Active detection of small quantities of shielded highly-enriched uranium using low-dose 60-kev neutron interrogation

Active interrogation with low-energy neutrons provides a search technique for highly-enriched uranium concealed in cargo. We describe the technique and show initial results using a low-dose 60-keV neutron beam. This technique produces a clear induced fission signal in the presence of small quantities of ²³⁵U. The technique has been validated with low-Z and high-Z cargo materials. The technique uses a forward-directed beam of 60-keV neutrons to induce fission in ²³⁵U. Detection of the fast fission neutrons with pulse-shape discriminating scintillators is then the signature for ²³⁵U. The beam of neutrons is generated with a 1.93 MeV proton beam impinging on a natural lithium target. The proton beam is produced by a radio-frequency quadrupole LINAC. The 60 keV neutron beam is forward-directed because the ⁷Li(p, n) reaction is just above threshold for a proton energy of 1.93 MeV.