Detecting Low Levels of Plutonium with HPGe Spectra, Compton Anticoincidencing, and Advanced Methods in Robust Fitting *

The problem of accurately detecting extremely low levels of plutonium is rapidly gaining increasing importance in applications of nuclear counter-proliferation, verification, and environmental and waste management. If methods to confidently detect trace signatures of plutonium based on gamma-ray spectra alone could prove sufficiently effective, remote-monitoring systems could be greatly simplified and confidence levels and throughput of environmental and waste management systems could be greatly increased. Relative to signal from very low levels of 239Pu, the background from natural terrestrial sources is so high that, as in space applications, consideration of Compton anticoincidencing is compelling. However, its use in materials management applications requires demonstration that the increased sensitivity be sufficiently high to overcome added cost and weight. In this paper we report the results of a laboratory experiment to determine sensitivity that can be gained in detecting low levels of 239Pu from Compton anticoincidencing in combination with newly-developed spectral analysis methodology, in which we collected a logarithmically-varying time series of spectra both with and without Compton anticoincidencing from a 98-nCi laboratory source. It was found that by using Compton anticoincidencing the collection time needed to achieve the same level of confidence could be decreased by a factor of 1.8.