Associated Particles Technique (APT) / Nanosecond Neutron Analysis (NNA)

The Associated Particle Technique / Nanosecond Neutron Analysis (APT/NNA) is a version of "neutron in, gamma out" technique, which allows one to suppress the background gamma-rays by a factor of 100, thus drastically reducing the detection time. Some versions of APT/NNA allow one to obtain a 3D image of chemical (elemental) composition of the inspected volume or object.

The main source of the background gamma-rays in "neutron in, gamma out" methods are gamma-rays that are produced by primary neutrons in the surrounding materials (floor, walls, construction materials, etc.) APT/NNA suppresses this background by detecting only those gamma-rays that are born in the inspected object. This is achieved by "tagging" the primary neutrons by an associated particle and accepting only those gamma-rays that coincide with those "tagged" neutrons.

In deuterium-tritium (DT) neutron generators "tagging" is done by detecting 3 MeV alpha-particles that are emitted simultaneously with, and at about 180 degrees ("back-to-back") to each of the 14 MeV neutrons produced in d(t,α)n reaction. These alpha particles are detected by a position sensitive detector, which provide information about emission time and direction of each of the 14 MeV neutrons.

For each of the detected gamma-quanta two parameters are determined: its energy and time-of-flight relative to the associated alpha-particle. Analysis of the energy spectrum of these gamma-rays yields chemical (isotopic) composition of the inspected object. Since direction of each alpha-particle (and thus for each neutron) is known, one can obtain a 3D image of the inspected object or volume in terms of its chemical composition.

More information about APT and NNA can be found here:

http://www.anl.gov/index.html,
http://www.nv.doe.gov/main.htm,
http://www.sciencedirect.com/