  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 absorbers on ambient or artificial external neutron fluxes by
surrounding them with neutron-reflective material.
