Joint Evaluation of Neutron and Gamma-ray multiplicity Analysis for Warhead
Attributes

We will present the concept of an attribute measurement system that uses the time-correlated detection of neutron-capture gamma rays together with neutrons to confirm the presence of several warhead attributes concurrently: (1) plutonium mass, (2) multiplication, (3) presence and type of high explosive (HE), and (4) HE thickness. This combination of attributes is especially useful for confirming the presence of a warhead and can also be used to confirm the dismantlement of a warhead (i.e., HE has been separated from the special nuclear material (SNM)). We will present results from recent demonstrations that were conducted with a capture-gamma multiplicity prototype comprising an array of eight 4”x4”x4” NaI detectors coupled to photomultiplier tubes (PMTs) together with a pair of MC-15 neutron multiplicity counters. Data acquired in a parametric study using a 4.5kg sphere of weapons-grade plutonium (the BeRP ball) moderated by various thicknesses of high-density polyethylene (HDPE) indicate that evaluating SNM mass and assembly multiplication as a function of thickness-dependent efficiency results in an agreement between neutron multiplicity data and capture-gamma multiplicity data at the correct thickness of moderating material. Additional measurements using mock-HE also demonstrate that the mass and multiplication can be reasonably estimated (relative to an MC-15) using correlated capture-gamma data indicating that this can be extended to warhead confirmation. This joint analysis technique promises to provide higher confidence than more traditional SNM mass attributes methods. For example, in addition to confirming that the total Pu mass present is greater than a threshold of 500g, this capability could potentially also assert that the multiplication is less than threshold X and/or greater than threshold Y, the HE signature is consistent with composition-Z (based on the capture-gamma spectrum), and the HE thickness is greater than T.