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Material properties of large-volume cadmium zinc telluride crystals and their relationship to nuclear detector performance

Description: The material showing the greatest promise today for production of large-volume gamma-ray spectrometers operable at room temperature is cadmium zinc telluride (CZT). Unfortunately, because of deficiencies in the quality of the present material, high-resolution CZT spectrometers have thus far been limited to relatively small dimensions, which makes them inefficient at detecting high photon energies and ineffective for weak radiation signals except in near proximity. To exploit CZT fully, it will be necessary to make substantial improvements in the material quality. Improving the material involves advances in the purity, crystallinity, and control of the electrical compensation mechanism. Sandia National Laboratories, California, in close collaboration with US industry and academia, has initiated efforts to develop a detailed understanding of the underlying material problems limiting the performance of large volume gamma-ray spectrometers and to overcome them through appropriate corrections therein. A variety of analytical and numerical techniques are employed to quantify impurities, compositional and stoichiometric variations, crystallinity, strain, bulk and surface defect states, carrier mobilities and lifetimes, electric field distributions, and contact chemistry. Data from these measurements are correlated with spatial maps of the gamma-ray and alpha particle spectroscopic response to determine improvements in the material purification, crystal growth, detector fabrication, and surface passivation procedures. The results of several analytical techniques will be discussed. The intended accomplishment of this work is to develop a low-cost, high-efficiency CZT spectrometer with an active volume of 5 cm{sup 3} and energy resolution of 1--2% (at 662 keV), which would give the US a new field capability for screening radioactive substances.
Date: September 1, 1997
Creator: James, R.B.; Lund, J. & Yoon, H.
Partner: UNT Libraries Government Documents Department

Analysis of Cd{sub 1{minus}x}Zn{sub x}Te microstructure

Description: The microstructure and chemical inhomogeneities of commercially available Cd{sub 1{minus}x}Zn{sub x}Te (CZT) have been evaluated using electron microscopy and microanalytical techniques. Since imperfections, such as inclusions, cracks and extended crystallographic defects are known to affect the performance of CZT gamma-ray spectrometers, understanding the nature and origins of such imperfections is vital to the improvement of device performance. CZT that is grown using a high-pressure Bridgeman method has a polycrystalline microstructure that contains numerous grain boundaries, twins and inclusions. In this study, scanning electron microscopy and X-ray energy-dispersive spectroscopy were used to analyze inclusions and cracks inside CZT material. Such analysis found regions of material rich in C, O, Si, Zn and Te. Transmission electron microscopy revealed small subgrains and thin platelets of a second phase material located inside the large-grain CZT matrix. Details of these microstructural features and their possible origins are discussed.
Date: August 1, 1997
Creator: Heffelfinger, J. R.; Medlin, D. L.; Yoon, H. & James, R. B.
Partner: UNT Libraries Government Documents Department

Spatial Mapping of Cadmium Zinc Telluride Materials Properties and Electrical Response to Improve Device yield and Performance

Description: Cadmium zinc telluride has experienced tremendous growth in its application to various radiation sensing problems over the last five years. However, there are still issues with yield, particularly of the large volume devices needed for imaging and sensitivity-critical applications. Inhomogeneities of various types and on various length scales currently prevent the fabrication of large devices of high spectral performance. This paper discusses the development of a set of characterization tools for quantifying these inhomogeneities, in order to develop improvement strategies to achieve the desired cadmium zinc telluride crystals for detector fabrication.
Date: January 26, 1998
Creator: Scyoc, J. M. Van; Brunet, B. A.; Yoon, H.; Gilbert, T. S.; Hilton, N. R.; Lund, J. C. et al.
Partner: UNT Libraries Government Documents Department

MODULAR AND FULL SIZE SIMPLIFIED BOILING WATER REACTOR DESIGN WITH FULLY PASSIVE SAFETY SYSTEMS

Description: OAK B204 The overall goal of this three-year research project was to develop a new scientific design of a compact modular 200 MWe and a full size 1200 MWe simplified boiling water reactors (SBWR). Specific objectives of this research were: (1) to perform scientific designs of the core neutronics and core thermal-hydraulics for a small capacity and full size simplified boiling water reactor, (2) to develop a passive safety system design, (3) improve and validate safety analysis code, (4) demonstrate experimentally and analytically all design functions of the safety systems for the design basis accidents (DBA) and (5) to develop the final scientific design of both SBWR systems, 200 MWe (SBWR-200) and 1200 MWe (SBWR-1200). The SBWR combines the advantages of design simplicity and completely passive safety systems. These advantages fit well within the objectives of NERI and the Department of Energy's focus on the development of Generation III and IV nuclear power. The 3-year research program was structured around seven tasks. Task 1 was to perform the preliminary thermal-hydraulic design. Task 2 was to perform the core neutronic design analysis. Task 3 was to perform a detailed scaling study and obtain corresponding PUMA conditions from an integral test. Task 4 was to perform integral tests and code evaluation for the DBA. Task 5 was to perform a safety analysis for the DBA. Task 6 was to perform a BWR stability analysis. Task 7 was to perform a final scientific design of the compact modular SBWR-200 and the full size SBWR-1200. A no cost extension for the third year was requested and the request was granted and all the project tasks were completed by April 2003. The design activities in tasks 1, 2, and 3 were completed as planned. The existing thermal-hydraulic information, core physics, and fuel lattice information was ...
Date: June 16, 2003
Creator: Ishii, M.; Revankar, S. T.; Downar, T.; Xu, Y.; Yoon, H. J.; Tinkler, D. et al.
Partner: UNT Libraries Government Documents Department

Analysis of CZT crystals and detectors grown in Russia and the Ukraine by high-pressure Bridgman methods

Description: Sandia National Laboratories (SNL) is leading an effort to evaluate vertical high pressure Bridgman (VHPB) Cd{sub 1-x}Zn{sub x}Te (CZT) crystals grown in the former Soviet Union (FSU) (Ukraine and Russia), in order to study the parameters limiting the crystal quality and the radiation detector performance. The stoichiometry of the CZT crystals, with 0.04 < x < 0.25, has been determined by methods such as proton-induced X-ray emission (PIXE), X-ray diffraction (XRD), microprobe analysis and laser ablation ICP mass spectroscopy (LA-ICP/MS). Other methods such as triaxial double crystal x-ray diffraction (TADXRD), infrared transmission spectroscopy (IR), atomic force microscopy (AFM), thermoelectric emission spectroscopy (TEES) and laser induced transient charge technique (TCT) were also used to evaluate the material properties. The authors have measured the zinc distribution in a CZT ingot along the axial direction and also its homogeneity. The (Cd+Zn)/Te average ratio measured on the Ukraine crystals was 1.2, compared to the ratio of 0.9-1.06 on the Russian ingots. The IR transmission showed highly decorated grain boundaries with precipitates and hollow bubbles. Microprobe elemental analysis and LA-ICP/MS showed carbon precipitates in the CZT bulk and carbon deposits along grain boundaries. The higher concentration of impurities and the imperfect crystallinity lead to shorter electron and hole lifetimes in the range of 0.5--2 {micro}s and 0.1 {micro}s respectively, compared to 3--20 {micro}s and 1--7 {micro}s measured on US spectrometer grade CZT detectors. These results are consistent with the lower resistivity and worse crystalline perfection of these crystals, compared to US grown CZT. However, recently grown CZT from FSU exhibited better detector performance and good response to alpha particles.
Date: January 10, 2000
Creator: Hermon, H.; Schieber, M.; James, R. B.; Lee, E. Y.; Yang, N.; Antolak, A. J. et al.
Partner: UNT Libraries Government Documents Department

Evaluation of CZT crystals from the former Soviet Union

Description: Vertical high pressure Bridgman (VHPB) Cd{sub 1{minus}x}Zn{sub x}Te (0.04 < x < 0.24) detector crystals grown in the Ukraine and Russia have been evaluated and compared to US-grown materials. Various analytical techniques were used to study the materials for trace impurities, precipitates, crystallinity, and electrical transport properties. Relatively high concentrations of carbon and trace impurities such as Se, Nd and Si have been detected in the crystals. In most cases, the crystals showed lower resistivity than US-grown CZT. However, recent crystals grown in Russia exhibited better detector performance than those grown in prior years, and good response to an {sup 241}Am radioactive source was found. Electron lifetimes below 1 {micro}s were measured in crystals having significant numbers of micro-defects, compared to lifetimes of 5--15 {micro}s found in spectrometer grade materials produced in the US. Furthermore, the zinc composition along the growth axis showed better homogeneity in comparison with the US material.
Date: January 26, 1998
Creator: Hermon, H.; Schieber, M.; James, R. B.; Antolak, A. J.; Morse, D. H.; Brunett, B. et al.
Partner: UNT Libraries Government Documents Department

Multi-parameter high-resolution spatial maps of a CdZnTe radiation detector array

Description: Resistivity results from a 48x48 pixelated CdZnTe (CZT) radiation detector array are presented alongside X-ray topography and detector mapping with a collimated gamma-ray beam. By using a variety of measurements performed on the same sample and registering each data set relative to the others, the spatial dependence of relationships between them was examined. The local correlations between resistivity and one measure of detector performance were strongly influenced by the positions of grain boundaries and other gross crystal defects in the sample. These measurements highlight the need for material studies of spatially heterogeneous CZT to record position information along with the parameters under study.
Date: November 7, 1998
Creator: Hilton, N. R.; Barber, H. B.; Brunett, B. A.; Eskin, J. D.; Goorsky, M. S.; James, R. B. et al.
Partner: UNT Libraries Government Documents Department