Determining the Effects of Construction Materials on Background Readings for Second Line of Defense Program Portal Monitoring Systems

摘要

The Nonproliferation program is an initiative of the Department of Energy’s (DOE) NationalNuclear Security Administration (NNSA) tasked with strengthening the capability of foreigngovernments to interdict the trafficking of illicit nuclear material across international borders andthrough maritime shipping channels. Radiation Portal Monitors (RPM) deployed around the worldas part of the program are used to passively detect both gamma and neutron radiation from cargoand pedestrian traffic. Portal monitoring systems in some locations are experiencing higher thanexpected background counts. The increased background measurements have been attributed to theconcrete surrounding the portal monitors. Higher background increases the minimum detectableactivity and can ultimately lead to more material passing through the RPM undetected. This work isfocused on understanding how the concrete surrounding the portal monitors is contributing to theanomalous data. For this study, six samples were taken from three different composition concreteslabs. Each sample was counted using a high purity germanium (HPGe) detector to determine itsnatural activity. Neutron activation analysis (NAA) was done using a Training, Research, Isotopes,General Atomics (TRIGA) research reactor to irradiate representative portions of each sample inorder to determine their elemental composition. The HPGe measurements and NAA results wereused 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 concreteto counts registered in polyvinyl toluene (PVT) scintillation detectors located in the RPM. InitialMCNP 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 todifferent types of concrete.