Neutron Slowing Down Time Based Inspection Method

The neutron slowing down time based inspection system extends the well-established differential die-away analysis (DDAA) to higher neutron energies and earlier fission neutron detection times. Simulation tools, mainly MCNPX, validated previously by experiments are used to further extend the domain of DDAA, which relies solely on thermal neutron-induced fissions, into the high-sub-MeV neutrons. Detailed time-energy correlated behavior of slowing-down neutrons, following a narrow pulse of 14-MeV neutrons, is calculated for ideal moderators like polyethylene and beryllium as well as for generic hydrogenous and metallic cargo materials. The time evolution and energy spectra of neutrons escaping the cargo-containing special nuclear materials (SNMs) (such as
²³³
U,
²³⁵
U,
²³⁹
Pu, and
²⁴¹
Pu) are shown to be very different and easily separable from the same quantities when an SNM is absent. The key to the technique is the “vanishing time”: the time when high-energy source neutrons are scattered down to energies below those of fission neutrons. If fissile material is present, high-energy neutrons (>1 MeV) reappear solely due to fissions induced by the slowed down source neutrons, with energy below 1 MeV. The fission neutrons, which escape cargo absorption, are then detected by a direct fast neutron detector. The detectors can be, in principle, organic or
⁴
He recoil-based scintillators, which are inherently insensitive to neutrons with lower energy than the fission neutrons. In addition, a successful implementation of this SNM inspection technique requires an intense medium-to-high duty factor narrow-pulse fast neutron generator, e.g., (d, T) or (d, D), and fast two-parameter (energy and time) data digitizers.