A ug 2 00 7 Detection of Neutron Sources in Cargo Containers

We investigate the problem of detecting the presence of clandestine neutron sources, such as would be produced by nuclear weapons containing plutonium, within cargo containers. Small, simple and economical semiconductor photodiode detectors affixed to the outsides of containers are capable of producing statistically robust detections of unshielded sources when their output is integrated over the durations of ocean voyages. It is possible to shield such sources with thick layers of neutron-absorbing material, and to minimize the effects of such ab-sorbers on ambient or artificial external neutron fluxes by surrounding them with neutron-reflective material. Terrorist nuclear weapons or special nuclear material may be shipped in cargo containers, and their detection is a matter of serious concern. If the special nuclear material is plutonium then its 240 Pu content is a significant source of spontaneous fission neutrons, depending on the quantity of plutonium and its isotopic composition. These spontaneous fission neutrons may be detectable. In an earlier report 1 we suggested applying 10 B-coated photodiode neutron detectors to the outsides of shipping containers to detect any spontaneous fission neutrons emitted inside. Simple analytic estimates showed that,