Elemental analysis by "neutron in, gamma out" technique

The main idea of the "neutron-in, gamma-out" technique is to irradiate the suspicious object or area with neutrons and to measure secondary g-radiation induced by neutrons in the material of the object. Different chemical elements found in HE produce well-known characteristic gamma-rays following scattering or absorption of neutrons. By decomposing the measured spectrum of gamma-rays into contributions from different chemical elements one can obtain the elemental composition of the inspected object, and thus determine whether it contains HE or other hazardous materials, or not.

The most important advantage of the method is that neutrons and gamma-rays have very high penetration ability, and explosives can be detected even if they are hidden behind concrete, metallic or other walls, while other techniques can not penetrate inside hermetic or metallic volumes.

Other strong side of "neutron-in, gamma-out" techniques is their capability to provide information on elemental composition of items within the inspected volume, which makes those methods highly selective. These methods are also effective even when area of interest can be accessed only from one side.

Efforts to develop devices based on "neutron-in, gamma-out" method have been made since 1970s around the world. While in some cases modifications of this method were successfully applied to specific tasks, the wide application of this potentially very powerful and universal technique has been hindered by a number of remaining unresolved issues.

The most important difficulties in traditional "neutron-in, gamma-out" methods are:
• different factors make gamma-ray background in the detector very high, leading to prohibitively long identification times of small amounts of explosives, which still may pose risk to the public. This problem is currently solved owing to the development of the APT and NNA techniques.
• neutron source must be powerful enough for a timely analysis, which raises concerns about public safety.

At present the best sources of neutrons for the multi-chemical element "neutron-in, gamma-out" method are portable neutron generators. They can provide a sufficient flux of fast neutrons while being non-radioactive when switched off; thus, they do not require any shielding when not in use. Portable neutron generators are commercially available from a few manufacturers such as SODERN in France, Thermo MF-Physics in the US, the All-Russia Research Institute of Automatics in Moscow, Russia.

Among commercial devices based on different varieties of the "neutron in, gamma out" method are PELAN, NIGAS, neutron analyzers by SODERN.