Monte Carlo Simulations of Solar Ion Implantation into Genesis Collection Materials

We have used the open source Monte Carlo code "Stopping and Range of Ions in Matter" (SRIM) to simulate implantation of He and Ne ions into Al and Au collection materials from the Genesis Mission. These simulations will aid in accurately interpreting measured results such as isotopic fractionation as a function of depth and correcting for gross effects such as back-scattering of ions and loss from ions that are stopped in the thin "brown stain" detected on many collection materials after the spacecraft was recovered(1). Comparison of Ne simulations with comparatively straight-forward Ne measurement results will build greater confidence in applying SRIM corrections to other measurements. Simulations in this work assumed the ion velocity distribution for He as reported for the SRC Lid of the Genesis spacecraft (2) for all ion species. Each simulation used 100,000 ions of a single isotope. Simulations recorded either the fact an ion was back-scattered or the stopping position of each ion. Several simulations also kept track of sputtered collector material ions. In cases where the "brown stain" was simulated, this was assumed to contain equal parts of Oxygen, Carbon and Hydrogen at a density of 1.2g/cm3, and at thicknesses of 10 and 100 angstroms, corresponding roughly to minimum and maximum thicknesses reported so far. At this stage of analysis, there was no strong technical basis for brown stain composition assumed. As reliable information is reported on the brown stain composition these simulations will be re-run. Simulations of Ne implantation into the Al kidney suggest a peak implantation depth of approximately 290 angstroms. The 21Ne/20Ne and 22Ne/20Ne ratios of ions stopping shallower than this implantation depth indicate isotopic fractionation of 0.93 and 0.87, respectively; and the ratios of ions deeper than this depth indicate fractionation of 1.03 and 1.06, respectively; approximately 0.6% of ions are backscattered into space and are fractionated by 0.82 and 0.65, respectively. When a 100 angstrom stain was included, approximately 1% of the ions stopped in the stain itself, but only 0.1% are scattered directly out of the system. A systematic shift is apparent toward more fractionation of the component trapped in Al at depths shallower than the peak, and slightly less fractionation deeper than the peak when the stain is included, but these differences are generally within the statistical uncertainties of the simulation. References: (1) Burnett et al (2005) Molecular Contamination on Anodized Aluminum Components of the Genesis Science Canister, Lunar and Planetary Science Conference, XXXVI. (2) Wiens et al (2005) Mission Solar-Wind Collection Conditions Document, Submitted to the Genesis Project.