Dual-Particle Imaging for Treaty Verification

Treaty verification frequently requires verifying the presence of special nuclear material (SNM) through radiation measurements. The measurements aim at detection, localization, and identification of SNM. The Dual-Particle Imager (DPI) is an instrument based on scintillation detectors (liquid organic and sodium iodide) that has been proposed as a device capable of fulfilling these objectives. The DPI is sensitive to fast neutrons and gamma-rays, with the ability to separately reconstruct images and emitted energy spectra for both particle types. To aid this effort, new advanced imaging techniques are being developed for use with the DPI that improve image resolution and allow for the isolation of emitted energy spectra in a multi-source environment. The system is fully scalable, and a handheld version of the DPI is also being developed. This smaller system relies on an array of stilbene scintillator bars, which are sensitive to both neutrons and photons. Each bar is read-out using silicon photomultipliers at either end, allowing for axial position sensitivity. This paper presents results from the first experiments using dual particle imaging on category-I SNM. The experiments were performed at the Device Assembly Facility (DAF) at the Nevada National Security Site (NNSS) and included a 4.5-kg sphere and 4.1-kg disk of weapons grade plutonium as well as a 13.1-kg sphere of highly enriched uranium. The advanced imaging methods will demonstrate the ability of the DPI to locate and characterize these SNM sources when other sources, which also emit neutrons and gamma-rays, are present in the field of view.