Spallation recoil II: Xenon evidence for young SiC grains
We have determined the recoil range of spallation xenon produced by irradiation of Ba glass targets with approximatly 1190 and approximatly 268 MeV protons, using a catcher technique, where spallation products are measured in target and catcher foils. The inferred range for (sup 126)Xe produced in silicon carbide is approximatly 0.19 µm, which implies retention of approximatly 70% for (sup 126)Xe produced in "typical" presolar silicon carbide grains of 1 µm size. Recoil loss of spallation xenon poses a significantly smaller problem than loss of the spallation neon from SiC grains. Ranges differ for the various Xe isotopes and scale approximately linearly as function of the mass difference between the target element, Ba, and the product. As a consequence, SiC grains of various sizes will have differences in spallation Xe composition. In an additional experiment at approximatly 66 MeV, where the recoil ranges of (sup 22)Na and (sup 127)Xe produced on Ba glass were determined using gamma-spectrometry, we found no evidence for recoil ranges being systematically different at this lower energy. We have used the new data to put constraints on the possible presolar age of the SiC grains analyzed for Xe by Lewis et al. (1994). Uncertainties in the composition of the approximately normal Xe component in SiC (Xe-N) constitute the most serious problem in determining an age, surpassing remaining uncertainties in Xe retention and production rate. A possible interpretation is that spallation contributions are negligible and that trapped (sup 124)Xe/(sup 126)Xe is approximatly 5% lower in Xe-N than in Q-Xe. But also for other reasonable assumptions for the (sup 124)Xe/(sup 126)Xe ratio in Xe-N (e.g., as in Q-Xe), inferred exposure ages are considerably shorter than theoretically expected lifetimes for interstellar grains. A short presolar age is in line with observations by others (appearance, grain size distribution) that indicate little processing in the interstellar medium (ISM) of surviving (crystalline) SiC. This may be due to amorphization of SiC in the ISM on a much shorter time scale than destruction, with amorphous SiC not surviving processing in the early solar system. A large supply of relatively young grains may be connected to the proposed starburst origin (Clayton 2003) for the parent stars of the mainstream SiC grains.