CODED APERTURE GAMMA-RAY IMAGING USING 3D POSITION-SENSITIVE SEMICONDUCTOR RADIATION DETECTORS FOR NUCLEAR SECURITY APPLICATIONS

摘要

In the past decade, there has been a significant increase in demand for radiation detectors that locate and identify potentially threatening nuclear materials. An 18 - detector array system was developed at the University of Michigan to be used in such applications. This portable detector system has the ability to detect and image gamma rays with energies between 30keV and 3MeV. Compton imaging is utilized to map out gamma-ray distributions in 4n space for energies above about 300keV. Due to the low Compton-interaction probability at energies below 300keV in the CdZnTe semiconductor detector material, gamma rays at these energies must be imaged using an alternative method which relies on photoelectric interactions. The system uses Coded Aperture Imaging (CAI) to image these lower-energy gamma rays. CAI relies on coded tungsten or lead masks with numerous open and closed elements. The size of these mask elements is chosen based on the spatial resolution of the detector, roughly one millimeter in this case. The coded mask is placed near the position-sensitive detector. The mask-to-detector distance is optimized with respect to the field-of-view and angular resolution of the image to be formed. When the mask-detector system is exposed to low-energy gamma rays, each mask element either attenuates or transmits gamma rays, resulting in a position-dependent count distribution in the detector. The count distribution and knowledge of the mask design and position are then used to reconstruct an image of the incident source distribution using algorithms such as backprojection, deconvolution, and maximum likelihood expectation maximization. CAI, coupled with Compton imaging, allows our group to extend imaging capabilities across the entire dynamic range of the electronic readout system. This work is supported by the U.S. Department of Homeland Security's Domestic Nuclear Detection Office and the National Science Foundation's Academic Research Initiative.