High Signal-to-Noise Pixelated CZT Directional Spectrometer for Generalized Holdup Measurements

Holdup is the residual special nuclear material, such as enriched uranium and plutonium, that accumulates in pipes, ducts, gloveboxes and other equipment at fuel processing facilities. Quantification of this material is important for the material balance of the plant, which is a key aspect of nuclear safeguards. However, holdup is difficult to measure, and conventional methods are subject to large error, primarily from the geometry of the equipment and unknown distribution of the material inside. This project will produce a portable instrument that can localize and quantify the materials by imaging their gamma emissions with a high signal-to-noise ratio. The CZT-based imaging spectrometer will use a specially designed coded-aperture mask to resolve low-energy gamma spectra as a function of direction, with angular resolution of a few degrees, and be coupled to a simultaneous localization and mapping tool for relative position and physical scene information. When left to dwell for long measurements, the image will reach high signal-to-noise ratio, improving materials quantification for complex holdup distributions. Phase I designed a new mask for holdup measurements. The coded-aperture mask was optimized using a ray-tracing simulation and then validated by laboratory measurements using check sources and nuclear materials resembling holdup. Directional spectroscopy software was also developed for the imager, which will be used to develop and experimentally test new quantitative imaging methods. The goal of Phase II is to develop a portable imaging spectrometer that can deliver quantitative images to users, i.e., estimates of material mass on a pixel-wise basis. Such a tool could save workers time and effort, as well as reduce radiation dose, since they would be able to set up and leave the imager to measure a piece of equipment rather than manually scan it. Further, the device will enable users to construct representative shielding terms by creating a point-cloud of the scanned objects for automated depth and orientation determination. The device may also find materials that were not otherwise detectable or noticed with manual instruments. The broader effect of a successful adoption of this technology is to reduce the uncertainty in special nuclear materials balance at a plant which would contribute to safeguards and civilian safety. Beyond fuel processing, this instrument will be useful in other measurement scenarios requiring high-quality images of isotopes for quantification, including waste management, nuclear security, and radiation protection.