GAMMA-RAY IMAGING SPECTROMETERS FOR NUCLEAR MATERIALS ACCOUNTING AND CONTROL

The goal of this work is to enable the Polaris technology platform to meet the needs of the nuclear materials accounting and control community. The current product, Polaris-H, can offer room- temperature high-resolution spectroscopy (1% FWHM at 662 keV) and omni-directional Compton imaging in a package that weighs less than 10 lbs with a 5 hour battery life. This capability makes the Polaris platform ideal to locate, quantify, and identify radiation sources and detect minor material diversions or changes in processes. However, there is a major limitation with the Polaris platform that was identified prior to the start of the phase I effort: at high count rates the quantitative analysis broke down due to high dead time and at dose rates above 100 mRem/hr the system could no longer form photopeaks. Through the phase I effort these issues were addressed to some extent, now a photopeak can be observed at dose rates as high as 1 Rem/hr. However, through interactions with potential users, especially at DOE labs such as Savannah River National Lab, it became apparent that the dynamic range in the image was as significant an issue as quantitative analysis at high count rates. The problem is that in these facilities there are many sources of radiation and it is difficult to only interrogate one subsystem without interference from other radiation. Without the ability to reject all other sources it is difficult to detect low source activities or small activity changes in the subsystem of interest. The results from phase I are the basis for a plan forward to adapt the Polaris-H technology into a product called the Polaris-P (P is for probe). In this product the CdZnTe crystal will be spatially separated from all the other electronics, including the readout boards and ADCs (analog-to-digital converters), so that it is possible to shield the CdZnTe crystal using a minimum amount of mass. This will allow the user to narrow their field of view with a collimator and boost the dynamic range between the directions of interest and all other directions by more than an order of magnitude. The Polaris-P will use a readout ASIC (application specific integrated circuit) developed by Brookhaven National Laboratory in order to further improve the quantitative analysis capability at high count rates. The Polaris-P will be handheld with a form factor similar to a power drill. The user will point the probe in the direction of the object of interest and measure until the system confirms that sufficient data has been collected. Drift in the system orientation over time will be corrected to enable handheld imaging. Tungsten shields will be sold with the probe and can be stacked to narrow the field of view to the user's desired extent. The result of these developments will be a product that has applications on material safety and security throughout the nuclear fuel cycle, with the strongest market in the commercial nuclear power sector.