Designing and Implementing a Variable Energy Neutron System to Search for Conventional Explosives.

Abstract

Research was conducted to design an active neutron interrogation system to search for conventional explosives. The work began with a review of the current methods for explosives detection, with a particular focus on active neutron interrogation. The next step was to design a layered shield for an isotopic neutron generator that limited the influence of unattenuated neutrons at undesirable angles. The final shielding design was then adapted for neutron and gamma ray detector collimation in an active neutron interrogation system, and a modular shielding arrangement was proposed. Focus then shifted to improving the flexibility of the active neutron interrogation system via introduction of a method to reliably vary the energy from nearly monoenergetic sealed-tube deuterium-deuterium or deuterium-tritium neutron generators. This was accomplished through single neutron elastic scatter off a target of known composition, resulting in neutrons with predictable energies that can be adjusted through manipulation of the interrogation target location relative to the scatter target and neutron generator. This neutron elastic scatter system was optimized through the adoption of a neutron time-of-flight (TOF) method, where the hydrogen nuclei in the start detector (scatter target) also served as the scattering medium, signaling neutron scatter and allowing for discrimination of neutrons that did not interact in the scatter target. Laboratory measurements and simulations were performed to characterize the new variable energy neutron elastic scatter (VENES) system and evaluate its effectiveness in performing fast neutron resonance radiography. Preliminary investigations on the application of VENES to fast neutron analysis and neutron backscatter were also conducted to direct future work. There was a particular interest in combining all three active neutron interrogation methods with the VENES system and proposed shield designs, allowing for investigations across multiple different neutron energies in the MeV range. The VENES system permits for reliable alteration of neutron energy in active interrogation systems, using relatively cheap, portable, and easily operated components that are either currently, or soon-to-be, commercially available. The improvements on current active neutron interrogation sources should result in its eventual adoption to a variety of applications.