Process tomography of diffusion, using PET, to evaluate anisotropy and heterogeneity

Anisotropy, compositional and structural heterogeneity of clays cause considerable deviations from homogeneous diffusion, in particular direction dependent transport rates and preferred transport zones. Conventional diffusion experiments, treating the sample as homogeneous black box in a concentration gradient, are interminable and inappropriate to elucidate these spatial effects. In contrast, tomographic imaging methods are capable both to shorten the required observation time and to reveal space-dependent variations of the diffusion process.
Here we applied positron-emission-tomography (PET) as quantitative spatiotemporal imaging method with perfect characteristics for non-destructive diffusion process observations. We adopted this nuclear medical imaging method and developed its geoscientific applicability in the past ten years [1-3]. GeoPET visualizes the concentration of certain positron-emitting radiotracers in opaque media with ultimate sensitivity (picomoles) and reasonable resolution (1 mm) on the laboratory scale (100 mm). For diffusion experiments longer living nuclides are applicable, like 58Co (T1/2=70.86 d) and 22Na (T1/2=2.603 a), and we have to consider the higher density of our specimens, which causes attenuation and scattering of the radiation [4].
GeoPET revealed spatial particularities in diffusion experiments that have been conducted on Opalinus clay samples of different sizes, as well as on other rock types. Applying the Comsol Optimization Module, we derived anisotropic diffusion parameters from the tomograms [5, 6].

References

1. Richter, M., et al., Radiochimica Acta 93, 643-651, 2005.
2. Gründig, M., et al., Applied Geochemistry 22, 2334-2343, 2007.
3. Kulenkampff, J., et al., Physics and Chemistry of the Earth 33, 937-942, 2008.
4. Zakhnini, A., et al., Computers and Geosciences 57 183-196, 2013.
5. Schikora, J., Dresden Technical University, Diploma thesis, 2012.
6. Kulenkampff, J., et al. Clays in Natural and Engineered Barriers for Radioactive Waste Confinement (O/11B/2), 2012. Montpellier, France.