May 2008 STXM analysis summary: no detectable Mg, Al, Ni, Ce. This off-normal track and its associated terminal particle (TP) were easily visible in the optical microscope. We obtained coordinates relative to features easily visible in the optical microscope view of the keystone (Slide 2), imaged the keystone in x-ray absorption, and located a target region containing the terminal particle. A quick XRF map of the nominal TP area showed an Fe-hot-spot, having a relatively high Ni/Fe ratio. We obtained full XRF spectra of the area containing this particle at higher spatial resolution (smaller step size, Slide 3), and extracted the spectra from two regions of interest: a 16 pixel region surrounding the TP (shown in red) and a 57 pixel region of aerogel as far from the TP as we could get (green; 57 pixels).Slide 4, which shows the ROI integrated spectra (divided by the number of pixels in the ROI to give equivalent Si and scatter peaks), demonstrates that we detected several elements, present at well above aerogel background, in this TP. The keystone was analyzed in a silicon nitride sandwich. The data reduction assumed Si3N4 with a density of 3.44 and 100 nm thickness at a take-off angle of 15 degrees, the tilt of the sample relative to the incident X-ray beam in the Sector 2_ID-D X-Ray Microprobe. At this angle, the fluorescence path length through the silicon nitride is 390 nm, resulting in an absorption of 15% at sulfur, 4% at K, and much less at increasing Z. Assessing the absorption of the overlying aerogel is more difficult, since we do not know the depth of the material being analyzed. If we assume the particle is 100 microns below the surface of the aerogel, this gives an aerogel path length of ~ 400 microns. This corresponds to about a factor of two absorption for sulfur, and indicates that there is considerable uncertainty in the abundances of the lightest elements. Aerogel absorption is minimal for the heavy elements (Cr and higher Z). No attempt was made to correct for the aerogel absorption Slide 5 is the abundance plot showing the elemental composition of the TP normalized to Fe and CI . The data are shown in three ways. First, they are plotted with no background subtraction (blue line). Second, they are plotted by subtracting the aerogel background corresponding to the same number of pixels as we have in the TP ROI (i.e., 16/57 times the aerogel background spectrum; purple line). Third, they are plotted assuming all the Si in the TP raw spectrum results from compressed aerogel having the same composition in the 57 pixel aerogel ROI (Si normalization; green line). Data for Cr, Ni and Cu are coincident for all three lines because of the negligible background contributions. Unless particle capture mobilized and redistributed contaminants in the aerogel or the contaminants were inhomogeneously distributed in the aerogel, the composition of the TP should be between the compositions shown for “No BG Subtraction” and “Si Norm BG Subtraction. The Cr/Fe, Mn/Fe, Ni/Fe and Cu/Fe in this TP are significantly enriched relative to CI. S/Fe, Ca/Fe, and Ti/Fe are all significantly below CI in this TP. This is very similar to the pattern we reported earlier for IS Candidate I1001,1,16 Object 1, which was an Fe hot-spot apparently embedded in high-density Si-rich region. For comparison, we show those data in Slide 6. In the case of I1001,1,16 Object 1 the absence of a clearly identifiable track left open the possibility that Object 1 was a contaminant cast in the aerogel. However, I1004,1,2 is the TP in a track, providing unambiguous identification that material with this composition hit the aerogel at relatively high velocity. We mapped the track from the exposed surface of the keystone to the TP, but found no significant concentration of material other than the TP.
| Notes By: | ndc\mrodrigu | Date: | 03/13/2013 |