The Absent-Minded Inspector: Confirming The Absence Of Nuclear Warheads Via Passive Gamma-ray Measurements

Arms-control agreements between the United States and Russia negotiated after the end of the Cold War have imposed limits on the number of deployed strategic nuclear weapons. Verification of these agreements has relied on on-site inspections, sometimes supported by radiation detection techniques to confirm that an object is non-nuclear. Such absence-confirmation measurements, so far, rely on the detection of neutron emissions associated with the presence of plutonium, but they would be inadequate for uranium devices. Alternative instruments relying on the detection of gamma emissions could simultaneously confirm the presence or absence of both plutonium-based and uranium-based weapons, complementing existing systems that detect neutrons, which can only confirm the absence of plutonium devices. Here, we demonstrate an inspection system and prototype device that uses only passive gamma radiation detection techniques to confirm the absence of gamma emissions from containerized objects. Such a system would be particularly valuable for next-generation arms-control agreements that limit total numbers of weapons, including those deployed, in storage, and slated for dismantlement. We have conducted extensive Monte Carlo simulations to support the development of a verification protocol and detection algorithm, and exemplify the viability of the proposed system using standard laboratory check sources and MCNP simulations for simplified configurations of special nuclear material.