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Nuclear Science, IEEE Transactions on

Issue 4  Part 1 • Date Aug. 2007

 This issue contains several parts.Go to:  Part 2  | Part 3 

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Displaying Results 1 - 25 of 29
  • [Front cover]

    Publication Year: 2007 , Page(s): C1
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  • IEEE Transactions on Nuclear Science publication information

    Publication Year: 2007 , Page(s): C2
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  • Table of contents

    Publication Year: 2007 , Page(s): 761 - 762
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  • High Temperature Properties of CdTe Crystals, Doped by Sb

    Publication Year: 2007 , Page(s): 763 - 768
    Cited by:  Papers (4)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (499 KB) |  | HTML iconHTML  

    Electrical properties of CdTe single crystals doped by Sb were studied in situ at high temperature point defect equilibrium under well defined Cd and Te vapour pressure. Up to samples revealed p-type conductivity both under Cd and Te saturation. The position of the deep acceptor level was determined using hole density temperature dependency at 350-650 K. The point defects responsible for hole density are supposed to be acceptors. The n-type conductivity above is determined by intrinsic point defects: electrons under Te saturation due to native disorder and Cd interstitials under Cd saturation. View full abstract»

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  • Crystal Growth and Characterization of Detector Grade (Cd,Zn)Te Crystals

    Publication Year: 2007 , Page(s): 769 - 772
    Cited by:  Papers (6)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (2007 KB) |  | HTML iconHTML  

    (Cd,Zn)Te crystals were grown from the melt by the Bridgman method with diameters of 25 and 75 mm. The material was doped with indium. The structural quality of the crystals was constantly improved to increase the single crystalline areas. These improvements reduced the number of grains in the 25 mm and 75 mm diameter, respectively. The crystals were characterized by electrical and optical methods to evaluate the relation between structural quality and detector performance. A reduction of the etch pitch density down to 2times103 cm-2 was achieved by the optimization of the crystal growth The resistivity of the crystals was in the range of 2times109 up to 5times1010 Omega cm. The detector performance was tested with different radiation sources. The product of mobility-lifetime of charge carriers was 3times10-3 cm2/V. The energy resolution for different radiation energies were measured for detector thickness of 1 mm and 10 mm. View full abstract»

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  • Growth of Thick Films CdTe From the Vapor Phase

    Publication Year: 2007 , Page(s): 773 - 776
    Cited by:  Papers (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (969 KB) |  | HTML iconHTML  

    100 thick films of CdTe were grown on semi-insulating (100) GaAs substrates by physical vapor transport (PVT) in a modified molecular beam epitaxy facility. The grown layers were highly oriented as revealed from X-ray pole figure measurements. Temperature- and intensity-dependent photoluminescence measurements were taken before and after the chemical removal of the substrate to determine the effect of the GaAs substrate and to estimate the crystallographic quality of the layers. Current-voltage characteristics were performed to obtain the resistitivity of the layers with . View full abstract»

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  • CdTe Detector Characteristics At 30°C and 35°C When Using The Periodic Bias Reset Technique

    Publication Year: 2007 , Page(s): 777 - 781
    Cited by:  Papers (7)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (407 KB) |  | HTML iconHTML  

    The energy resolution of CdTe detectors typically deteriorates in several tens of minutes due to the polarization effect at 30degC. In this paper, the energy resolutions were evaluated at 30degC and 35degC, and the periodic bias reset technique was investigated as a way to stabilize the detector performance. Two different bias reset circuits were used, one was an exponential voltage change type and the other was a linear voltage change type. The 500 V bias with 0.5 s to 60 s reset durations and 1 min to 20 min reset intervals was applied to 1 mm thick CdTe detectors. It was found that the 0.5 s reset duration and 5 min reset interval were able to stabilize the detector performance at 35degC. View full abstract»

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  • Composition Study of CdTe Charges Synthesized by the Travelling Heater Method

    Publication Year: 2007 , Page(s): 782 - 785
    Cited by:  Papers (4)  |  Patents (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (346 KB) |  | HTML iconHTML  

    By means of the traveling heater method it was possible to synthesize CdTe polycrystalline charges with an extremely low Tellurium deviation. This result is somewhat unexpected, due to the fact that at the temperature of the synthesis process the phase diagram foresees a large incorporation of Tellurium excess. In order to study in detail this phenomenon, "ad hoc" synthesis processes are carried out and the composition of the obtained CdTe charges carefully measured. Possible explanations for the observed data are discussed. View full abstract»

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  • Preparation of Semi-Insulating CdTe:In by Post-Grown Annealing After Elimination of Te Inclusions

    Publication Year: 2007 , Page(s): 786 - 791
    Cited by:  Papers (5)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (959 KB) |  | HTML iconHTML  

    Annealing under Cd-saturated overpressure within the temperature range of 700-800degC was used for elimination of Te inclusions in high resistive In-doped CdTe. Te inclusions were significantly reduced by annealing and only residual inclusions with dimensions less than 1 mum remained in the material. All annealed samples were converted to the high conductive n-type after Cd-saturated annealing. Annealing finished by quenching at 550degC, 650degC, and 750degC under various Cd/Te overpressures was used to find an annealing conditions leading to the preparation of a semi-insulating material. The range of optimal overpressures was found to be very narrow and keeping these conditions during annealing and quenching is practically impossible. The semi-insulating material was prepared by means of Te-rich annealing finished by slow cooling. The annealing temperature, In doping concentration, and cooling regime were observed to strongly affect the preparation of semi-insulating material. View full abstract»

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  • Dynamics of Point Defects in Tellurium-Rich CdTe

    Publication Year: 2007 , Page(s): 792 - 797
    Cited by:  Papers (3)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (188 KB) |  | HTML iconHTML  

    The evolution of defect structure occurring during the cooling of Tellurium-rich CdTe is studied theoretically and experimentally. Classical nucleation theory is applied to comprehend the precipitation of excess Te and a formation of Te precipitates both in pure lattice and on extended defects is taken into account. Numerical simulations demonstrate significant effect of the cooling process on the room temperature precipitate magnitude and density. Theoretical results are compared with high temperature in-situ measurements of Te-rich CdTe:In. It is shown that the cooling rate significantly affects the temperature dependence of the conductivity. Characteristic temperatures, at which excess Te precipitates, are identified as apparent bows in measured dependencies. Optimum annealing-cooling treatment to obtain detector grade CdTe is suggested. View full abstract»

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  • Boron Oxide Encapsulated Vertical Bridgman: A Method for Preventing Crystal-Crucible Contact in the CdZnTe Growth

    Publication Year: 2007 , Page(s): 798 - 801
    Cited by:  Papers (7)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1639 KB) |  | HTML iconHTML  

    One of the reasons for the formation of twins and grain boundaries during the CdZnTe (CZT) crystal growth is the crystal-crucible interaction, typical of the vertical Bridgman technique. Particularly detrimental seems to be the use of quartz crucibles that ease the sticking of the crystal to the crucible walls. Due to this reason, many authors suggest the use of graphite crucibles or of carbon coated quartz crucibles. In order to avoid the contact between the growing crystal and the crucible, it was proposed to opportunely control the wetting angle. However, it was shown that this is possible only under microgravity conditions or by imposing a pressure difference between the melt and the solidifying crystal. In this work, we show that it is possible to avoid the contact between the crystal and the crucible by interposing a thin liquid boron oxide layer. This condition can be obtained in a vertical Bridgman furnace by covering the polycrystalline charge with a boron oxide pellet. In this way, crystals with large single grains were obtained also in quartz crucibles. Moreover, the crystals show low dislocation density, as expected in the case of crystals grown without contact with the crucible. View full abstract»

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  • Characterization of Low-Defect Cd0.9Zn0.1Te and CdTe Crystals for High-Performance Frisch Collar Detectors

    Publication Year: 2007 , Page(s): 802 - 806
    Cited by:  Papers (21)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1123 KB) |  | HTML iconHTML  

    Low dislocation density, high-purity, and low inclusion concentration Cd0.9Zn0.1Te (CZT) and CdTe crystals were grown by a vertical Bridgman technique using in-house zone refined precursors. The grown crystals were sequentially processed using optimized chemo-mechanical processes to fabricate planar and Frisch collar detectors. Infrared transmission and scanning electron microscopy studies have shown that EIC grown CZT and CdTe crystals have significantly lower Te inclusions and defect densities than commercially available spectrometer grade crystals. The charge transport properties (electron and hole mobility-lifetime products, mutaue & mutauh) of various detectors have been evaluated by Hecht analysis. The detectors have been tested for spectral response using 59.5 and 662 keV gamma-ray sources. The CZT detectors with planar electrodes showed 2.6% FWHM at 662 keV. By adding a Frisch collar, the detectors' spectra improved significantly. The Frisch collar detectors proved to be very promising for assembling large-area arrays with excellent energy resolution at relatively low manufacturing cost. View full abstract»

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  • Charge Collecting Properties of Proton Irradiated CdTe Detectors

    Publication Year: 2007 , Page(s): 807 - 810
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (118 KB) |  | HTML iconHTML  

    We have investigated the effects of proton irradiation on the charge collecting properties of planar CdTe detectors. The spatial distribution of the charge collection and of the electric field inside the material has been studied by photocurrent spectroscopy and surface potential analyses. The results indicate that a localized damaged layer is formed under the irradiated electrode due to the presence of a high concentration of electron traps. The induced p+-type region affects the distribution of the electric field: after irradiation the field is no longer uniform inside the detector but it rapidly decreases to zero towards the damaged layer. View full abstract»

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  • Characterization of Traveling Heater Method (THM) Grown Cd0.9Zn0.1Te Crystals

    Publication Year: 2007 , Page(s): 811 - 816
    Cited by:  Papers (30)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (2275 KB) |  | HTML iconHTML  

    High-performance semi-insulating single crystals of n-type (CZT) were grown using the traveling heater method (THM). X-ray and -ray detector configurations fabricated from this material have a room-temperature mean energy resolution of 4.3% FWHM for a source (122 keV) and uniform pixel-to-pixel response on monolithic 20205 pixellated detectors. Energy resolution of 1% FWHM for (662keV) has been measured on virtual Frisch-grid 4411 devices useful for homeland security applications. Additional characterization techniques including mobility-lifetime measurements, infrared microscopy, X-ray topography, and OPTICAL Deep Level Transient Spectroscopy (ODLTS) have demonstrated the superior quality of this THM CZT. View full abstract»

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  • Characterization of CdTe/n+ -Si Heterojunction Diodes for Nuclear Radiation Detectors

    Publication Year: 2007 , Page(s): 817 - 820
    Cited by:  Papers (8)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (201 KB) |  | HTML iconHTML  

    CdTe/n+-Si heterojunction diodes were fabricated and characterized for the development of gamma ray detectors. With the careful control of the growth parameters thick single crystal CdTe epilayers of high-crystalline quality were grown directly on the (211) Si substrates in a metalorganic vapor phase epitaxy. The heterojunction diode was fabricated by growing a 5 mum thick n-type CdTe buffer layer on the n+-Si substrate, followed by the growth of 100 mum thick undoped p-CdTe layer. The diode fabricated showed very good rectification property with a low value of the reverse bias leakage current, typically 1.26 times 10-7 A/cm2 for an applied reverse bias of 60 V. The diode clearly demonstrated the gamma radiation detection capability by resolving energy peaks from the 241Am radioisotope during the radiation detection test performed at room View full abstract»

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  • Performance-Limiting Defects in CdZnTe Detectors

    Publication Year: 2007 , Page(s): 821 - 827
    Cited by:  Papers (32)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (2110 KB) |  | HTML iconHTML  

    We studied the effects of small, < 20 mum, Te inclusions on the energy resolution of CdZnTe gamma-ray detectors using a highly collimated X-ray beam and gamma-rays, and modeled them via a simplified geometrical approach. Previous reports demonstrated that Te inclusions of about a few microns in diameter degraded the charge-transport properties and uniformity of CdZnTe detectors. The goal of this work was to understand the extent to which randomly distributed Te-rich inclusions affect the energy resolution of CZT detectors, and to define new steps to overcome their deleterious effects. We used a phenomenological model, which depends on several adjustable parameters, to reproduce the experimentally measured effects of inclusions on energy resolution. We also were able to bound the materials-related problem and predict the enhancement in performance expected by reducing the size and number of Te inclusions within the crystals. View full abstract»

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  • Deep Traps Induced by 700 keV Protons in CdTe and CdZnTe Detectors

    Publication Year: 2007 , Page(s): 828 - 833
    Cited by:  Papers (4)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (692 KB) |  | HTML iconHTML  

    We have studied the effects of 700 keV protons at increasing fluences (up to 2times1012 p/cm2) on CdTe:Cl and Cd0.9Zn0.1Te detectors. The charge transport properties of the detectors have been studied by mobility-lifetime (mutau) product measurements, gamma spectroscopy and photo induced current transient spectroscopy (PICTS) analyses. The correlation of the macroscopic transport behaviour with the information on the microscopic defective states allowed for the identification of the role played by the dominant traps: levels Z and H1 resulted related to electron trapping effects, while levels K and X showed a hole trapping character. View full abstract»

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  • Room-Temperature Replacement for Ge Detectors—Are We There Yet?

    Publication Year: 2007 , Page(s): 834 - 842
    Cited by:  Papers (6)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (336 KB) |  | HTML iconHTML  

    The search for a semiconductor detector capable of room-temperature operation with similar performance to liquid- nitrogen-cooled Ge detectors has been going on for several decades. The main challenge is in achieving the very high degree of spatial uniformity in the detector response that is needed for high-resolution gamma-ray detection. There exist two distinct paths that one can take to meet this goal. One path is to develop materials with near-perfect charge transport properties similar to that of Ge. This requires materials with high mobilities and lifetimes for both electrons and holes, which means extremely low concentrations of electrically-active and carrier-trapping defects. The other path is to employ special device technologies that can provide a highly uniform detector response given the charge transport properties of existing room-temperature semiconductor materials. Many types of device structures and readout techniques have been proposed and investigated, but only a few have the potential to achieve near-Ge spectral performance. We will discuss the basic material and device requirements, the obtainable performance, the state of the technology, and the challenges of these different approaches. View full abstract»

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  • A Prototype Three-Dimensional Position Sensitive CdZnTe Detector Array

    Publication Year: 2007 , Page(s): 843 - 848
    Cited by:  Papers (25)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (2451 KB) |  | HTML iconHTML  

    A new CdZnTe gamma-ray spectrometer system that employs two layers of modular detector arrays Is being developed under the collaboration between the University of Michigan and the Pacific Northwest National Labaratory (PNNL). Each layer can accommodate up to three by three 3-dimensional position sensitive CdZnTe gamma-ray spectrometers. This array system is based on the newly developed VAS_UM/TAT4 ASIC readout electronics. Each of the nine detector modules consists of a pixellated CdZnTe detector and a VAS_UM/TAT4 ASIC front-end board. Each 1.5times1.5times1.0 cm3 CdZnTe detector employs an array of 11 by 11 pixellated anodes and a planar cathode. The energy depositions and 3-dimensional positions of individual interactions of each incident gamma ray can be obtained from pulse amplitude, location of each pixel anode and the drift time of electrons. Ten detectors were tested individually and half of them achieved resolution of <1.0% FWHM at 662 keV for single-pixel events (~30% of all 662 keV full energy deposition events). Two of them were tested in a simple array to verify that the upgrade to an array system does not sacrifice the performance of individual detectors. Experimental results of individual detectors and a two-detector array system are presented, and possible causes for several worse performing detectors are discussed. View full abstract»

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  • Readout System for Arrays of Frisch-Ring CdZnTe Detectors

    Publication Year: 2007 , Page(s): 849 - 853
    Cited by:  Papers (3)  |  Patents (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (786 KB) |  | HTML iconHTML  

    Frisch-ring CdZnTe detectors have demonstrated good energy resolution for identifying isotopes, <1% FWHM at 662 keV, and good efficiency for detecting gamma rays. To facilitate the application of this detector in radiation detection, we are designing a portable device that can incorporate with an 8times8 Frisch-ring detector array. A prototype system has been designed, which includes detector modules, front-end electronics, signal processing circuit, USB interface and high-voltage power supply. This paper describes the design and assembly of the detector modules and the structure of the prototype system. Some test results are also reported. View full abstract»

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  • Simulation and Design of Orthogonal Capacitive Strip CdZnTe Detectors

    Publication Year: 2007 , Page(s): 854 - 859
    Cited by:  Papers (4)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (655 KB) |  | HTML iconHTML  

    Orthogonal strip detectors are single-carrier devices that allow reduction of the amount of read-out channels while obtaining a good spatial resolution. They are especially interesting for low count rate imaging applications like gamma-ray medical imaging. Nevertheless, applying a bias voltage between and strips is necessary in order to localize correctly the interactions. Unfortunately, this bias voltage induces a leakage current, thus generating an unwanted shot noise. We propose in this paper a new setup using capacitive contacts, which suppresses leakage currents between collecting and non-collecting strips. In this paper, we first briefly present the state-of-the-art relative to coplanar grid and strip detectors, their advantages and their drawbacks. In a second part, we will focus on the theoretical background of the capacitive contact. In a third part, a simulation study of the electrode geometry is reported. Thanks to the spectral approach, we have computed the 3D applied and weighting electric fields, bulk resistances and capacitances for various geometries of the design. Afterwards, we have used these data to simulate pulse waveforms and study the associated electronics. Finally the fabrication of the devices is presented and our first experimental results are reported. As a conclusion, we will discuss the interests and the limitations of this kind of device. View full abstract»

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  • Temperature Dependence of Spectroscopic Performance of Thallium Bromide X- and Gamma-Ray Detectors

    Publication Year: 2007 , Page(s): 860 - 863
    Cited by:  Papers (3)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (182 KB) |  | HTML iconHTML  

    Thallium bromide (TlBr) is a compound semiconductor characterized by wide band-gap energy (2.68 eV), high atomic numbers (Tl: 81, Br: 35) and high density (7.56 g/cm3). Mobility-lifetime products for electrons and holes in TlBr crystals are 1times103 cm2/V and 3times10-4 cm2/V, respectively, which are comparable to for carriers in CdTe crystals. Therefore, TlBr is a suitable material for fabrication of X- and gamma-ray detectors. In this study, TlBr detectors have been fabricated from crystals grown by the traveling molten zone method. The detectors have thicknesses of approximately 0.4 mm, and have gold electrodes (3 mm in diameter) on both surfaces of the crystals. In order to evaluate the temperature dependence of the charge transport properties of the detectors, (mutau)e and (mutau)h in the detectors have been estimated by the Hecht analysis at various temperatures of the detectors (-40, 20, 0, 20 and 40degC). The (mutau)h in the detectors was of the order of 10-5 cm2/V at -40degC, and increased with increasing temperatures. The maximum value of (mutau)h the was obtained at 20degC, and was about ten times larger than the value at -40degC. On the other hand, the (mutau)e was approximately 1times10-3 cm2/V between -40degC and 20degC. The temperature dependence of spectral response of the detectors has been evaluated by acquiring energy spectra of gamma-rays from 241Am at -40, 20, 0, 20 and 40degC. At -40degC, the full-energy peak corresponding to 59.5 keV gamma-rays has been obtained from the detectors, but the energy resolution of the peak was degraded due to the hole tailing on the low-energy side of the peak. The energy resolution of the detectors gradually improved with increasing temperatures of the detectors. The best FWHM energy resolution was measured to be 5.4 keV (9.1%) at 20degC. View full abstract»

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  • Simulation of Photoelectric Transport in High-Resistivity CdTe for X-Ray Detectors

    Publication Year: 2007 , Page(s): 864 - 867
    Cited by:  Papers (3)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (148 KB) |  | HTML iconHTML  

    Photoelectric transport in high resistivity CdTe at room temperature was numerically simulated based on the self-consistent steady state solution of coupled drift-diffusion equations and the Poisson equation of the photo-excited electron-hole system. Relative influence of recombination and space charge induced distortion of electric field on the reduction of charge collection efficiency was investigated. It was shown that space charge effects play significant role, if concentration of deep levels exceeds 1013 cm-3 at typical values of carriers' capture cross-section. Possibility to reduce the screening effects decreasing a detector thickness to a minimum value guaranteeing full absorption of incident X-ray radiation was investigated. View full abstract»

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  • Electric Field Properties of CdTe Nuclear Detectors

    Publication Year: 2007 , Page(s): 868 - 872
    Cited by:  Papers (21)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1567 KB) |  | HTML iconHTML  

    Recently, diode-like In/CdTe/Pt detectors have been realized which look very promising thanks to their enhanced spectroscopic performance. Scope of this work is to investigate the electric field distribution inside these detectors and its temporal evolution by means of the Pockels effect. The implemented set-up allows us to map the electric field and thus to extract the field profiles between the contacts. The analysis shows that the field is mainly confined below the anode. After applying the bias, these detectors are not very stable at room temperature, and they present a pronounced degradation in spectroscopic performance. This degradation is correlated in this work to the evolution of the electric field which has been observed to dasiamovepsila towards the anode. We attribute this dasiapolarizationpsila behavior in CdTe detectors to the concomitance of two factors: the presence of deep levels in the bulk material, and the high hole barrier height of In on CdTe. View full abstract»

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  • Development and Characterization of Large La-Halide Gamma-Ray Scintillators for Future Planetary Missions

    Publication Year: 2007 , Page(s): 873 - 878
    Cited by:  Papers (9)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (659 KB) |  | HTML iconHTML  

    Future planetary missions such as BepiColombo are resource limited in both mass and power. Due to the proximity of the spacecraft to the Sun, the instrumentation will encounter harsh environments as far as radiation levels and thermal loads are concerned. Only radiation hard detectors that need little or no cooling will be able to successfully operate after long cruise times and over the expected mission lifetimes. The next generation of lanthanum halide scintillators promises to provide sufficient resolution in the spectral range between 1 and 10 MeV where most of the elemental gamma-ray emission lines can be detected. In order to be suitable for planetary gamma-ray spectrometers with sufficient sensitivity it had to be proven that larger crystals of size 3 can be produced and that they maintain their resolution of 3% at 662 keV. For that purpose we have produced and characterized several larger crystals and assessed their radiation hardness by exposing the crystals to radiation doses that are representative of the expected conditions in the space environment. Systematic measurements on several crystals allowed the determination of the activation potential and the performance verification from which the consequences for instrument flight performance can be derived. From these investigations we conclude that these scintillators are well suited for planetary missions, with excellent and stable performance. View full abstract»

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