THE COSMIC-RAY HE-3 HE-4 RATIO FROM 200 MEV PER NUCLEON(-1) TO 3.7 GEV PER NUCLEON(-1)/

The abundances of cosmic-ray helium isotopes between 0.2 and 3.7 GeV n ucleon(-1) were measured by the Isotope Matter Antimatter Experiment ( IMAX) during a flight from Lynn Lake, Manitoba, Canada on 1992 July 16 -17. The IMAX balloon-borne magnetic spectrometer realized a direct me asurement of the charge, the velocity, and the rigidity of cosmic rays using plastic scintillators, a high-resolution time-of-flight system, and two silica-aerogel Cerenkov counters in conjunction with a drift chamber/multiwire proportional chamber tracking system. About 75,000 h elium isotopes are identified by their mass using the velocity versus magnetic rigidity technique. The measured He-3/He-4 ratios are correct ed to the top of the atmosphere, and a comparison with previous data i s given. The observed isotopic composition is found to be generally co nsistent with the predictions of a standard leaky box model of cosmic- ray transport in the Galaxy.