| Abstract: | Abstract— We have determined the recoil losses from silicon carbide (SiC) grain‐size fractions of spallation Ne produced by irradiation with 1.6 GeV protons. During the irradiation, the SiC grains were dispersed in paraffin wax in order to avoid reimplantation into neighboring grains. Analysis for spallogenic 21Ne of grain‐size separates in the size range 0.3 to 6 μm and comparison with the 22Na activity of the SiC + paraffin mixture indicates an effective recoil range of 2–3 μm with no apparent effect from acid treatments, which are routinely used in the isolation of meteoritic SiC grains. Our results indicate that the majority of presolar SiC grains in primitive meteorites, which are micrometer‐sized, will have lost essentially all spallogenic Ne produced by cosmic‐ray interaction in the interstellar medium. This argues against the validity of previously published presolar ages of Murchison SiC (～10 to ～130 Ma, increasing with grain size; Lewis et al., 1994), where recoil losses had been based on calculated recoil energies. It is argued that the observed variations in meteoritic SiC grain‐size fractions of 21Ne/22Ne ratios are more likely due to the effects of nucleosynthesis in the He‐burning shell of the parent AGB stars which imposes new boundary conditions on nuclear parameters and stellar models. It is suggested that spallation‐Xe produced on the abundant Ba and REE in presolar SiC, rather than spallogenic Ne, may be a promising approach to the presolar age problem. There is a hint in the currently available Xe data (Lewis et al., 1994) that the large (>1 μm) grains may be younger than the smaller (<1 μm) grains. The retention of spallogenic 21Ne produced by the bombardment of SiC grains of different grain sizes with 1.6 GeV protons, avoiding reimplantation into neighboring grains by dispersing the SiC grains in paraffin wax, has been derived from a comparison of mass spectrometrically determined 21Ne, retained in the grains, with the 22Na activity of the grains‐plus‐paraffin mixture. Compared to estimates of retention used in previous attempts to determine presolar ages for SiC (Tang and Anders, 1988b; Lewis et al., 1990, 1994), the results indicate significantly lower values. They do, however, agree with retention as expected from previous measurements of recoil ranges in similar systems (Nyquist et al., 1973; Steinberg and Winsberg, 1974). The prime reason for the discrepancy must lie in the energy of the recoiling nuclei entering in the calculation of retention by Tang and Anders (1988b), which is based on considerations by Ray and Völk (1983). Based on the results, it appears questionable that spallation contributes significantly to the observed variations of 21Ne/22Ne ratios among various SiC grain‐size separates (Lewis et al., 1994). We rather suggest that the variations, just as it has been observed for Kr and Ba already (Lewis et al., 1994; Prombo et al., 1993), have a nucleosynthetic origin. Confirmation needs input of improved nuclear data and stellar models into new network calculations of the nucleosynthesis in AGB stars of elements in the Ne region. Finally we argue that, to determine presolar system irradiation effects, spallation Xe is more favorable than is Ne, primarily because of smaller recoil losses for Xe. Although preliminary estimates hint at the possibility that the larger (>1 μm) grains are younger than the smaller (<1 μm) ones, the major uncertainty for a quantitative evaluation lies in the exact composition of the Xe‐N component thought to originate from the envelope of the SiC grains' parent stars. |
