Synchrotron-based microanalysis can be used to analyse fragile and minute samples because it offers a unique way to study the bulk morphology, internal structure, crystallography, and trace composition of sub-millimetre grains at the micrometre scale. Some scientific applications, however, require even higher resolution than currently available which leads to an advancement of the techniques. For instance, nondestructive observation of Earth and Planetary Science materials requires multiscale analysis because samples can often feature chemical and structural heterogeneities that span scales from the millimetre to the nanometre [1].

Here we present the new imaging capabilities of the nano-imaging beamline end-station (ID22NI) for the non-invasive study of planetary science samples. The setup is based on a focusing system permitting large bandwidth and fast nanoscale mapping optimised for fluorescence imaging.

The optics scheme of ID22 beamline has to be suitably tuned in order to obtain decananometre X-ray spot sizes with comparable horizontal and vertical dimensions. First, the horizontal source was reduced to about 25 micrometres by means of slits, thus creating a secondary horizontal source 28 metres away from the synchrotron source. The loss of intensity is compensated for by switching to the “pink beam” mode, whereby there is no crystal monochromator in the beam. Thus the only optical element between the source and the focusing device is a flat Si mirror with Pd or Pt stripes that filters out high incident energies. This dramatically increases the photon flux and improves the performance and stability of the focus at the expense of the energy resolution E/E which increases by two orders of magnitude up to 10–2. At 63 metres away from the synchrotron source, the beam is focused using a Kirkpatrick-Baez (KB) system that consists of two independent mirrors for focusing the beam vertically and horizontally (Figure 146). The primary mirror, coated with a graded multilayer, acts both as a vertical focusing device and a monochromator, yielding very high flux of about 1012 photons/s for X-rays in the 15-20 keV range. The minimal spot size achieved was 80 nm in two dimensions and 40 nm in one dimension [2]. The nanoprobe features a flux density of up to 5.1013 ph/s/µm2, largely superior to the microprobe’s 1011 ph/s/µm2, thus exhibiting much higher sensitivity for (ultra-) trace element detection and allowing mapping with a greatly reduced dwell time.

Samples are placed in the focal spot of the KB system on a 4-axis stage and can be raster scanned on three axes plus one rotation. At each position a fluorescence spectrum which gives information about the sample elemental composition is collected. The spectra are measured using a SII Nanotechnology Vortex 50 mm2 silicon drift diode (SDD) collimated detector placed in the horizontal plane at 75 degrees from the incident beam. SDD detectors provide an excellent trade-off between output count rate (approximately 600 kcps at 0.25 µs peaking time) and energy resolution (140 eV), although displaying lower peak-to-valley ratios than standard Si(Li) detectors. A scanning mode based on the acquisition of data while continuously translating the sample with a piezo-driven stage speeds up the measurement by up to 50%.

A cometary grain from the NASA Stardust cometary mission collection was studied [1]. The sample was measured in situ, in the original low density (approx. 0.01 g/cm3) silica aerogel, in which it was trapped after its few millimetres penetration track.

Fig. 146: Schematic view of the experimental setup of the ID22NI end-station.

Figure 147 shows the superposition of the Fe 2D map with the Ca and Ni maps, respectively, the two other major elements in the grain. A chemical heterogeneity at the sub-micrometre scale is evident in this view. No post-processing of the image has been done so that strong heterogeneities from one 50 nm pixel to the other are shown. Micrometre Ca-rich zones and hundreds of nanometres Ni-rich phases are embedded in an Fe-rich matrix. This chemical view starts to be consistent with the classical petrology of the cometary grains composed of micrometre Fe-rich crystals of olivines and pyroxenes, Ca-rich pyroxenes and plagioclases, nanometre Fe-Ni precipitates, all embedded in a silica matrix. These excellent results suggest that XRF studies of Stardust grains could be used to obtain petrology information in place of the alternative more destructive analyses.

Fig. 147: Colour-coded map of the distribution of Fe, Ni and Ca in the cometary grain.

Further analyses must be carried out to better establish the shape and 3D quantitative chemical composition of the grain. They include 2D/3D morphology, crystallinity and oxidation state by means of nano-tomography, diffraction or spectroscopy.


Principal publication and authors

P. Bleuet (a,d), A. Simionovici (b), L. Lemelle (c), T. Ferroir (c), P. Cloetens (a), R. Tucoulou (a), J. Susini (a), Applied Physics Letters 92, 213111 (2008).
(a) ESRF
(b) LGIT, OSUG, Université J. Fourier, CNRS UMR 5559, Grenoble (France)
(c) LST, Université de Lyon, Ecole Normale Supérieure de Lyon, UMR5570-USR3010 (France)
(d) Present address: CEA, LETI, MINATEC (Grenoble)


[1] G.J. Flynn et al., Science 314, 1731 (2006).
[2] R. Ortega, P. Cloetens, G. Devès, A. Carmona, and S. Bohic, PLoS ONE 9, e925 (2007).