Estimation of the Elemental Composition of Low-Z Neutron-attenuating Materials Using Scintillator Pulse-height Data

Neutron spectrum unfolding is the notoriously difficult inverse problem that must be overcome to derive spectral information about fast neutrons from scintillator pulse-height analysis. This difficulty, along with the relative paucity of information carried by neutron spectra from typical sources, has generally relegated neutron spectrum unfolding to dosimetry applications. But new treaty-verification challenges like warhead verification call for the characterization of various treaty- relevant source attributes without the revelation of design information, and neutron spectroscopy may offer some hope toward this end. Two attributes of interest are the presence of high explosive (HE) and common neutron-reflecting materials like beryllium metal. By exploiting the enhanced spectroscopic capabilities of deuterated liquid scintillators, and utilizing a priori information about the energy-dependent cross sections of the low-Z constituents of HE and neutron reflectors, we can dramatically constrain the parameter space for the inverse problem of spectrum unfolding. This may enable estimation of the elemental compositions of warhead components surrounding the neutron-emitting plutonium pit using pulse-height data from scintillators which are otherwise insensitive to warhead design information. Here we describe techniques used, present proof-of- principle measurements and Monte Carlo simulations, and describe future studies to further develop this capability.