Relative abundance of the helium isotopes He-3 and He-4 in the cosmic radiation

The geomagnetic field near the equator is used to measure the He-3/He-4 abundance ratio near 6 GeV/nucleon. This ratio is found to be 0.24 + or - 0.05, under the assumption that the helium rigidity spectrum has the form dI/dR = about R exp(-2.65). If all He-3 is of secondary origin, this implies an interstellar mean escape-path length of about 15 g/sq cm, a value considerably larger than that determined from observations of heavier elements but compatible with measurements of the helium isotopic ratio at lower energies.