Development of Chemical Procedures to Cleanly Separate Solar Wind Osmium Embedded in Genesis Silicon Collector

We have developed a method to cleanly extract Solar Wind (SW) osmium (Os) embedded in silicon (Si) wafers that were contaminated during crash landing of the GENESIS capsule. It forms part of a larger NASA funded research study aimed at determining the Re and Os concentrations of the solar photosphere. Solar wind Os is buried >13 nm (1nm = 10-9 m) below the wafer surface. Our procedure involves removal of the top 13 nm of the wafer surface followed by clean extraction of SW Os. Removal of the top 13 nm of silicon is accomplished by sequential oxidation of Si wafers with HNO3 and removal of the resulting oxide layer with dilute HF. Extensive experimentation with Nd-implanted flown samples shows a linear relationship of the oxidation rate constant with the concentration of SW hydrogen, which varies with depth. Using this relationship, we find that a flown wafer needs to be immersed in nitric acid for 422 minutes to oxidize topmost 9 nm. To oxidize the next 4 nm the wafer needs to be immersed for 196 minutes. The oxide layers can be removed by clean 2M HF to create atomically smooth surface (Imai et al., 2008) on silicon not physically damaged during the crash. Once cleaned, the wafer surface is etched using acidic bromine solution, which indiscriminately dissolves Si (Bressers et al., 1996). The acidic bromine solution provides not only a very clean method for extracting Os, but also simplifies processing of samples. More flight wafers will be required to optimize these results and to evaluate any heterogeneity in behavior among the flight samples. Methods and cleaning protocols developed here will also help the GENESIS community at large for removing contamination on samples prior to analysis.