Standoff Performance of HPGe Detectors in Identification of Gamma-Ray Radiation Sources

The detection and identification of radiation sources at distances in the range of 15 meters or more is becoming increasingly important for illicit materials interdiction and the location of lost or orphan sources. In most locations, there is a considerable gamma-ray flux from natural background (NORM) and cosmic-induced nuclides. This gamma-ray flux varies with time, weather conditions, location, and changes in the materials at a location such as a portal. All of these contribute to changes in the detector total count rate unrelated to the nuclides of interest making a system based on count rate unreliable. The high resolution of HPGe enables the nuclide-specific peak and background counts to be extracted from the spectrum of the suspect object or area without relying on background spectrum subtraction techniques using background spectra necessarily collected at either different places or times. Collimators can be used to reduce the detector field of view and improve the signal-to-noise ratio by reducing the NORM contribution. This will reduce the minimum identifiable activity (MIA). Data were collected with and without collimators. A straightforward summing technique allows the data from multiple detectors to be aggregated to improve the signal. Data have been collected for 137 Cs at distances up to 80 meters and used to predict the performance at 100 m. The MIA has been calculated for specified false positive and false negative rates for systems with up to 8 HPGe detectors.