counting semiconductor detector and features two energy thresholds for energy selective imaging. The aim of this study is to present the development of optimized threshold adjustment masks with small energy windows of about 3 keV width using a monochromatic X-ray source for equalization and to demonstrate the benefits of spectroscopic X-ray imaging using an integrated circuit as object. View full abstract»

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  • Status of Direct Conversion Detectors for Medical Imaging With X-Rays

    Page(s): 1800 - 1809
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (671 KB) |  | HTML iconHTML  

    Imaging detectors for medical X-ray and computed tomography (CT) applications have undergone many improvements and technology changes over time. But most (dynamic) detectors sold in this field still rely on indirect conversion, using scintillators and photodiodes to convert the X-ray quanta ultimately into electrical signals. Direct conversion detectors promise very high spatial resolution and high signal-to-noise ratios. Some direct conversion materials may allow for counting or even energy resolving detection of the X-ray quanta. Based on this, for example spectrally resolving CT systems are becoming an interesting option for the next decade. This contribution highlights the requirements of advanced medical X-ray and CT imaging and reviews examples of status and progress in the field. The emphasis is on the direct conversion sensors for pixelated detectors, but considerations on read-out concepts and on associated challenges such as interconnects will also be presented. Finally, the most burning issues, such as count rate limitations and polarization effects, will be discussed from an application point of view. View full abstract»

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  • Polycrystalline Mercuric Iodide Films on CMOS Readout Arrays

    Page(s): 1810 - 1816
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (5899 KB) |  | HTML iconHTML  

    We have created high-resolution x-ray imaging devices using polycrystalline mercuric iodide (HgI2) films grown directly onto CMOS readout chips using a thermal vapor transport process. Images from prototype 400 times 400 pixel HgI2-coated CMOS readout chips are presented, where the pixel grid is 30 mum times 30 mum. The devices exhibited sensitivity of 6.2 muC/Rcm2 with corresponding dark current of ~2.7 nA/cm2, and a 80 mum FWHM planar image response to a 50 mum slit aperture. X-ray CT images demonstrate a point spread function sufficient to obtain a 50 mum spatial resolution in reconstructed CT images at a substantially reduced dose compared to phosphor-coated readouts. The use of CMOS technology allows for small pixels (30 mum), fast readout speeds (8 fps for a 3200 times 3200 pixel array), and future design flexibility due to the use of well-developed fabrication processes. View full abstract»

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  • Temperature Dependence in the Long-Term Stability of the TlBr Detector

    Page(s): 1817 - 1822
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (302 KB) |  | HTML iconHTML  

    In this paper, TlBr detectors with three-electrodes were prepared and their long term stability evaluated as a function of the temperature. Systematic measurements of counting rate were carried out to observe the random pulse formation, due to the polarization effect. Three-electrode detectors presented stability of about 112 h at near room temperature and more than 325 h at 0degC. Although the three-electrode detector presents a better performance compared to the two-electrode detector, its stability has been still dependent on the temperature. View full abstract»

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  • Study of Surface Treatment Effects on the Metal-CdZnTe Interface

    Page(s): 1823 - 1826
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (349 KB) |  | HTML iconHTML  

    The quality of a CdZnTe-based X-ray detector is highly related to the interface between semiconductor and metal contact. One of the factors that increase leakage currents in CdZnTe based X-ray detectors is the presence of a conductive surface layer. In this paper the result of the passivation of the CdZnTe surface by means of an aqueous solution of NH4F/H2O2 is studied by optical ellipsometry and by the current-voltage characteristics of gold contacts deposited on the oxidized surface. Collected data show that leakage currents can be reduced and contact stability improved by the combined use of the passivation layer and a guard ring. View full abstract»

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  • Higher Voltage Ni/CdTe Schottky Diodes With Low Leakage Current

    Page(s): 1827 - 1834
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (275 KB) |  | HTML iconHTML  

    A significant improvement in electrical characteristics of Schottky diodes designed for X- and gamma-ray detectors has been achieved using semi-insulating CdTe single crystals and unified technology, where both Schottky and near-ohmic contacts were formed by the deposition of the same metal (Ni) on the opposite surfaces of the crystal pre-treated by chemical etching and Ar ion bombardment with different parameters. Reduction of injection of minority carriers from the near-ohmic contact in the neutral part of the diode and high Schottky barrier for holes provides low leakage current even at high bias voltage (<50 nA/cm2 at 2000 V and at room temperature). The current-voltage characteristics of the detectors with Ni/CdTe/Ni electrode configuration in the low-voltage range are described by the generation-recombination Sah-Noyce-Shockley theory. The results of the reproducibility and time stability of the fabricated diodes are reported. View full abstract»

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Aims & Scope

IEEE Transactions on Nuclear Science focuses on all aspects of the theory and applications of nuclear science and engineering, including instrumentation for the detection and measurement of ionizing radiation; particle accelerators and their controls; nuclear medicine and its application; effects of radiation on materials, components, and systems; reactor instrumentation and controls; and measurement of radiation in space.

Full Aims & Scope

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