Neutron Detection With Gamma-Ray Spectrometers for Border Security Applications

Development of technologies for neutron detection that do not require ³He is important because the supply of ³He is very limited, and the cost of the gas is becoming prohibitive for many applications. This study evaluates the ability to detect neutron sources with gamma-ray spectrometers that are already present in many radiation measurement systems. Detection is based on count rates for gamma rays in the 3 to 8 MeV range, which are produced by the emission of fission gamma rays and neutron capture reactions in vehicles and their cargo. For materials in the normal stream of commerce, gamma rays above 3 MeV are produced only by sources that also emit neutrons. Therefore, unless the gamma-ray count rate is high enough to produce excessive random pileup, the detection of high-energy gamma rays provides an unambiguous indication of the presence of a neutron source. As part of this investigation, several shields that are suitable for use in radiation portals were constructed and characterized for their abilities to produce additional high-energy, neutron-capture gamma rays. A shield (composed of alternating layers of polyethylene and steel) enhances the ability to detect neutrons without producing detrimental effects for gamma-ray measurements. Calculations show that when shielded by neutron-detection-enhancing materials, NaI detectors can be as sensitive to the presence of a concealed neutron source as moderated ³He detectors.