Gamma-ray and neutron background comparison of US metropolitan areas

Gamma-ray and neutron background surveys were performed by the Naval Research Laboratory (NRL) in U.S. cities, including Washington, D.C.; Kansas City and St. Louis, Missouri; Chicago, Illinois; Richmond, Virginia; Boston, Massachusetts and Baltimore, Maryland. Measurements covered a range of industrial, residential and commercial areas. Germanium grade gamma-ray data over the energy range of 0.05–3.0MeV and neutron count rates with unmoderated He-3 sensitivity were recorded as a function of latitude, longitude and elevation in one second intervals. Typical Potassium Uranium Thorium (KUT) backgrounds were seen along with several anomalies. For example, a decrease in the thermal neutron flux in large urban canyons was seen and verified via Monte Carlo simulations. The data were collected to provide natural background models for simulation work. Germanium grade spectroscopy is required, because it provides sufficiently detailed isotopic information of the gamma-ray background. As expected a comparison of the background shows significant differences between the individual cities.