Nuclear based explosive inspection techniques can detect a wide range of substances of importance for a wide range
of objectives. For national and international security it is mainly the detection of nuclear materials, explosives and
narcotic threats. For Customs Services it is also cargo characterization for shipment control and customs duties. For
the military and other law enforcement agencies it could be the detection and/or validation of the presence of
explosive mines, improvised explosive devices (IED) and unexploded ordnances (UXO).
The inspection is generally based on the nuclear interactions of the neutrons (or high energy photons) with the
various nuclides present and the detection of resultant characteristic emissions. These can be discrete gamma lines
resulting from the thermal neutron capture process (n,γ) or inelastic neutron scattering (n,n'γ) occurring with fast
neutrons. The two types of reactions are generally complementary. The capture process provides energetic and
highly penetrating gamma rays in most inorganic substances and in hydrogen, while fast neutron inelastic scattering
provides relatively strong gamma-ray signatures in light elements such as carbon and oxygen. In some specific
important cases unique signatures are provided by the neutron capture process in light elements such as nitrogen,
where unusually high-energy gamma ray is produced. This forms the basis for key explosive detection techniques.
In some cases the elastically scattered source (of mono-energetic) neutrons may provide information on the atomic
weight of the scattering elements.
The detection of nuclear materials, both fissionable (e.g., 238U) and fissile (e.g., 235U), are generally based on the
fissions induced by the probing neutrons (or photons) and detecting one or more of the unique signatures of the
fission process. These include prompt and delayed neutrons and gamma rays. These signatures are not discrete in
energy (typically they are continua) but temporally and energetically significantly different from the background,
thus making them readily distinguishable.
The penetrability of neutrons as probes and signatures as well as the gamma ray signatures make neutron
interrogation applicable to the inspection of large conveyances such as cars, trucks, marine containers and also
smaller objects like explosive mines concealed in the ground.
The application of nuclear interrogation techniques greatly depends on operational requirements. For example
explosive mines and IED detection clearly require one-sided inspection, which excludes transmission based
inspection (e.g., transmission radiography) and greatly limits others.
The technologies developed over the last decades are now being implemented with good results. Further advances
have been made over the last several years that increase the sensitivity, applicability and robustness of these systems.
The principle, applications and status of neutron-based inspection techniques will be reviewed.