Determining the Effect of Concrete Roadways on Gamma-ray Background for Radiation Portal Monitoring Systems

The dissolution of the Soviet Union coupled with the growing sophistication of international terror organizations has brought about a desire to ensure that a sound infrastructure exists to interdict smuggled nuclear material prior to it leaving its country of origin. To combat the threat of nuclear trafficking, radiation portal monitors (RPMs) are deployed around the world to intercept illicit material while in transit by passively detecting gamma and neutron radiation. Portal monitors in some locations have reported abnormally high background counts with little or no consistency. The higher background data has been attributed, in part, to the concrete surrounding the portal monitors. Higher background increases the minimum detectable activity and can ultimately lead to more material passing through the RPMs undetected. This work was focused on understanding the influence of the concrete surrounding the monitors on the total gamma-ray background for the system. The study combined destructive and nondestructive analytical techniques with computer simulations to form a qantitative model that is adaptible to any RPM configuration. Six samples were taken from three different composition concrete slabs. The natural radiological background of these samples was determined using a high-purity germanium (HPGe) detector in conjunction with the Canberra In-Situ Object Counting System (ISOCSTM) and GenieTM 2000 software packages. The composition of each sample was determined using neutron activation analysis (NAA) techniques. The results from these experiments were incorporated into a Monte Carlo N-Particle (MNCP) photon transport simulation to determine the number of gamma-rays from the different concrete slabs detected by the RPM.