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With the signing of a comprehensive test ban treaty in 1996, the ability to detect the presence of anthropogenic radionuclides in the atmosphere for verification purposes is increasingly important. A trigger detector is a concept designed to aid in this purpose by giving timely notice of suspicious concentrations of radionuclides in the atmosphere adjacent to a radionuclide monitoring station (RMS) [1] (DeGeer, Nuclear Detection Group, Division of Nuclear Weapons Physics, National Defence Research Establishment, S-17290 Stockholm, Sweden, De Geer, Paper presented at the ARPA sponsored meeting on CTBT Monitoring Technologies, San Diego, 26–29 September 1994). In this research, the evaluation and performance analysis of two different trigger detectors was studied. Point source experiments were performed to characterize the detector response. Limits of detection for each detector were determined using a simulated atmosphere (∼100 gal of water) contaminated with a gamma-ray emitting radioisotope. Three different analytical models of radioactive clouds were developed to predict detectability: (1) uniformly contaminated hemisphere; (2) uniformly contaminated slabs; and (3) a series of infinitely long line sources (representing rectangular segments of a uniformly contaminated atmosphere). The simulation of a contaminated atmosphere yielded a detectable concentration, for 1.0 MeV photons, of approximately 0.5 Bq/m3 by the NaI detector. The study indicates that the NaI crystal is the detector of choice for a RMS triggering system. |
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