The II-VI compound, cadmium zinc telluride (CdZnTe), is the most promising semiconductor material for fabricating room-temperature gamma-ray spectrometers for different nuclear engineering applications. Its unique properties, e.g. high-average atomic number, large enough band-gap, high resistivity and good charge transport ability, allow CdZnTe to work at room temperature with high detection efficiency and great energy resolution. However, the presence of various material defects, such as the vacancy-related ones, has a significant impact on the electrical properties of CdZnTe and can distinctly affect the detector performance. A deep understanding is highly desired to facilitate the precise control of these defects during detector material growth and device fabrication processes.



In this work, we propose to use positron annihilation lifetime spectroscopy (PALS) measurement to study CdZnTe crystals, aiming at achieving a better understanding of origins and natures of vacancy-related defects. As a powerful non-destructive technique, the PALS is very sensitive to local electron density and the nanopore properties of materials. More specifically, the positrons, after being implanted into materials, can naturally diffuse to and be trapped in free volumes, and subsequently decay with characteristic lifetimes, which can be associated with the properties of vacancy-type defects. Our proposed PALS measurements will be conducted in a wide temperature range from 77K to 750K. Furthermore, such efforts will be coupled with systematic annealing experiments in various vapor environments, enabling a controllable treatment to improve the performance of CdZnTe detectors. The 16 samples will have certain vacancy density and distribution after going through those specially-designed annealing treatments. For each sample, detailed temperature-dependent PALS data will be performed and collected, using 20 sampling points from 77K to 750K.



It should be noted that we already achieved some preliminary PALS results of CdZnTe (see attachment). These results are very exciting, which highlights the importance of a systematic PALS study of CdZnTe radiation detectors. As a result, we propose to use the NSUF facilities to carry out the corresponding study. The availability of positron user facility of North Carolina State University’s PULSTAR reactor program will allow us to conduct this new research endeavor.



The scientific output of this project will provide pivotal evidence in terms of the effects of vacancy-related defects on the electrical compensation and charge transport process of CdZnTe detectors. If the research is successful, it will enable us to make a step-change improvement in room-temperature radiation detectors. As a result, the accomplishment of the work will significantly promote the development of CdZnTe room-temperature radiation detectors and accelerate their wide deployment in nuclear engineering applications.

The proposed research lays the groundwork for developing advanced radiation sensors to address crosscutting technology gaps relating to measurements at existing and advanced nuclear power plants as well as within their fuel cycles. Such efforts directly advances DOE’s Office of Nuclear Energy mission to promote nuclear power as a resource capable of meeting the Nation's energy, environmental, and national security needs by resolving technical, cost, safety, proliferation resistance, and security barriers through research, development, and demonstration. More specifically, the proposed research supports and advances two DOE NE topics: “Advanced Sensors and Instrumentation” and “Proliferation and terrorism risk assessment”. The accomplishment of the work is expected to have a transformational influence on the radiation-sensor research. Such work focuses on filling the critical knowledge gaps to develop advanced methodologies for monitoring to achieve high reliability and availability of nuclear energy systems and to understand, limit, and manage the risks of proliferation and physical security for fuel cycle options. It fully meets the critical mission need of DOE NE.