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The National Security Technologies, LLC, Remote Sensing Laboratory has recently used an array of six smallfootprint
(1-inch diameter by 3-inch long) cylindrical crystals of thallium-doped sodium iodide scintillators to obtain
angular information from discrete gamma ray–emitting point sources. Obtaining angular information in a near-field
measurement for a field-deployed gamma sensor is a requirement for radiological emergency work. Three of the
sensors sit at the vertices of a 2-inch isosceles triangle, while the other three sit on the circumference of a 3-inchradius
circle centered in this triangle. This configuration exploits occlusion of sensors, correlation from Compton
scattering within a detector array, and covariance spectroscopy, a spectral coincidence technique.
Careful placement and orientation of individual detectors with reference to other detectors in an array can provide
improved angular resolution for determining the source position by occlusion mechanism. By evaluating the values
of, and the uncertainties in, the photopeak areas, efficiencies, branching ratio, peak area correction factors, and the
correlations between these quantities, one can determine the precise activity of a particular radioisotope from a
mixture of radioisotopes that have overlapping photopeaks that are ordinarily hard to deconvolve. The spectral
coincidence technique, often known as covariance spectroscopy, examines the correlations and fluctuations in data
that contain valuable information about radiation sources, transport media, and detection systems. Covariance
spectroscopy enhances radionuclide identification techniques, provides directional information, and makes weaker
gamma-ray emission—normally undetectable by common spectroscopic analysis—detectable. A series of
experimental results using the concept of covariance spectroscopy are presented.
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