A Gamma Ray Detector of Variable Geometry for Whole Body Counting of Small Animals

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Abstract
A gamma ray detector for counting small animals in metabolism cages has been constructed. The cage is centered between horizontal coaxial NaI crystals. By varying the distance of separation between the crystals the counting efficiency of the detector for 131I can be varied from approximately 6 per cent to 0.24 per cent making it possible to count a wide range of source activities. The detector is shielded by 4 in. of lead which allows for considerable stability in background counting rates. Various aspects of the counter performance have been studied and are presented in graphical form with some discussion of the characteristics. These include determinations of efficiency as a function of crystal separation, gamma ray energy and point vs. bulk source. A short treatment of the effect of scattering is also included.