Depth Determination of Buried Caesium-137 and Cobalt-60 Sources Using Scatter Peak Data

Abstract

An investigation into an alternative approach to 3-D source mapping is proposed by combining the insights of two existing techniques. The first of these is a 3-D “imaging” tool, N-Visage, that has been developed by REACT Engineering, Ltd. (Whitehaven, U.K.). This technique is efficient and robust, but is not a true 3-D technique as it relies on user-supplied 2-D 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 that 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 photons rather than X-ray attenuation is preferable where depths are of a sufficient thickness to effectively eliminate a measurable X-ray photopeak.