FIGARO: detecting nuclear materials using high-energy gamma-rays

The potential diversion of nuclear materials is a major international concern. Fissile (e.g., U, Pu) and other nuclear materials (e.g., D, Be) can be detected using 6–7 MeV gamma-rays produced in the 19F(p,αγ) 16O reaction. These gamma-rays can induce neutron emission via photoneutron and photofission processes in nuclear materials. However, they are not energetic enough to generate significant numbers of neutrons from common benign materials. Neutrons are counted using an array of BF3 tubes in a polyethylene moderator. A strong increase in neutron count rates is seen when irradiating depleted uranium, Be, D2O, and 6Li, with little or no increase for other materials (e.g., H2O, SS, Cu, Al, C, 7Li). Experiments using both photon and neutron shielding show that the technique is resistant to countermeasures. We have reduced the neutron background from proton beam reactions (thus increasing the system's sensitivity) and have tested a high-current gas cell which should be capable of operating at proton beam currents of up to 100 μA.