Imaging of objects at shallow depths in soil : tests of a proposed method

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SR_1996-024-pdf
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Coote, G.E. 1996 Imaging of objects at shallow depths in soil : tests of a proposed method . Lower Hutt: Institute of Geological & Nuclear Sciences Institute of Geological & Nuclear Sciences science report 96/24 8 p.

Abstract: The aim of this project was to test the suggestion that objects buried a few centimetres below the soil surface (in particular plastic anti-personnel land mines) might be located by a method based on measuring gamma rays from naturally occurring radioisotopes in soil - one isotope of potassium and several members of the uranium and thorium decay series. A nonradioactive object, of any material, would displace its own volume of soil and reveal its presence through an area of ground with lower gamma ray emission than adjacent areas. To test the idea a test bed was built from wood, steel sheet, and plastic wheels. The main component was an airtight wooden box containing a standard 7.5 cm by 7.5 cm scintillation crystal and shielded with 5 cm of lead in every direction except for a 4 cm square opening directly beneath it. This box was positioned just above soil or sand in a large box and could be moved by hand in a two-dimensional pattern. The shielding and collimation proved adequate and tests were performed with one sodium iodide and two bismuth germanate detectors. A diminution in gamma ray yield above buried objects was observed, but was quite small. This small reduction has been ascribed to effects related to scattering and attenuation of gamma rays in soil which act to oppose the expected much larger effect. Following tests with the best available detector and a variety of buried objects it was concluded that a practical device could not be based on this principle. (auth)