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A method to determine the depth of buried localized radioactive contamination nonintrusively and nondestructively using principal component analysis is described. The γ-ray spectra from two radionuclides, cesium-137 and cobalt-60, have been analyzed to derive the two principal components that change most significantly as a result of varying the depth of the sources in a bespoke sand-filled phantom. The relationship between depth (d) and the angle (θ) between the first two principal component coefficients has been derived for both cases, viz. d(Φ) = x + y log(e) Φ where x and y are constants dependent on the shielding material and the γ-ray energy spectrum of the radioactivity in question, and φ is a function of θ. The technique enables the depth of a localized radioactive source to be determined nonintrusively in the range 5 to 50 mm with an accuracy of ±1 mm.

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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 -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 -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 -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.