Passive Gamma-Ray Imaging as a Verification Tool for Nuclear Weapons Treaties

Treaties that include nuclear weapons as treaty accountable items (TAI) will invariably have to grapple with the question of how to verify that the TAI is indeed a nuclear weapon or a nuclear weapon component. While this may be a question that can be readily addressed with sufficient technical information about the TAI, the reality is that many of the technical details that allow an item to achieve nuclear yield are considered too sensitive for distribution, even among prospective treaty partners that are already nuclear weapons states. Nevertheless, to enforce such a treaty one will need technical means to inspect a TAI and verify that it is a nuclear weapon or weapon component. In particular, to provide confidence to the inspecting party, technical means must address TAI attributes that indicate nuclear yield is possible. One such attribute is the distribution of nuclear and overlying materials, and these can be determined using passive, coded-aperture, gamma-ray imaging. This paper addresses the fundamentals of gamma-ray imaging, and how it might be applied in TAI inspections.