Accurate Analysis of Shallow Solar Wind Ion Implants by Sims Backside Depth Profiling

Introduction: The NASA mission “Genesis” collected solar wind (SW) for laboratory analysis [1]. SW can serve as a proxy for the Sun’s composition, and thus the average composition of the solar nebula [2], provided that fractionation processes during SW formation [e.g., 3] can be understood and quantitatively modeled. The Geneisis objective is to obtain elemental and isotopic abundances with precisions and accuracies better than available spacecraft data, which have uncertainties of up to several tens of percent [4]. SW ions travel with an average speed of 400km/s, (~1keV/amu) at 1 AU; thus, SW is implanted with a mean depth of ~40nm into exposed collectors. The highest energy SW ions (up to 1200km/s) penetrate up to ~600nm deep. Despite the purity of the collector materials, contamination of their surfaces is signficant: a natural oxide layer, a molecular film deposited in space [5], and particles deposited during the hard landing are all present. Here, we present a technique that is capable of analyzing nearly complete depth distributions of many elements in the SW even in the presence of high levels of surface contamination: backside depth profiling by secondary ion mass spectrometry (SIMS), which we have successfully applied to the analysis of bulk SW with fluence as low as 210 atoms/cm and also of all three SW regimes for some more abundant elements. A low impact energy primary ion beam and oxygen flooding are used to minimize ion beam mixing of surface contamination and transient effects on SIMS ion yields. We discuss sample preparation, analytical conditions, standardization, and data reduction as well as potential systematic errors.