Passive gamma spectroscopy has been successfully used for nuclear warhead inspection systems based on the template-matching approach. The most prominent example of such a system is Sandia’s Trusted Radiation Identification System (TRIS), which is based on an earlier system used at Pantex since 1994 to confirm the identities of containerized plutonium pits. Remarkably, TRIS uses only 16 energy bins, i.e., 16 numbers, to accomplish this task. Additional experiments have shown that such a template-matching method could be performed in a way that does not reveal classified information. To be used in a real inspection setting, however, inspectors must gain confidence that the system hardware and software work as designed and display genuine measurements through a process known as authentication. It also requires establishing and maintaining confidence in the template, i.e., that the data characterizing the treaty accountable item is genuine and has not been altered. In the case of TRIS, the template data are stored electronically and signed as a whole, such that no information about the template can ever be shared with inspectors as a confidence-building measure. Here, we propose an inspection protocol that uses a different approach: Information is stored in the form of punched cards that encode the secret template. Public masks can be used to reveal selected features of the template, e.g., total counts in particular energy bins, while keeping others secret, constraining certain physical properties of the treaty accountable item and providing increasing levels of transparency. We illustrate our approach using Princeton’s Information Barrier Experimental II based on a vintage 6502 processor.
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