- by L. JACOBSON, F.C. DE BEER, R. NSHIMIRIMANA, L. K. HORWITZ and M. CHAZAN
“This study presents the application of neutron tomography to the analysis of ironstone slabs found in a late Earlier Stone Age context (Fauresmith industry) at the back of Wonderwerk Cave, Northern Cape Province, South Africa. These slabs have markings on the surface that might be anthropogenic, and thus significant to understanding the emergence of human symbolic behaviour. Neutron tomography proved to be an effective tool for distinguishing surface incisions from lines that are the expression of internal fissures in the rock. In recent years, a range of non-destructive imaging techniques based on penetrating radiation has been developed for the non-invasive three-dimensional (3D) study of the internal structure of objects. Imaging with penetrating radiation entails the set-up geometry of source–sample–detector. Radiation from the source is collimated towards the sample and detector. As the radiation passes through the sample, it is attenuated (absorbed and/or scattered) to a higher or lesser degree due to characteristics related to the type of radiation used, as well as characteristics of the sample’s composition. A two-dimensional (2D) ‘shadow image’ of the sample is cast on the detector (in electronic and digital format), from where the image (2D-radiograph) is relayed for inspection and observation on a PC monitor. Among these methods, neutron tomography (NT) has established itself as an important diagnostic tool for non-destructive examination and analysis of historical, archaeological and fossil objects (e.g., Schillinger et al. 1996; Schwarz et al. 2005; Rant et al. 2005, Fiori et al. 2006; Kardjilov et al. 2006; Kusche et al. 2007; Banhart 2008; de Beer et al. 2009). NT is a radiation-based analytical imaging technique that shares many features with other tomographic imaging methods: it uses serial slices and facilitates virtual reconstruction of both the internal and external structure of 3D objects on a 2D or 3D scale. That it is non-invasive is a critical factor, since many of the samples studied may be rare or unique, or alternatively breaking/cutting them will undermine the goals of the investigation (Rant et al. 2005).
The specific importance of NT lies in the ability of neutrons to penetrate many materials, including metals, which other imaging techniques cannot (Johansen 2005). Moreover, NT has proved especially successful in providing high-resolution images, in the order of magnitude of tenths of a millimetre, for features within a rock sample that is several centimetres in size, and also of imaging hidden interior features that X-ray and gamma-ray imaging have failed to reveal (Schillinger et al. 1996). Although most applications of NT have focused on metals, several studies have centred on imaging the internal composition of geological samples (e.g., Winkler et al. 2002; Vontobel et al. 2005; de Beer et al. 2007; Longridge et al. 2009). Indeed, NT has proved to be an important complement to X-ray tomography in determining or validating existing data concerning some of the important physical properties of rock” (read more).
(Source: Archaeometry 55:1-13, 2013)