High-resolution observations of the isotopic composition of carbon and silicon in the galactic cosmic rays

High-resolution (sigma/sub M/<0.25 amu) satellite observations of the galactic cosmic rays (approx.100--300 MeV amu/sup -1/) have yielded determinations of near-Earth isotopic composition as follows: /sup 13/C//sup 12/C = 0.070 +- 0.006; /sup 29/Si//sup 28/Si = 0.109(+0.024, -0.014); and /sup 30/Si//sup 28/Si = 0.084(+0.020, -0.014). Employing a standard propagation model, source values greater than solar are derived for each of these ratios. The /sup 13/C excess may be the result of cross section uncertainties, but the /sup 29/Si and /sup 30/Si appear to reflect real differences between the galactic cosmic-ray source and the solar system.