Scanning Transmission X-Ray Microscopy as a Tool for Analysis of Interstellar Dust Captured in Aerogel

Introduction: The Stardust mission returned to earth in January 2006 with two aerogel collectors. Cometary dust samples from the Jupiter-family comet Wild2 were studied during the first months after recovery in the so-called Cometary Preliminary Examination [e.g. 1,2]. Like the cometary collector, the interstellar (IS) collector contains 132 aerogel tiles and 240 foils The IS collector was exposed to the interstellar dust stream for 196 days. In comparison with the hundreds of Wild2 comet dust impact tracks containing thousands of particle fragments, we expect the interstellar dust to be orders of magnitude smaller and rarer. Volunteers in the Stardust@Home public project have scanned more than half a million microscope images searching for rare features in the aerogel which may be impact tracks from dust particles [3]. The candidates identified so far will be carefully extracted from the aerogel using computer controlled micromanipulators and analyzed [4]. Some candidates might turn out to be impact tracks of interstellar origin particles. These are the first contemporary samples of interstellar dust ever collected. Because we lack laboratory analogs, we cannot truly predict what they will look like in the simplest terms like mechanical strength and track morphology, degree of crystallinity, and elemental composition. Stardust Interstellar Preliminary Examination (ISPE): The ISPE [5] will investigate candidates extracted from the aerogel collector using only noninvasive and non-destructive techniques. This means that all fragments of particles will remain in a small volume of aerogel, thin enough for optical and X-ray microscopy. Coined “picokeystones” for the picogram mass of the impactor, these tiny aerogel wedges are about 50 μm thick and contain an entire track, maybe <30 μm in diameter. We will use the Scanning Transmission X-ray Microscope (STXM) at the Advanced Light Source (ALS) Beamline 11.0.2, at Lawrence Berkeley National Laboratory (LBNL) to analyze Stardust interstellar dust candidates. Here we describe the capabilities of the beamline for analyzing samples in aerogel, and demonstrate a method for quantifying radiation damage to susceptible organics in a sample. Radiation damage to interstellar organics is possible due to the high absorption coefficient of C,N,O by soft X-ray energies. STXM Beamline Specifications: The STXM at ALS Beamline 11.0.2.2 has an elliptically polarized undulator soft X-ray source, providing an energy range from 200 to 2000 eV. The resolving power (E/ΔE) is 2500-7000 (ΔE = 80 meV at C K-edge). The scanning microscope focuses the synchrotron beam to a <40 nm spot using a Fresnel zone plate. The very high spatial resolution is well suited to searching for sub-micron dust fragments in aerogel tracks. The microscope may also be slightly defocused in order to efficiently map relatively large areas. A photon-counting detector behind the sample records the intensity of the transmitted radiation, generating an image pixel by pixel during rastering. Elemental maps are produced by acquiring an image of the sample just below and just above the edge for each element. Detecting very low concentrations of elements, while limiting photon dose to the sample usually requires that we use a strong resonance peak for mapping, giving an increase in sensitivity of up to 5 times over the edge jump. The height of the resonance peak depends on the chemical nature and crystallography of the target material. For example, all simple silicates for which we have so far obtained Mg K-edge XANES, have a strong absorption at 1314 eV, which is consistent with Mg-XANES of olivines measured and modeled by Wu et al. [6] Radiation Damage of Epoxy Resin: Epoxy resin (EMBED-812) is easily damaged by low energy X-ray photons. Figure 1 shows C and O K-edge XANES spectra.. We made several overlapping maps for different elements on the epoxy as shown in figure 2. We used a beamline set-up exactly like analyses we have made on Stardust cometary aerogel.