UC Berkeley

There is an ever-growing need for instrumentation that provides both high-resolution and sensitive three-dimensional (3-D) gamma-ray imaging capabilities across a wide range of photon energies under near-field conditions. Such technology is particularly critical to the fields of emergency response and contamination remediation, nuclear security and safeguards, and nuclear medicine. To meet this technological demand, this dissertation presents a proof-of-principle gamma-ray imaging prototype that functions as both a coded aperture and Compton imager, with the former modality suited to energies below a few hundred keV and the latter suited to energies above a few hundred keV. This prototype integrates a novel coded aperture design with a Compton camera that consists of two high-purity germanium (HPGe) double-sided strip detectors (DSSDs). The two imaging modalities are operated serially in the near field via a single detection system. The design and pattern optimization of the coded aperture as well as the methodologies developed for coded aperture and Compton image reconstruction are discussed. Furthermore, this work includes 3-D gamma-ray images of sources of various shapes and energies ranging from about 100 keV to1 MeV in the near field to demonstrate the broad imaging capabilities of the system.

This dissertation investigates the collective use of coded aperture and Compton imaging in the fields of nuclear safeguards and nuclear medicine. In nuclear safeguards, uranium holdup is one of the more insidious problem of materials accounting and control. Both coded aperture and Compton imaging can be applied to solve this problem, offering the possibility of visualizing and quantifying uranium holdup via the 186-keV gamma-ray emission of ²³⁵U and 1001-keV gamma-ray emission of ²³⁸U, respectively. Three-dimensional coded aperture and Compton images of highly-enriched uranium (HEU) pellets are included in this work.

Another important application of the proposed technology is facilitating the development of a powerful cancer treatment known as targeted alpha-particle therapy (TAT). Arguably the most promising TAT radionuclide that has been proposed is ²²⁵Ac. The development of ²²⁵Ac-based radiopharmaceuticals has been hampered due to the lack of effective means to study the daughter redistribution of these agents in small animals at the preclinical stage. The ability to directly image the daughters, namely ²²¹Fr and ²¹³Bi, via their gamma-ray emissions would be a boon for preclinical studies. That said, conventional medical imaging technologies, including single photon emission computed tomography (SPECT) based on pinhole or parallel-hole collimation, cannot be employed due to sensitivity limitations. As an alternative, this dissertation investigates the use of coded aperture and Compton imaging as complementary modalities to image ²²¹Fr via its 218-keV gamma-ray emission and ²¹³Bi via its 440-keV gamma-ray emission, respectively. This work includes images of ²²¹Fr and ²¹³Bi in tumor-bearing mice injected with ²²⁵Ac-based radiopharmaceutical. These results are the first demonstration of visualizing and quantifying the ²²⁵Ac daughters in small animals via gamma-ray imaging and serve as a stepping stone for future radiopharmaceutical studies.