The IS Candidate was identified from the optical microscope images taken normal to the aerogel surface. The note from the JSC Curatorial Facility indicated that after extraction the IS Candidate feature that had been identified prior to extraction could not be located. A relatively large "search box" was identified by JSC on the photograph of the picokeystone. The 2-ID-D beamline at the APS has a 250 nm, zone-plate focused beamspot. At this beam size it would be impractical to map the entire search box indicated on Slide 1. We performed mid-IR mapping of the entire area of this picokeystone using the Continuum FTIR micro-spectroscopy instrument on beamline U2B of the NSLS prior to the APS run in order to see if we could detect any track-like feature in the 4000 cm-1 to 650 cm-1 wavelength range. No features were identified. High resolution optical microscope examination of the search area by B. Lai located a single dark spot, possibly a terminal particle (TP) near one edge of the search box (Slide 2). Coarse scale XRF mapping of the region around this dark spot identified two Fe hot-spots (spots A and B in Slide 3), within 20 microns of one another, in the approximate location of the spot in the optical image. Both spots have Ni/Fe much lower than CI. Note that there are also two Ca hot-spots in this area, not co-located with the Fe hot-spots. We mapped at 250 nm resolution the entire region between these two Fe hot-spots and the surface of the aerogel (Slide 4). The high-resolution map identified a possible entry hole or small bulb at the surface, which is most easily visible in the Al, Cr, and Fe images, and four Fe hot-spots (the two previously identified as Spot A and Spot B, and two additional ones labeled Spot C and Spot D). The four Fe hot-spots are distributed along an almost straight line that points back to the linear feature at the surface. This has all the characteristics of an entry track, except that no track is visible in the optical microscope. We concentrated our efforts on this potential track. The possible bulb or entry hole was analyzed using the well-defined linear feature in the Al map to define the ROI for the feature. The spectrum of this feature is compared to the spectrum of the aerogel in a feature-free region in Slide 5. The possible bulb hasan approximately CI Ni/Fe ratio, but the K/Fe, Ti/Fe, V/Fe, Cr/Fe, Mn/Fe, Cu/Fe and Zn/Fe ratios are all significantly elevated in this feature (Slide 6). Spots A, B, and C were separately analyzed at high resolution (100 nm step size). The particle and background ROIs are shown in red and green respectively on Slides 7, 9, and 11. The XRF spectrum of each particle, as well as the spectrum of the aerogel in a hot-spot free region, is shown in Slides 8 (Spot A), 10 (Spot B), and 12 (Spot C). The CI and Fe normalized element abundance patterns are shown in three ways in Slide 13. First, they are plotted with no background subtraction. Second, they are plotted by subtracting the aerogel background corresponding to the same number of pixels as we have in the TP ROI. Third, they are plotted assuming all the Si in the Fe hot-spot raw spectrum results from compressed aerogel having the same composition in the aerogel ROI. Unless particle capture mobilized and redistributed contaminants in the aerogel or the contaminants were inhomogeneously distributed in the aerogel, the composition of the particle should be between the compositions shown for “No BG Subtraction” and “Si Norm BG Subtraction.” Spot A: V is 2 orders of magnitude above CI and the Fe/Ni ratio is 100 times that of CI. Spot B: Only V, Cr, Fe, Ni and Cu are above background. The abundance pattern is similar to that of Spot A. Spot C: located about half-way between Spots A and B and the surface of the aerogel, gave a lower count rate. Fewer elements were detected, but it has a similar composition to Spots A and B. Spot D was not analyzed at high resolution. Its composition was derived from the 250 nm resolution map. Spot D: much closer to the surface than the other three spots, has a composition pattern similar to the “bulb,” including a CI-like Ni/Fe ratio. Overall, Spots A, B, and C all have similar compositions, with Ni/Fe, Cr/Fe and in two cases (Spots B and C) Mn/Fe approximately CI, but with Ni/Fe more than an order of magnitude lower than CI. The V/Fe ratio is two orders of magnitude greater than CI in all three spots, something we have not seen in any of other the IS Candidates we analyzed previously. Slide 13 also shows the abundance pattern if the elemental masses of all analyzed features are summed.This estimate of the total track composition is similar to that of the surface feature which dominates the mass.The Ni/Fe ratio is almost identical to that of CI. If Fe is 18.5%, then the masses of particles A, B, C and D suggest diameters of about 1.6, 1.0, 1.2, and 0.5 microns in diameter. These count-derived sizes agree well with the number of pixels in each particle in the Fe maps. Particle A is located about 150 microns from the surface of the aerogel, which is generally consistent with the expected length of a hypervelocity track produced by a 1.6 micron diameter particle. Overall, these results are suggestive of an incident particle that deposited material near the surface then broke up during capture and deposited particles along a capture track. Recommendations for future work on this picokeystone: Since we were not able to map the entire picokeystone, it would be desirable to map the entire “search box” at moderate resolution (possibly at the NSLS or ALS) to see if there are other hot-spots of similar composition outside of the rectangle we analyzed. If any are found, that would suggest the material we found is a contaminant, and the linear distribution of hot-spots and the “entry bulb” is just a chance occurrence.
| Notes By: | ndc\mrodrigu | Date: | 03/15/2013 |