Performance of a Radiation Portal Freight Monitor based on Integrated Germanium Detector Modules

Portal monitors, container scanners, and mobile search systems, constructed with high resolution germanium (HPGe) detectors, are currently being installed at locations world-wide. This reflects a general desire for improved performance and a reduction in the time to make a good decision in interdiction cases. An integrated gamma-ray spectrometer, incorporating a mechanically-cooled HPGe detector, digital signal processing electronics, MCA, and communications has been developed to meet the detection and environmental needs of these systems. Specially developed software allows these spectrometers to be configured in a variety of portal monitoring applications. The HPGe detectors are designed to have good lowand medium-energy detection efficiency and excellent spectral peak resolution in order to eliminate peak overlaps and thereby remove problems of masking of SNM by common industrial and medical radionuclides found in all types of monitors. Systems using detectors with inferior resolution, regardless of efficiency, are unable to separate the radiation signals from NORM and illicit nuclides. In portal monitoring or in freight scanning, the Field of View (FOV) of a detector and transit speed determine the time the radioactive material contribute data to the spectrum. The absolute full-energy peak efficiency of the detector and background count-rate in the peak energy region determine the signal-to-noise ratio, and thus the minimum detectable (or identifiable) amount of material passing through the FOV. The performance of the integrated spectrometers and their performance in pedestrian portal configurations have been previously reported. A freight portal monitor was constructed with 8 HPGe detectors in cooperation with Thermo Fisher and the performance measured using 133 Ba, with and without high NORM background and with varying amounts of shielding. Test sources were moved through the portal at 8 km/h on a commercial truck and the response was recorded. Measurements presented show the impact of shielding and masking on the performance of the portal. The results illustrate applicability of the design to a variety of monitoring situations for the detection of illicit material.