The cosmic-ray 3He/4He ratio from 100 to 1600 MeV amu-1
The Superconducting Magnet Instrument for Light Isotopes (SMILI) flew for 19 hours on 1989 September 1 with a residual overburden of 5 g cm-2. It measured the charge, rigidity, and velocity of 30,000 cosmic-ray helium nuclei, with velocity determined by time-of-flight and Cerenkov techniques. Using these data, the flux and isotopic composition of helium as a function of energy were determined. The observed isotopic composition is consistent with that expected from interstellar propagation models inferred from the secondaries of CNO, in contrast to earlier observations which indicated an overabundance of 3He. We discuss constraints that this result places on cosmic-ray transport and solar modulation models.