Quantitative Compton Imaging in 3D

Much of the verification work performed in international nuclear safeguards, including the evaluation of declared materials through non-destructive assay, relies on radiation detection instrumentation. Specifically, the penetrative and characteristic nature of gamma rays from nuclear materials have made advanced spectrometers a key tool for such activities, and more recently, imaging spectrometers have been investigated for this purpose. Such instruments are less impacted by background radiation and are significantly more forgiving of alignment requirements. They are also particularly useful in situations where material location is unknown, e.g. gloveboxes. However, to fulfill validation requirements, the images must be quantitative with regards to the types and amount of nuclear materials present. To obtain quantitative results, distances to the materials must be known and these can be generated by combining the imager with 3D scene capturing hardware and software. In this paper we will describe work towards quantitative gamma-ray imaging for non-destructive assay and evaluation of declarations using commercial gamma-ray imagers and a suite of sensors that provide 3D scene information. Specifically, we present our work using the H3D H420 imaging system augmented with a 16-beam lidar, depth camera and inertial measurement unit. While the high energy resolution and 3D spatial resolution of the H420 enable both high-precision coded-aperture and Compton imaging, we focus here only on the Compton-imaging modality. We will present on all necessary components to perform this type of analysis, including data readout, response characterization, 3D scene mapping and advances in list-mode reconstruction algorithms. We will emphasize the progress made with image reconstruction techniques, including multi-static-view 2D to 3D imaging, background handling and contextual data fusion. We demonstrate the current capabilities of the system with laboratory measurements and several source standards. We will conclude with future work and how the developed approach integrates with the coded-aperture-imaging modality to present a robust tool that can be used across a broad range of gamma-ray energies relevant to the nuclear safeguards domain.