Year
2006
Abstract
This paper evaluates the feasibility of using a high resolution gamma ray spectrometer to detect the presence of weapons grade plutonium aboard a small craft in a littoral environment. It is envisioned that such a spectroscopy system would be deployed on a SPARTAN unmanned surface vessel (USV) in such a way as to provide at least two minutes of monitoring time at distances near enough to achieve very high statistical reliability. Greater statistical confidence is clearly a high priority in order to minimize the logistical and manpower burdens created by false positive alarms. Four hypothetical scenarios were envisioned and analyzed. Each scenario placed a plutonium source on the target vessel and varied source location, distances to the detector aboard the USV, and types of obstacles between source and detector. Actual background measurements were taken aboard a USNA Yard Patrol boat at sea and incorporated into the analyses. A Monte Carlo probabilistic approach was also conducted for the same four smuggling scenarios, with the detector positioned at a range of 10 meters from the source. These probabilistic analyses resulted in predicted count rates very close to those generated by the deterministic approach. Results from the two approaches to the problem compared very favorably, within a factor of two in most cases, and suggest the validity of both methodologies. Overall, the evidence suggests that the spectroscopy system appears to be viable for detecting plutonium aboard a vessel at sea, given sufficient counting time and close enough range. Maximum effective ranges varied from about 13 meters to more than 40 meters depending on the amount and type of material shielding the source.