Isotopic composition of helium in the cosmic radiationabove 11 Gigavolts

We have measured the 3He/4He abundance ratio above 11 GV with a balloon-borne experiment that employs the geomagnetic method of isotope separation near Earth's equator. Above a kinetic energy of approximately 6 GeV per nucleon, we measure the 3He/4He ratio to be 0.24 +- 0.05 under the assumption that the helium spectrum at Earth is a power law in rigidity of the form R/sup -2.65/. The implied interstellar mean escape pathlength of approx.15 g cm-2, with this assumption, is considerably greater than values determined from observations of heavier elements. Propagation models more complex than the standard simple leaky box would be required to simultaneously understand the propagation of both the lighter and heavier components of cosmic rays