Combating nuisance alarms caused by the “ship effect” in helium 3 based neutron detection radiation portal monitors

CHED 955

Anna F. Oliveri, oliveaf05@juniata.edu1, Elise Buckley2, James Borgardt2, Richard T. Kouzes, richard.kouzes@pnl.gov3, Allen Seifert3, Edward R. Siciliano3, and Lindsay Windsor3. (1) Department of Chemistry, Juniata College, 1700 Moore Street, Huntingdon, PA 16652, (2) Department of Physics, Juniata College, 1700 Moore Street, Huntingdon, PA 16652, (3) Pacific Northwest National Laboratory, 902 Battelle Blvd, Richland, WA 99352
Ship effect neutrons can present unique challenges in ongoing efforts to interdict illicit nuclear trafficking at border crossings. 3He neutron proportional counters can generate false positives due to these neutron spikes. This work explores methods to mitigate these nuisance alarms through a better understanding of the effects manifested in different materials, the role of a target materials' neutron density, and data analysis techniques to account for its effects. We used a mobile RPM equipped with 3He tubes to detect the neutron flux from several commercial NORM. While neutrons from illicit nuclear sources are Poisson in their frequency distribution, those from background are not. Ship effect neutrons deviated from a Poisson distribution when binned over 0.1 sec time intervals, however when averaged over 2.0 sec intervals a Poisson distribution is recovered. These findings provide knowledge and suggest an analysis algorithm to distinguish between innocent ship effect neutrons and neutron-emitting illegal sources.