[PPS10-15] Electron nanoprobe analysis of H2+/He+-irradiated alumina to understand the space weathering process
Keywords:space weathering, solar wind irradiation, hydrogen analysis, electron microscopy/spectroscopy, alumina
In this study, we employed H2+/He+ irradiation to alumina (α-Al2O3), which was analyzed in nanometer scale using a scanning transmission electron microscope equipped with electron energy loss spectrometer (STEM-EELS). The target mineral, alumina, having a most basic oxide crystal structure (corundum structure), is a common mineral found around dying stars and in presolar grains identified from primitive chondrites .
The H2+/He+ irradiation was conducted at the Wakasa-wan Energy Research Center. 20 keV He+ and 40 keV H2+ were irradiated to two target alumina samples, respectively, both at the dose of ~1018 ions/cm2. Cross-sectional ultra-thin sections were prepared from the irradiated samples using a focused ion beam (FIB, HITACHI ETHOS NX5000), the sections of which were analyzed with a 200 kV-TEM (JEOL JEM-2100) to obtain BF/DF-TEM images and STEM-EEL spectrum imaging (SI) data.
High densities of dislocations, nano-voids, and nano-cracks due to void coalescence were observed over the area from the surface to the depth of 300 nm for both the H2+-irradiated and He+-irradiated samples. The SI dataset from the H2+-irradiated sample alone indicated generation of O-H species accompanied by reduction of Al, highly localized adjacent to the nano-cracks. This suggests that a chemical reaction between hydrogen and alumina selectively occurred on the unstable surface along the nano-cracks. The EEL spectra from the He+-irradiated sample, on the other hand, exhibited small He-K edges over the irradiated region, and their peak positions were slightly shifted from the reference position toward the higher energy side. The total helium gas volume fraction incorporated was estimated to be less than 3% of the total void volume, using the helium gas pressure and amount evaluated from the peak shift and intensity of the He-K edge. The injected helium ions likely clustered to form bubbles in the α-alumina, almost all of which escaped away from the thin section of ~100 nm.
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