Detection of illicit nuclear materials in urban environments is difficult due to the large amount of background radiation from naturally occurring radioactive materials (NORM) in the roadways and buildings. Mobile searches suffer from low count rates so the detection algorithms must be carefully balanced between missing real sources and reporting too many false alarms. The Modeling Urban Scenarios and Experiments (MUSE) project aims to create a virtual testbed for the simulation of radiation detection to predict realistic background and threat source detection events, so that detection algorithms can be better optimized to find illicit sources. The MUSE project is a collaboration of Oak Ridge National Laboratory (ORNL), Lawrence Berkeley National Laboratory (LBNL), and the Remote Sensing Laboratory (RSL). The project also works with staff at Lawrence Livermore National Laboratory (LLNL) to support the development of the Optimization Planning Tool for Urban Search (OPTUS) software package . The first step in creating a virtual testbed is to determine what level of detail is required in the models in order to match actual measurements.
Several measurement campaigns were conducted in 2015 and 2016 at the Fort Indiantown Gap (FTIG) Combined Arms Collective Training Facility (CACTF) that contains a representative urban environment, with a dozen buildings and several streets. Detectors commonly used in search operations (2”×4”×16” NaI(Tl)) were used to measure count rates from background and sources . Measurements with a shielded high-purity germanium detector of the roadways, sidewalks, and building walls were made to determine the concentration of NORM that make up the bulk of the background radiation [3,4].
This paper will show some comparisons of modeling simulations to measurements taken during the OPTUS-3 campaign in November 2015. Measurements with and without an 81 µCi cesium source were made at several points along the center line of the main road. Initial results show good agreement between the measured and simulated spectra above 300 keV. Additional studies to determine the causes of mismatch below 300 keV have been started and will also be discussed.