C.M. Ryan, C.M. Marianno, W.S. Charlton, A.A. Solodov, and J. Livesay, “Determining the Effects of Construction Materials on Background Readings for Second Line of Defense Program Portal Monitoring Systems”, 51st Annual Meeting of the Institute of Nuclear Materials and Management, Baltimore, MD, USA, July 11-15, 2010.
The Nonproliferation program is an initiative of the Department of Energy’s (DOE) National Nuclear Security Administration (NNSA) tasked with strengthening the capability of foreign governments to interdict the trafficking of illicit nuclear material across international borders and through maritime shipping channels. Radiation PortalMonitors (RPM) deployed around the world as part of the program are used to passively detect both gamma and neutron radiation from cargo and pedestrian traffic. Portal monitoring systems in some locations are experiencing higher than expected background counts. The increased background measurements have been attributed to the concrete surrounding the portal monitors. Higher background increases the minimum detectable activity and can ultimately lead to more materialpassing through the RPM undetected. This work is focused on understanding how the concrete surrounding the portalmonitors is contributing to the anomalous data. For this study, six samples were taken from three different composition concrete slabs. Each sample was counted using a high purity germanium (HPGe) detector to determine its natural activity. Neutron activation analysis (NAA) was done using a Training, Research, Isotopes, General Atomics (TRIGA) research reactor to irradiate representative portions of each sample in order to determine their elemental composition. The HPGe measurements and NAA results were used to create a source term and material card, respectively, in a Monte Carlo N-Particle (MCNP) representation of a RPM. The MCNP model was run to determine the contribution of the concrete to counts registered in polyvinyl toluene (PVT) scintillation detectors located in the RPM. Initial MCNP results show that the concrete does contribute to the background gamma counts in the RPM. This will allow for a quantitative model to be developed showing the response of any RPM to different types of concrete.