Neutron Spectrum Unfolding with Organic Scintillators for Arms-control Verification.

Abstract

Verification of warhead dismantlement will require verification of certain warhead attributes without the disclosure of sensitive design information. Traditional measurements like high-resolution gamma-ray spectroscopy would contain a wealth of classified information if applied for warhead verification. Neutron spectroscopy with organic scintillators, on the other hand, may enable the distinction of some treaty-relevant warhead attributes without the acquisition of design information. However, due to the weakly interacting nature of neutrons with matter, neutron spectra must be unfolded from scintillator pulse-height data, and this is a difficult inverse problem. Despite well-developed and diverse spectrum-unfolding algorithms, neutron spectra previously cannot be unfolded with the fidelity and stability that would be required for verification applications. In this thesis, a new direction is pursued which focuses on improving detector performance in order to arrive at a better-conditioned response matrix. This requires detailed characterization of detector performance using accelerator-based time-of-flight measurements. Three organic scintillators are fully characterized at the Nuclear Structure Laboratory at University of Notre Dame: the hydrogen-based liquid EJ309, the hydrogen-based plastic EJ299-33, and the deuterated liquid EJ315. Unfolding trials are conducted with fine-structured neutron spectra measured alongside time-of-flight data to be used for calculating reference spectra. It is shown that deuterated scintillators outperform standard hydrogen-based liquids at spectrum unfolding due to their improved matrix condition. This improvement, along with stabilization of various detector-performance characteristics, allows stable spectrum unfolding with 100-keV-wide energy groupings - a considerable improvement over prevailing results in unfolding literature. The elemental compositions of various low-Z neutron-attenuating materials are estimated to an accuracy of 10% from scintillator pulse-height data. This could be useful in verifying the presence of high-explosive and neutron-reflecting materials in a treaty-sanctioned nuclear weapon.