However, one of main properties of x-rays that makes them powerful -- their high penetrating power – also limits the scope of application of this imaging technique. In particular, imaging thick or heterogeneous samples at the micron scale generally requires that the sample be thinned, which is often impractical or impossible.
A paper highlighted on the cover of the March issue of the Journal of Analytical Atomic Spectroscopy describes a new solution to this problem, enabled by technology developed at CHESS. The paper, “Superior spatial resolution in confocal X-ray techniques using collimating channel array optics: elemental mapping and speciation in archaeological human bone,” by Sanjukta Chaudhury et al, describes the use of collimating channel arrays (CCAs), designed and fabricated by co-authors David Agyeman-Budu and Arthur Woll, for a 3D version of XFI called confocal XRF imaging (CXFI). Other implementations of CXFI use polycapillary optics rather than CCAs, which limits the resolution to about 10 microns at 10 keV. Furthermore, the resolution in these implementations varies with energy, such that the image resolution obtained with polycapillaries can be wildly different for different elements.
Using CCAs rather than polycapillaries as the collection optic enables CXFI at a spatial resolution approaching 2 microns in the largest linear dimension. The paper demonstrates this technique on samples of archaeological human bone, part of a University of Saskatchewan examination of remains obtained from Royal Naval Hospital Cemetery (ca. A.D. 1793-1822) near English Harbor in Antigua. Such remains can provide great insight into the diet and medicinal practices in use at the time. But the samples are both rare and brittle, rendering them difficult to thin and polish for traditional 2D XFI.
Further details about CCAs and their fabrication, which takes place at the Cornell center for NanoScale Science and Technology, can be found here. Thanks to a long-standing collaboration among CHESS, the University of Saskatchewan and CLS, these optics are now available for general users of 20ID-B.