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

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 and detector can potentially be inferred. Using scattered p- - hotons rather than x-ray attenuation is preferable where depths are of a sufficient thickness to effectively eliminate a measurable x-ray photopeak.