Performance of a Small Portal Using Integrated Germanium Detector Modules

Portal monitors and mobile search systems are most effective in positive identification when constructed with high resolution detectors. To satisfy the need for this type of detector, an integrated gamma ray spectrometer, incorporating a germanium detector with integral mechanical cooling, digital signal processing electronics, MCA, and communications has been developed. This modular subsystem has been used in a several different applications, including use in stationary and mobile systems for the detection and identification of radioactive materials. To have the sensitivity needed for these applications the detectors must have good low and medium energy detection efficiency coupled with excellent spectral peak resolution. The high resolution is needed to remove peak overlaps and overcome the problems of masking of SNM with common nuclides. In either situation where the spectrometer and the source are moving relative to one another, the Field of View (FOV) determines the time the radioactive material contributes to the data in the spectrum. The absolute efficiency and background (i.e., signal-to-noise ratio) determine the minimum detectable or identifiable quantity for the material in the FOV. The uniformity of the performance of several units over the energy range of 80 keV to 1.8 MeV was previously reported. A portal monitor was constructed with 8 detectors and the performance measured for several common test nuclides. The total height of the detection zone is 2 meters with 4 detectors uniformly spaced on each side. In this portal, the horizontal FOV is limited by steel collimators. The vertical FOV was not collimated. The background was measured in low background and elevated (NORM) background situations. The measurements presented show this unit can be applied to a wide variety of monitoring situations for the detection of illicit material