Genesis Sodium and Potassium Bulk Solar Wind Fluences

Introduction: We present preliminary measurements of solar wind (SW) Na and K abundances measured by secondary ion mass spectrometry (SIMS). Genesis B/C (bulk SW) array silicon (Si) and diamondlike-carbon (DlC) on Si (DoS) collectors were analyzed using backside depth profiling (BDP). This is the first reported use of SIMS for BDP of thinned Genesis DoS collectors. Background: It is thought that the outer convective layer (OCZ) of the Sun is decoupled from nucleosynthetic processes leaving it essentially unchanged in composition from that of the early solar nebula for elements heavier than 3 amu [1]. The solar wind (SW) is generated as particles are ionized and accelerated by the solar magnetic field out of the solar photosphere (our spectroscopic view of the OCZ). Thus, SW samples the solar photosphere, and the solar photospheric composition is a proxy for the protoplanetary disk that ultimately formed the planets. A Genesis baseline has a better founding in theory than the current use of CI chondrites, and has potential for much more accurate numbers for volatile or chemically mobile elements. In situ spacecraft data suggests that SW formation causes elemental fractionation relative to the solar photosphere for elements with high first ionization potentials (FIP) and long first ionization times (FIT) (e.g., [1],[2]), but low FIP, short FIT elements are expected to show minimal to no fractionation relative to photospheric abundances [3]. However, this hypothesis needs to be tested. SW Na and K abundances are of interest in two ways. First, their FIP and FIT values will help to define the details of SW fractionation necessary for understanding photospheric composition. Second, precise Na and K abundances are needed for cosmochemical models; Na and K abundances in CI chondrites vary by almost 20%. Measurements of Na were performed previously by SIMS [4,5] and RIMS [6,7]. In all cases, surface contamination interfered with the SW signal. Only one preliminary measurement on Genesis Na abundance was reported [5]. There are no prior reports for K. Here we present results for Na and K SW measurements. Backside Approach: Ubiquitous and pervasive presence of surface contamination and the length of time required for the analysis to reach steady state have interfered with previous analyses, so all SW measurements for this study used backside depth-profiling. This method is relatively common for SIMS analysis of electronic devices; services for thinning silicon wafers are commercially available. This technique had already been successfully applied to Genesis Si samples [cf., 8,9], however BDP of SW through DoS is a new technique, and Genesis-specific issues preclude commercial thinning. In particular, to mitigate incorporation of contaminates in the near-surface of the DlC collector, an annealing step was skipped in the flight material. Accordingly, the Genesis DlC is extremely fragile, neither gentle grinding nor solution etching (ammonia) of the Si-backing were successful. On approaching the Si-DlC interface the DlC would inevitably and spontaneously fragment and curl. Here, we mechanically thinned our DoS Genesis-flown fragment (60630), but well before reaching the DlC-Si interface we implemented a technique currently under development at ANL, whereby DoS wafers are etched with XeF2 to gently remove the Si-backing, leaving the ~1μm layer of DlC intact (Fig.1). Fig. 1. (a) DoS Genesis fragment is affixed face-down onto a substrate (e.g., a graphite planchet). (b) XeF2 etching has removed all of the Si, leaving DlC, as well as epoxy and carbon (not shown) completely intact. The new front surface can, in theory, be implanted with a reference fluence even for monoisotopic elements like Na without interfering with SW analysis.