Sims-ams Method for Measuring Solar Wind Silicon in Dlc Genesis Collectors

Introduction: SIMS analysis is widely used for Genesis sample measurements at various institutions, but only two laboratories [1,2] offer accelerator-based SIMS. The use of a tandem accelerator allows for breakup of molecules during the acceleration process, thus assuring removal of molecular interferences without significant loss in sensitivity. Additionally, both mass spectrometers [1,2] are capable of simultaneous analysis of different ion species. The MegaSIMS [1] instrument has already successfully provided highquality oxygen isotope ratio results. We are illustrating the use of the Naval Research Laboratory (NRL) facility [2] to determine the amount of solar wind (SW) silicon retained in the diamond-like carbon (DLC) collectors. We are also encouraging the Genesis science community to consider the method presented below as a possible alternate solution in various other cases of interest, e.g. higher-mass elements where molecular interferences are a more serious issue, making the SIMS-AMS technique better suited. The DLC for the Genesis collectors is a thick (1.53 μm) layer on a Si substrate and was produced at Sandia National Laboratories by pulsed laser deposition from high density graphite (pellets of hot-pressed carbon powder). It was intended for analysis of nitrogen and noble gas isotopes, but can be also considered for SW Si analysis, which can not be done in Si or Sicontaining substrates. The expected mean implantation depth of SW Si in DLC is close to 100 nm with a total expected fluence of 1.9 10 atoms/cm from the 2year flight period. Previous XPS and SIMS data [3] show ubiquitous Si on DLC but only as a surface layer, up to about 30 nm, making it a reasonable candidate for Si analysis. More terrestrial contamination can appear as Si-rich areas in deeper layers, due to annealing steps during fabrication of the thick DLC layer from successive thin layers. Other options for Si analysis would be Ge substrates, mostly destroyed in the landing, or sapphire, which would need Au coating, which in turn might introduce additional contamination. Experimental Technique: The SIMS-AMS facility at NRL is using a modified Cameca 6F to provide a Cs primary beam. Desired ions, in this case Si and C, are selected through a mass-filtering recombination magnet and injected simultaneously in a 3-MV tandem Pelletron accelerator. After acceleration, only ions in a given charge state are sent to the spectrograph magnet for parallel detection. The C matrix beam was monitored in an ETP14150 electron multiplier mounted in a shielding box. For the low-concentration Si we had a large-area position-sensitive microchannel plate (MCP) detector, used in transmission mode, and followed by a silicon-implanted energy detector. The combination of two detectors allows for removing possible mass-to-charge ambiguities. The power of background reduction using position/mass and energy coincidence filtering, as well as the ability of the SIMS-AMS system to break apart injected molecules and analyze resulting atomic fragments were described in more detail Ref. [4] and references therein. Initial Implanted Standard Analysis: Initial results were obtained from a standard consisting of 4 10 atoms/cm of 84 keV Si implanted in a 1μmthick DLC layer on a Si substrate and a blank consisting of unimplanted DLC. The ion-implanted standard and control DLC material were cleaned with a modified RCA process using semiconductor-grade chemicals. The selection mask in the injection magnet had two approximately 1-u wide openings, the terminal voltage was set at 2.4 MV, and charge state was 3+ was selected, yielding 9.6 MeV ions. We verified that there is no background at mass 28 u in the spectrograph by showing that the energy spectrum contains only the 9.6 MeV peak. A depth profile for Si in shown in Fig. 1 together with the time dependence of the C matrix beam.