Presolar grains are nanometer- to micrometer-sized particles that are present at ppmlevel
abundances in various primitive solar system materials. Careful analysis of the composition of these grains can provide information about their origin in stellar environments. Particularly challenging is the analysis of nanometer-sized presolar diamonds where most of our compositional knowledge comes from bulk measurements that are averaged over
extremely large numbers of grains. An insufficient spatial resolution of many standard
analytical techniques (e.g., SIMS) can also be a limiting factor in the analysis of ‘larger’ (i.e., micrometer-sized) presolar grains, when small subgrains are studied. Since such inclusions are cogenetic with their host material, a detailed characterization of these sub-grains (and possibly their rims) can provide important additional constraints about the formation conditions in stellar environments. These examples indicate the need for a novel analytical approach that allows detailed elemental and structural characterizations at an atomic resolution.
We have recently begun to develop methodologies for atom-probe tomographic studies of presolar grains. Atom-probe tomography allows the analysis of sample volumes up to 100x100x100 nm3 with the same atomic detection efficiency of >50% for all elements. The
three-dimensional position of each atom is recorded along with its mass-to-charge ratio by positionsensitive time-of-flight mass spectrometry. This makes it possible to reconstruct the full three-dimensional structure of the analysis volume and visualize elemental distributions in their spatial context. It is also possible to determine elemental (and to a limited
extent isotopic) compositions of specific sub-volumes. Even though atom-probe tomography is a well-established analytical technique in material sciences and the actual measurements are fairly routine, the sample preparation can be difficult, especially in the case of loose small particles. Here we describe our ongoing efforts to analyze presolar nanodiamonds and SiC grains in the atom-probe and present first results from the analysis of presolar SiC.
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