The goal of this project is to develop high-resolution Cadmium Manganese Telluride (CMT) and Cadmium Zinc Telluride (CZT) materials for nuclear and radiological detection applications in homeland security. These materials have emerged as promising advanced detectors for X-ray and gamma-ray spectroscopy and imaging without cryogenic cooling. Advances in research have led to the development of CZT for commercial devices, but there is still the presence of defects such as Tellurium (Te) inclusions that limit the performance of large-volume CZT and CMT detectors that are needed for nuclear and radiological detection applications. The results of this project will provide benefit to society, with high impact on the science and technology of semiconductor nuclear detectors for room-temperature applications in homeland security (maritime and port security, border security, transportation security, nonproliferation and domestic nuclear security). The outcomes also include new capabilities that are very important to the success of emerging detector technologies and analysis tools needed to support next-generation nuclear materials management and safeguards. This project advances discovery and understanding while promoting teaching, training, and learning.
The project team will use theory-based design, knowledge-based processing and fabrication, and novel experimental techniques to develop improved cadmium manganese telluride (CMT) and cadmium zinc telluride (CZT) materials for high-resolution nuclear detection applications. The project will enhance the science and expand the overall knowledge in this area by using a combination of theory, modeling and experiments to complete the following tasks: 1) optimization of the Bridgman methods and Traveling Heater Method (THM) for growth of improved CMT and CZT crystals; 2) a novel post-growth annealing and doping process for removing performance-limiting defects caused by tellurium inclusions and associated impurities in CMT and CZT detector materials; and 3) improved surface passivation and detector fabrication techniques to produce better detectors. These methods employ state-of-the-art instrumentations that incorporate 3D-infrared transmission spectroscopy and advanced measurement tools to probe and collect data during the post-growth annealing process. The development of in-situ tools to monitor crystal annealing adds a new experimental dimension that will lead to any improved understanding of the migration of tellurium inclusions and novel methods to minimize their impact on electron trapping. The end result will be CMT and CZT detectors with better resolution, improved detection efficiency and better directional sensitivity.
The outcomes from the modeling aspects of this project will provide an understanding of fundamental phenomena associated with Bridgman and THM growth of ternary II-VI compounds and suggestions regarding post-growth treatments to improve the microstructural properties of these crystals. The anticipated impact of in-situ probing and data collection techniques will include new insights into the science and dynamical properties of post-growth annealing, uniform doping of detector materials, migration of Te secondary phases and impurities, and methods to process detector surfaces. This project advances discovery and understanding while promoting teaching, training, and learning. It is multidisciplinary with investigators from the following collaborating entities: Alabama A&M University, University of Minnesota ? Twin Cities, Brookhaven National Laboratory (BNL), FLIR Radiation Inc, and the Interdisciplinary Consortium for Research and Educational Access in Science and Engineering (INCREASE). The workforce development component of this project will provide opportunities for women and under-represented minorities to build careers and earn graduate degrees in areas critical to the development of cutting-edge nuclear and radiological detection technology.
The results from this project made impact on the development of improved cadmium manganese telluride (CMT) and cadmium zinc telluride (CZT) materials for high resolution and room-temperature nuclear detection applications in homeland security. The major advantage of CZT and CMT detectors is their capability to operate at room temperature without cryogenic cooling. The major areas of impact are: Crystal growth: optimization of carbon coating thickness to produce CZT crystals with less dislocations and hence more efficient detectors. Post-growth annealing for removing performance-limiting defects caused by tellurium inclusions and associated impurities in CZT and CMT crystals. Surface processing for improved surface passivation and detector fabrication techniques to produce better detectors. Research infrastructure and workforce development across science, technology, engineering and mathematics (STEM) disciplines with focus in nuclear radiation detection and identification of radioisotopes. Applications areas in homeland security include maritime and port security, border security, transportation security, nonproliferation and domestic nuclear security. In growing CZT crystals using the Bridgman method, our etch pit density (EPD) measurements showed that carbon coating the ampoule with a thickness of about 2.0 micrometers gives fewer dislocations than smaller coating thicknesses of about 0.2 micrometer. This result will help optimize carbon coating thickness to produce CZT crystals with less dislocations and hence more efficient detectors. The post-growth annealing of CZT in cadmium vapor reduced the tellurium inclusions in the crystal. This led to a reduction of the number of charge carriers trapped by tellurium inclusions, and hence more efficient detectors. The sizes of tellurium inclusions were reduced up to 80% in a 60-hour annealing of CZT at 510 oC in cadmium vapor. Tellurium inclusions were completely eliminated in CMT samples annealed at 570 oC in cadmium vapor for 26 hours. We also observed that annealing the samples under temperature gradient causes the migration of tellurium inclusions from low temperature region to high temperature region of the crystal. In many cases the migrating tellurium inclusion leaves a void behind. For CZT samples annealed at 700 oC in a 10 ?C/cm temperature gradient, we observed Te inclusions migration from low to high temperature region at a rate of 0.022 micrometer per second. Our surface processing and detector fabrication results show that Chemo-Mechanical Polishing with bromine-methanol-ethylene glycol solution proved to be a better method for reducing surface leakage current, compared to just polishing in alumina powder or polishing in alumina powder plus etching in bromine-methanol. Hydrogen peroxide and a mixture of ammonium fluoride and hydrogen peroxide passivation agents also decreased the surface-leakage current and improved the detection efficiency of CZT detectors. The above technical results provide benefit to society, with impact on the science and technology of semiconductor nuclear detectors for room-temperature applications in homeland security (maritime and port security, border security, transportation security, nonproliferation and domestic nuclear security). These outcomes are also important to the success of developing technologies and analysis tools to support next-generation nuclear materials management and safeguards for future U.S. fuel cycles. While advancing discovery and understanding, this project also promoted teaching, training, and learning. It enhanced infrastructure for research and education through the establishment of a nuclear engineering and radiological science laboratory at Alabama A&M University for faculty and students’ research. In addition to research capabilities, the laboratory is equipped to train students in nuclear radiation detection and identification of radioisotpes. The multidisciplinary composition of the investigators, from materials science, nuclear engineering, physics, and computational sciences, enabled the project to contribute to student training and workforce development across STEM disciplines. The project integrated research and education through class-group projects, capstone/senior projects, and graduate thesis and dissertation research activities. The capstone/senior design projects for students in electrical engineering and mechanical engineering were developed in the areas of vacuum systems, furnaces and heat transfer, photonic materials and radiation detection. Fifteen minority students were trained to acquire technical skills in the development of semiconductor nuclear radiation detectors: twelve undergraduates and three graduate students. Out of the fifteen African American students, four are female and eleven are male. The thesis and dissertation research projects of two of the three graduate students were fully supported by this project (the students graduated in 2013, one with a Ph.D. degree and the other with a thesis-based M.S. degree). This project also resulted in the expansion of our research and students training collaborations with Brookhaven National Laboratory (BNL). The average number of participants supported per year by BNL (through the Office of Educational Services and the Nonproliferation and National Security Department) in these collaborations includes five undergraduates, two graduate students, and three faculty members. Long-term research and training activities with BNL were established with access to various Nonproliferation and National Security Department laboratories for crystal growth, detector fabrication and characterization, and other user facilities, including the National Synchrotron Light Source (NSLS) and the Center for Functional Nanomaterials (CFN).
