Active Detection of Shielded SNM With 60-keV Neutrons

Fissile materials, e.g.,
²³⁵
U and
²³⁹
Pu, can be detected non-invasively by active neutron interrogation. A unique characteristic of fissile material exposed to neutrons is the prompt emission of high-energy (E > 1 MeV) fission neutrons. One promising mode of operation subjects the object to a beam of low-energy (E < 1 MeV) neutrons, generated by a proton beam impinging on a Li target. The emergence of high-energy secondary neutrons then clearly indicates the presence of fissile material. Our interrogation system comprises a low-dose 60-keV neutron generator (5 times 10
⁶
/s ), and a 1 m
²
array of scintillators for high-energy neutron detection. Preliminary experimental results demonstrate the detectability of small quantities (376 g) of
²³⁵
U shielded by steel (200 g/cm
²
) or plywood (30 g/cm
²
), with a typical measurement time of 1 min.