An investigation into sulfur isotopes in the Galactic cosmic rays

Measurements made by the Ulysses High Energy Telescope (HET) have made it possible for the first time to resolve the individual isotopes of sulfur S-32, S-33, and S-34. The measured ratio of Galactic cosmic-ray S-34/S-32 in the heliosphere is 24.2% +/- 2.7%. Based on this value, the Galactic source composition for S-34/S-32 has been found to be 6.2% +/- 2.6% +/- less than or equal to 2.9%. The former error reflects the statistical and instrumental uncertainties, while the latter is an estimate of the uncertainty in the derived source abundance arising from the propagation calculation. The S-34/S-32 source abundance agreement with the solar system abundance of this ratio (4.4%) is in accord with conclusions from other measurements that the overall isotopic cosmic-ray source is solar system-like, with only a couple of notable exceptions. The HET also resolves S-33. The ratio of Galactic cosmic-ray S-33/S-32 in the heliosphere is 19.0% +/- 2.4%, corresponding to a source abundance of 2.6% +/- 2.4%, since S-33 is mostly secondary. This ratio is higher than, but consistent with, the solar system abundance of 0.8%. Assuming similar systematic errors to those found for S-34/S-32, the S-33/S-32 ratio illustrates the difficulty in finding source abundances for mostly secondary cosmic-ray isotopes. The overall implications of the S-33 and S-34 measurements and their derived source abundances are included in the report.