Differential Die-Away Analysis to Detect the Presence of Nuclear Material in Cargo

Cargo scanning plays an essential role in the Department of Homeland Security's national security strategy. The improvement of nuclear and radiological material detection is imperative to maintain a strong defense against the smuggling of illicit materials. The University of Tennessee has developed a cargo container scanning system using differential die-away analysis to indicate the presence of shielded fissile material.

In preparation for measurements to be conducted at the Y-12 National Security Complex, our initial experimental setup uses natural uranium as simulated contraband within the cargo container. The container can be filled with hydrogenous materials, such as wood or paper, or steel in various configurations, replicating cargo contents of different densities. Scenarios will be considered that test the detection sensitivity of uranium via differential die-away due to the shielding of the interrogation source and/or the prompt fission neutrons. In the initial setup, a Sodern Genie 16 deuterium-tritium neutron generator is used to produce 14.1 MeV neutrons, which are moderated by a custom-designed moderator assembly. Stimulated neutrons are detected by fast neutron detectors, which include a liquid scintillator, a stilbene crystal scintillator, and an organic glass scintillator. The described setup and methodologies enable iterative experiments, which accommodate variations in the contraband position, filler materials, and neutron generator operating parameters. These experiments show the optimal system configuration and highlight challenges associated with determining the presence natural uranium.