Year
2007
Abstract
In the interdiction of illicitly trafficked Special Nuclear Material (SNM) the neutron signal is a valuable adjunct to the gamma-ray signature. Neutrons are not attenuated by heavy metal shielding which might be used to shield the gamma rays emitted by the source. However, “gross-counting” neutron detectors are unable to distinguish neutrons in the environmental background from those emitted by a neutron source. The most challenging interdiction situation for neutron detection is the case where the neutron count rate is up to ten times the average background. Count rates higher than this are readily detectable with gross counting instruments. Below this level however, gross counters have major problems due to innocent alarms. A portable instrument has been developed which incorporates 30 moderated 3He tubes, multiplicity counting electronics and software capable of discriminating against noncorrelated neutron events, thereby greatly increasing the sensitivity of detection of SNM. The instrument is packaged in a way that is easily transported in a motor vehicle and to be rapidly deployed as needed. The instrument is described and performance data are presented.