Laser Post-Ionization Mass Spectrometry Analysis of Genesis Solar Wind Collectors

The samples returned to Earth by the NASA's Genesis Mission contain a record of the elemental and isotopic abundances of the Solar Wind (SW). This record is formed by the SW ions implanted in the near-surface regions of the Genesis sample collectors, so that the SW material can be distinguished from a terrestrial contamination, which occurred due to the crash landing of the spacecraft Sample Return Capsule. At Argonne National Laboratory, we are conducting analyzes of the Genesis SW collectors using a specially developed Laser Post-Ionization Secondary Neutral Mass Spectrometer (LPI SNMS), SARISA. This approach, based on ion sputtering of a SW collector surface and laser post-ionization of the neutral atoms sputtered from it, has proved to be sensitive, accurate and well suited for the quantitative analysis of the Genesis samples. We will report in this work the abundances of SW Mg and Ca measured with SARISA in two types of SW collector materials, silicon and diamond-like carbon (DLC). These LPI SNMS measurements were conducted in Resonance-Enhanced Multi-Photon Ionization (REMPI) regime using a sputter depth profiling method. In order to make our analyzes quantitative, we used specially prepared standards, made from exactly the same materials as the flown Genesis SW collectors and implanted with known fluencies of Mg and Ca ions. The REMPI analyzes of these standards allowed us to characterize the actual efficiency and detection limits of the SARISA instrument: for Mg, its useful yield peaked at about 20% and detection limits corresponded to < 50 part-per-trillion. We measured concentration vs depth profiles for Mg and Ca in SW collectors (Si and DLC, respectively) and compared them to the corresponding implant standards. One striking feature of the SW implants (compared to the standards) was that maxima of the SW element concentration vs depth profiles were broad, with apparent diffusion of the implanted atoms towards the surface and into the bulk. Since these collectors (1) were subjected to intense bombardment by more abundant SW ions (H and He), and (2) the Solar light heated them to the temperature of ~160° C during the SW collection, which lasted up to 852.83 days, this feature can be explained by radiation- enhanced thermal diffusion processes. In order to evaluate how much the terrestrial contamination can affect the accuracy of our LPI SNMS measurements, we conducted a special series of experiments with sputter depth profiling of Genesis SW collectors from their backside (i.e. opposite to the one exposed to the SW). By these measurements, we have demonstrated that (1) the implanted solar wind can be identified by backside sputter depth profiling, (2) the apparent widening of depth profiles into the bulk of SW collectors is mostly due to ion mixing effects, and (3) that widening and shifting of concentration maxima towards the collectors surface is real. We also concluded that the depth resolution of our measurements has to be improved in order to accurately profile the near-surface regions of SW collectors. At the Fall Meeting, we will present these experimental results and discuss what needs to be (and what is being) done in order improve accuracy of measurements of elemental abundances by ion sputtering based analytical methods. This work is supported by NASA under Work Orders W-19,895 and W-10,091 and by the U.S. Department of Energy (BES-Materials Sciences), under Contract No. DE-AC02-06CH11357.