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A Comparison of γ-ray Spectroscopy with Accelerator Mass Spectrometry for the Environmental Assay of Plutonium

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A variety of analytical techniques have been developed and applied to the analysis of plutonium, including α-particle spectroscopy, Hybrid K-edge densitometry, X-ray fluorescence, γ-ray spectroscopy and various embodiments of mass spectroscopy. For bulk analysis requirements of environmentally-derived substances, such as soils in the context of the assessment of plutonium in contaminated land, methods that require several stages of sample preparation are generally not suitable because such preparatory work cannot be done easily, in-situ, in the field. Further, specific localized arisings in the land, i.e., particles, might be missed by a sample-based methodology. Neutron assay is generally not sufficiently sensitive for trace plutonium assay in bulk samples because the abundance of 240 Pu (favored in this regard for its spontaneous fission neutron emission) is usually too small to overcome the inherent contribution from 238 U. By contrast, high-resolution, γ-ray spectroscopy offers the potential to assess relatively large sample quantities, given that a correction is made for self-shielding by the sample. Broad energy germanium (BEGe) counters offer unparalleled performance in terms of both i) energy resolution (particularly in the energy region 50 - 150 keV relevant for 241 Am as a conduit for 241 Pu) and ii) detection efficiency. Whilst arguably the most sensitive analytical method with isotopic sensitivity for trace plutonium is accelerator mass spectrometry, high-resolution, γ-ray spectrometry can provide an independent route for bulk assay of samples without the need for extensive sample preparation. This paper will discuss the challenges associated with these approaches drawing on data taken with soil samples from the United Kingdom.