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Publications (42)25.89 Total impact

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    ABSTRACT: A functional prototype two-plane Compton camera array for localization and identification of remote radiation sources, consisting of four PVT and four NaI(Tl) scintillation detectors with PMT readout, is presented. The large-volume, 76x76x76 mm scintillators provide a broad field of view for scattered photons and facilitate maximum efficiency at moderate cost. Each detector is equipped with a voxelSPEC, a compact electronic module that provides high voltage for the PMT, signal processing, detector stabilization, and an Ethernet communication interface. The voxelSPEC delivers list-mode event data with nanosecond precision timing over non-proprietary Ethernet and makes a system extension very easy. A software package has been developed for real-time data processing and image reconstruction. Advantages in hard- and software allow stable, unattended operation of the camera array for many days, and provide easy-to-read information on the radiation source in real time. Measurements with the prototype array have been performed for a few standard scenarios and geometries to verify the model predications obtained by Monte-Carlo simulations. Simulations have been further performed to explore larger camera arrays with 2x4x4, 2x8x8, 2x10x10, 2x13x13 and 2x16x16 detectors.
    IEEE Transactions on Nuclear Science 04/2013; 60(2):1066-1071. · 1.22 Impact Factor
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    ABSTRACT: We analyzed two CZT crystals cut from as-grown CdZnTe (CZT) ingots, the only difference between them being the rate of cooling after the crystal growth process. Using White Beam X-ray Diffraction Topography (WBXDT) and Infrared (IR) Transmission Microscopy, we identified and quantified the extended defects, e.g., Te inclusions, dislocations, and sub-grain boundaries. The effects of cooling rate on the size distribution and concentration were studied. The WBXDT and IR images of the fast-cooled crystal revealed very high density of dislocations and sub-grain boundaries, crisscrossing throughout its entire volume, extending from deep inside almost to the surface. In addition, IR analyses showed that the concentration of Te inclusions in the fast-cooled crystal (106 cm−3) was higher than that in the slow-cooled one (105 cm−3). For the latter, both the WBXDT and the IR images were bright and clear with low concentration of defects. We concluded that slow cooling rate can greatly reduce the number of Te inclusions and inclusion-decorated extended defects in as-grown CZT ingots.
    Journal of Crystal Growth 09/2012; 355(1):84–87. · 1.55 Impact Factor
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    ABSTRACT: We have employed photoluminescence mapping at room temperature of detector grade CZT grown by modified Traveling Heater Method (THM) technique and estimated the spatial variation of Zn composition. The composition was found to be fairly uniform over the major portion of the ingot length except near the conical part of the ingot and the upper part of the ingot. Unlike Bridgman growth technique, no detectable striations were observed. The composition was within ±0.3% over the length little more than 4 cm of the ingot. The growth interface revealed from the composition mapping was found to be reasonably flat.
    Journal of Crystal Growth 05/2012; 347(1):53–55. · 1.55 Impact Factor
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    ABSTRACT: Portable radiation detection systems will greatly benefit from the use of the same detector for gamma and neutrons, on the roadmap towards miniaturization, provided such devices are capable of robust discrimination of gammas against neutrons. One of the most promising and yet commercially available inorganic scintillator, delivering different pulse shapes for thermal neutrons and gammas is the Cs2LiYCl6:Ce (CLYC) [1,2]. The detection for thermal neutrons follows through the 6Li(n,alpha)3H capture reaction, and with the appropriate levels of 6Li enrichment, which have currently reached 95%, the CL YC crystals could become very efficient neutron detectors. In order to successfully integrate this new scintillator in our digital devices, the variability of the energy resolution across various crystals, the dependence of the energy resolution on energy, shaping time and temperature, the pulse shape parameters variability across the crystals and the thermal neutron sensitivity were investigated.
    Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC), 2012 IEEE; 01/2012
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    ABSTRACT: CdZnTe, commonly known as CZT is the material of paramount importance for hard X-ray and gamma ray spectroscopy and imaging applications at room temperature. Over the years, the quality of CZT crystals and its charge transport properties has improved significantly making it an attractive detector material especially for homeland security applications. The applications for homeland security demand large and thick detectors to provide a sufficient stopping power for fast detection of high energy gamma photons. In this present report we have grown two inch diameter CZT by Traveling heater method (THM) technique. The as-grown crystals were characterized through photoluminescence (PL) mapping for composition uniformity, growth interface study, Te precipitations/inclusions studies. In order to evaluate our as-grown samples, charge transport characteristics have been studied for thick samples up to ~16mm thick. It has been demonstrated that by controlling the growth interface, visualization of large and thick (~16mm thick) detectors with fairly good response is possible from as-grown CZT detectors grown by THM technique.
    Proc SPIE 09/2011;
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    ABSTRACT: The concentration and size distribution of Te inclusions/precipitates in CZT are key factors in a device's performance. High concentrations can degrade the performance drastically, especially for long drift-length devices (more than about 10-mm thick). Here, we extend our previous findings on the concentration and size distribution of Te inclusions/precipitates in CZT wafers grown by the THM technique, by considering the rate of cooling of the ingots. We measured their distribution along the diameter of the wafers in a conventional slow-cooled and a fast-cooled ingot. The overall average concentration of Te inclusions/precipitates for the slow-cooled sample was less than 1×105cm−3, attesting to their suitability for fabricating thick radiation-detection devices.
    Journal of Crystal Growth 01/2011; 332(1):34-38. · 1.55 Impact Factor
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    ABSTRACT: In this present work we report the growth of Cd{sub 0.9}Zn{sub 0.1}Te doped with In by a modified THM technique. It has been demonstrated that by controlling the microscopically flat growth interface, the size distribution and concentration can be drastically reduced in the as-grown ingots. This results in as-grown detector-grade CZT by the THM technique. The three-dimensional size distribution and concentrations of Te inclusions/precipitations were studied. The size distributions of the Te precipitations/inclusions were observed to be below the 10-m range with the total concentration less than 10 cm³. The relatively low value of Te inclusions/precipitations results in excellent charge transport properties of our as-grown samples. The (){sub e} values for different as-grown samples varied between 6-20 x 10³ cm²/V. The as-grown samples also showed fairly good detector response with resolution of 1.5%, 2.7% and about 3.8% at 662 keV for quasi-hemispherical geometry for detector volumes of 0.18 cm³, 1 cm³ and 4.2 cm³, respectively.
    MRS Online Proceeding Library 01/2011; 1341.
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    ABSTRACT: In this present work we have grown Cd0.9Zn0.1Te doped with indium by the traveling heater method (THM) technique. Large 2in diameter CZT ingots of more than 1kg each were successfully grown by the THM technique in vertical configuration. In order to evaluate our as-grown CZT samples, charge transport characteristics were studied at and below room temperature. The key parameter investigated for as-grown CZT samples was the mobility–trapping time product and its temperature variation. Mobility–trapping time values as high as 9×10−3cm2/V at 30°C were measured for samples exhibiting resistivities in the 1–2×1010Ωcm range. The as-grown samples showed moderately good resolution of 1.5–3.5% at 662keV when fabricated. The variation of the internal electric field along the depth of the detector was studied for as-grown material to evaluate deformations inside the crystal due to the presence of residual stress or other defects.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 01/2011; 652(1):162-165. · 1.14 Impact Factor
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    ABSTRACT: Compton cameras are of general interest in various fields of operation. Because of the ability to locate and identify remote sources, homeland security supports the development of such devices in a rugged and reliable form. The decisions upon appropriate materials for the scatter- and absorber plane depend on performance and economical trade-offs. In order to estimate the expected performance of the Compton camera, simulations are necessary. Certain experimentally determined parameters have to be fed into simulations, such as the energy resolution of the detector. Two materials with low effective atomic number (Z<sub>eff</sub>), CaF<sub>2</sub> and plastic, promise to be good candidates for the scattering plane. Those scintillators are known for quite some time, but not very well characterized with respect of energy resolution and nonlinearity. A modified Compton coincidence technique using a high purity Germanium (HPGe) detector in coincidence with the investigated is discussed in this paper: The wide-angle Compton-coincidence (WACC) setup provides a fast and reliable means for characterization of low-Z scintillators. For quality control purposes, the actual scatter detector can be monitored in-house using the WACC technique. This work presents results of different scintillator materials and sizes for validation and exploration of this method.
    Nuclear Science Symposium Conference Record (NSS/MIC), 2010 IEEE; 12/2010
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    ABSTRACT: Radiation detector systems for homeland security applications have been usually equipped with <sup>3</sup>He tubes to detect the distinguished neutron signature of Special Nuclear Materials (SNMs). The serious shortage of <sup>3</sup>He gas, however, recently initiated substantial efforts to develop alternative neutron detectors, particularly for large-area Radiation Portal Monitors (RPMs). Most activities are currently directed to detectors comprising <sup>6</sup>Li or <sup>10</sup>B - beyond doubt with remarkable success. Nevertheless, their broad deployment poses an economic challenge. Our contribution presents a different technique - the detection of neutron capture gammas. In contrast to other attempts we do not focus on characteristic gammas or conversion electrons in the low-energy range, or on the detection of single high-energy capture gammas. Rather we propose to measure the sum energy of multiple gammas released after neutron capture reactions in a semi-calorimetric approach. This method allows simultaneous measurements of neutron and gamma radiation with a single detector (even including spectroscopic information for nuclide identification). A first prototype of such a Neutron Capture Detector (NCD) was developed based on proven standard detector materials and technologies. It consists of thin Cadmium sheets surrounded by four BGO scintillation crystals. The detector response was studied in measurements with cold neutrons extracted at the BER II reactor, and with fission neutrons from <sup>252</sup>Cf. The NCD performance is discussed in comparison with those of a <sup>3</sup>He tube. Simulation calculations have been performed to estimate the detection efficiency as a function of the detector size. A complete database to model the multiple-gamma emission from excited <sup>114</sup>Cd nuclei was composed by a semi-empirical approach which combines the gamma energies and yields of well resolved transitions with information from integral measurements. The - - simulation results are validated experimentally and allow optimizing more complex NCD systems for RPM applications.
    Nuclear Science Symposium Conference Record (NSS/MIC), 2010 IEEE; 12/2010
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    ABSTRACT: A complete simulation framework that combines Monte Carlo simulation with the digital-signal-processing algorithms developed in-house is presented for designing and optimizing scintillation detector systems. The framework covers basic components of a practical scintillation detector system and includes all relevant physical processes. With the complete simulation individual physical factors that can affect the performance of a detector system, such as detector geometry, surface finishing, decay time and scintillation yield of scintillator as well as responses of photodetector and front-end electronics, can be taken into account. As a demonstration of the simulation framework, the light collections of plastic (EJ200), NaI(Tl) and BGO scintillators with different surface types have been simulated as a function of the reflectivity of a diffuse reflector. Optical simulations were further performed to investigate the homogeneity of light transportation and detection in plastic scintillators with different surface finishing properties and coupled to one or two PMT(s). Finally, detector signals simulated for a NaI(Tl) detector have been compared with measured mean pulses. The simulated signal can be further digitized and used as input of digital signal processing.
    Nuclear Science Symposium Conference Record (NSS/MIC), 2010 IEEE; 12/2010
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    ABSTRACT: Homeland security applications demand high performance Compton-camera systems, with high detector efficiency, good nuclide identification and able to operate in-field conditions. A low-Z scintillator has been proposed and studied as a promising candidate for use in the scattering plane of a scintillator-based Compton camera: CaF<sub>2</sub>(Eu). All the relevant properties for the application of this scintillator in a mobile Compton camera system have been addressed: the energy resolution and the non-linearity at room temperature and in the temperature range of -20°C to +55°C, the photoelectron yield and the relative light yield in the relevant temperature range. A new method of inferring the relative light output of scintillators as a function of temperature has been proposed.
    Nuclear Science Symposium Conference Record (NSS/MIC), 2010 IEEE; 12/2010
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    ABSTRACT: Performance of a He-3 counter and a B-10 loaded liquid scintillator EJ309B5 has been studied in terms of neutron detection efficiency. The measurements were carried out in a mixed field of neutron and gamma radiation from an intense ( ~ 10<sup>6</sup> neutrons/s/4π)<sup>252</sup>Cf source. The response of both detectors to background and high intensity gamma radiation ( ~ 100 μSv/h at a detector) from a <sup>60</sup>Co source has been measured to establish background count rate and gamma rays cut-off point, respectively. The analysis showed that the properties of a He-3 counter are significantly better than that of EJ309B5. However, it has been pointed out how to improve the performance of a liquid scintillator in order to reduce gamma radiation sensitivity.
    IEEE Transactions on Nuclear Science 11/2010; · 1.22 Impact Factor
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    ABSTRACT: Cadmium Zinc Telluride (CZT) and Cadmium Manganese Telluride (CMT) crystals have emerged as promising advanced materials for X-ray and gamma-ray detection at ambient temperature. An understanding of the bulk properties in relation to energy resolution and device performance has led to the development of CZT into commercial devices, and is contributing to the improvement of CMT. The key detector attributes are large band-gap energy (˜1.6 eV, tunable by Zn or Mn concentration), high atomic number, and high density (˜6 g/cm^3). The techniques used in this study include infrared transmission microscopy, synchrotron X-ray diffraction topography, micro-scale X-ray mapping, and Pockels effect. We found that point defects and Te inclusions in CZT and CMT can trap the charge carriers generated by the absorption of X-rays, gamma-rays, and charged particles. We propose the use of thermal annealing and doping techniques to eliminate the deleterious effects caused by Te inclusions.
    10/2010;
  • U. N. Roy, S. Weiler, J. Stein
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    ABSTRACT: The Travelling Heater Method (THM) for CdZnTe growth is a widely accepted technique for mass production of detector grade material. Compared to other characterizations of THM grown CdZnTe, study of the growth interface has been neglected in the past. In the present report, large grain/single crystalline as-grown detector grade CdZnTe crystals have been grown by THM technique with diameter up to 52 mm. The crystals were grown from Te solution and the structure of the growth interfaces were investigated for both slow cooled and rapid cooled ingots. The macroscopic shape of the interface was studied and correlated with the grain growth of the ingots. A detailed microscopic morphology of the interface was studied in order to investigate the formation of the Te inclusions at the interfaces.
    Proc SPIE 08/2010;
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    ABSTRACT: This paper presents a novel technique developed for linearizing the energy spectra of radiation detectors in commercial radioisotope identification devices. Based on few spectrum measurements with standard radio-nuclide sources, this method allows generation of individual nonlinear calibration functions at minimum expense in the routine instrument setup. Instead of fitting peak positions, the measured raw data are compared with simulated spectrum templates, and local gain factors providing the best correspondence are taken as reference points for the calibration function. This approach avoids the problem of fitting multiple peaks with intensity ratios influenced by absorbing layers and assures an accuracy of 1% in the energy range of 30 keV to 3 MeV.
    IEEE Transactions on Nuclear Science 07/2010; · 1.22 Impact Factor
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    ABSTRACT: Boron-10 loaded liquid scintillators were studied in order to improve n/¿ separation. Pulse shape discrimination (PSD) was implemented by means of a zero-crossing (ZC) method to distinguish between ¿-rays and fast/slow neutrons. Significant progress was done for BC523A2 and EJ339A2 scintillators loaded with reduced amount of <sup>10</sup>B (2% and 2.5%, respectively), as compared to the results obtained earlier with BC523A loaded with 4.4% of <sup>10</sup>B. The improvement was probably caused by the reduction of <sup>10</sup>B content, as indicated in the reported study. A <sup>10</sup>B loaded scintillator EJ309B5 based on non-flammable liquid was also studied, showing the best separation of thermal neutron capture events from fast neutrons, ¿-rays and noise.
    IEEE Transactions on Nuclear Science 03/2010; · 1.22 Impact Factor
  • U. N. Roy, S. Weiler, J. Stein
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    ABSTRACT: Growth interface of large diameter CdZnTe ingots grown from Te solution by travelling heater method have been studied. Both macroscopic and microscopic investigations were carried out. The results indicated that the shape of the interface strongly governs the grain growth on the ingot, while the microscopic morphology of the growth interface is responsible for Te inclusions in the grown crystal.
    Journal of Crystal Growth 01/2010; 312(19):2840-2845. · 1.55 Impact Factor
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    ABSTRACT: Compton cameras have been used for astronomical and medical imaging applications as early as the 1970s. Recent interest in their potential for the detection and localization of special nuclear material (SNM) has led to increasing investigations. In this work, a specialized algorithm was developed for the optimization of a two-plane Compton camera. The MCNP-PoliMi code was utilized to simulate photon interactions within the detectors and coupled with an analytical technique used to estimate angular uncertainties. Using our specialized algorithm, a large area (approximately 1 m<sup>2</sup>) Compton camera consisting of two planar arrays of photon detectors was evaluated for several scintillators: LaBr<sub>3</sub>, CaF<sub>2</sub>, NaI, and plastic (C<sub>9</sub>H<sub>10</sub>). Optimization of plane thickness and voxel size were conducted for Compton camera efficiency and angular uncertainty at source energies of 100-400 keV.
    Nuclear Science Symposium Conference Record (NSS/MIC), 2009 IEEE; 12/2009
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    ABSTRACT: The concept of a two-plane planar Compton camera, consisting of scintillation detector elements, is presented. Several materials as C<sub>9</sub>H<sub>10</sub>, CaF<sub>2</sub>, YAlO<sub>3</sub>, NaI, and LaBr<sub>3</sub> are considered for operation in the scatter and/or absorption plane. The performance of the Compton camera is optimized by means of Monte Carlo simulations to meet the requirements for Homeland Security applications. For a low-threshold detector system we propose to utilize C<sub>9</sub>H<sub>10</sub> or CaF<sub>2</sub> for the scatter plane and NaI or LaBr<sub>3</sub> for the absorption plane. Particular effort must be focused to achieve low energy thresholds in particular for the detectors of the scatter plane if photons of incident energies below 200 keV are to be detected with reasonable efficiencies.
    Nuclear Science Symposium Conference Record (NSS/MIC), 2009 IEEE; 12/2009