Passive one-dimensional self-transmission imaging of subcritical metallic plutonium assemblies

We present a new passive technique to assess the spatial extent of subcritical metallic assemblies of plutonium. This technique could be used in disarmament verification where the size of the assembly is a verification attribute. The technique is based on measurements of neutron and gamma-ray coincidences from fission events within special nuclear material (SNM) and is analogous to active transmission imaging where the interrogating source is the SNM itself. Experimental measurements were performed using a 252Cf source and a 4.5 kg sphere of weapons-grade plutonium metal. These measurements and Monte Carlo simulations demonstrate that this technique is highly sensitive to the outer radius of metallic spherical assemblies of plutonium and moderately sensitive to the presence of a void inside the plutonium.