Scaling beta-delayed neutron measurements to large detector areas

We explore the performance of a cargo screening system that consists of two large-sized composite scintillation detectors and a high-energy neutron interrogation source by modeling and simulation. The goal of the system is to measure β-delayed neutron emission from an illicit special nuclear material by use of active interrogation. This task is challenging because the β-delayed neutron yield is small in comparison with the yield of the prompt fission secondary products, β-delayed neutrons are emitted with relatively low energies, and high neutron and gamma backgrounds are typically present. Detectors used to measure delayed neutron emission must exhibit high intrinsic efficiency and cover a large solid angle, which also makes them sensitive to background neutron radiation. We present a case study where we attempt to detect the presence of 5 kg-scale quantities of ²³⁵U in a standard air-filled cargo container using 14 MeV neutrons as a probe. We find that by using a total measurement time of ∼11.6 s and a dose equivalent of ∼1.7 mrem, the presence of ²³⁵U can be detected with false positive and false negative probabilities that are both no larger than 0.1%.