Depth determination of buried caesium-137 and cobalt-60 sources using scatter peak data

摘要

An investigation into an alternative approach to 3D source mapping is proposed, by combining the insights of two existing techniques. The first of these is a 3D ‘imaging’ tool, N-Visage^™ that has been developed by REACT Engineering Ltd. This technique is efficient and robust, but is not a true 3D technique as it relies on user-supplied 2D manifolds to constrain source locations. The second technique uses the γ-photopeak and an x-ray peak to determine radionuclide source depth using a relative attenuation method. We look at the possibility of combining both techniques to constrain both the location and depth of a radiological source buried under shielding. It is believed a combined method using spectra recorded above the shielding object will be of use in the nuclear decommissioning and land contamination industries. N-Visage^™ has previously been used to map source distributions of mixed radionuclides with complex geometries through shielding media. The software works by producing a computer model which recreates the experimental setup. A survey is imported, comprising a set of γ-spectra recorded with an instrument of known efficiency and isotropy taken at a variety of locations around the area of interest. A survey plan recording the location and orientation of the instrument for each reading is also reconstructed. N-Visage^™ is then able to determine the locations of the source(s) without prior knowledge of exactly where they are located, by building and inverting a simple physical model relating potential source locations to the recorded spectra. This research sets out to investigate the possibility of combining the geometric insights of N-Visage^™ with a method of extracting depth information from scatter data, rather than the x-ray peak. By combining the γ-photopeak and scatter areas of a spectrum, the thickness of the shielding media between source -n-nand detector can potentially be inferred. Using scattered photons rather than x-ray attenuation is preferable where depths are of a sufficient thickness to effectively eliminate a measurable x-ray photopeak.