Publications Details

Detector Response Function and Directional Gamma-Ray Source Calculations for Polaris

Mitchell, Dean J.; Horne, Steven M.; Thoreson, Gregory G.; Harding, Lee T.; O'Brien, Sean O.

A Directional Unfolded Source Term (DUST) method was developed to compute directionally resolved gamma-ray source terms based on back-projection spectra synthesized by Compton Cameras. Spectral features in the unprocessed spectra are indistinct primarily because the rotational angles for the conical projections cannot be determined, so probability distributions are constructed from overlapping cones. The DUST method uses an angular response function to compute a covariance matrix, which is used to process count rates in back-projection spectra by linear regression to partition the gamma-rays among several spatial regions. This method was applied to analyze data collected by the Polaris detector during an evaluation that was conducted at Oak Ridge National Laboratory (ORNL). The evaluation includes measurements of calibration sources with angular separations ranging from 1° to more than 50°. Measurements were also performed for cylindrical depleted uranium castings and a ¹³⁷Cs source inside a large polyethylene sphere. The DUST algorithm was able to differentiate gamma-rays emitted by ¹³⁷Cs and ⁶⁰Co when the sources were separated by less than 2°, but separation greater than 10° was required to isolate the ¹³³Ba emission from gamma-rays emitted by the other sources. The computed source terms were consistent with emission profiles from the calibration sources and from models of the spatially-extended sources. Methods for viewing radiation profiles were also evaluated because user input is required to select spatial regions of interest.