Relative Abundance of the Helium IsotopesHe3andHe4in the Cosmic Radiation

The geomagnetic field near the equator is used to measure the abundance ratio of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$/${}_{}{}^4{}_{}{}^{}\mathrm{He}$ near 6 GeV/nucleon. This ratio is found to be 0.24±0.05, under the assumption that the helium rigidity spectrum has the form $\frac{\mathrm{dI}}{\mathrm{dR}}\sim R^{-2.65\mathrm{}}$. If all ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ is of secondary origin this implies an interstellar mean escape path length of ≈ 15 g/${\mathrm{cm}}^2$, a value considerably larger than that determined from observations of heavier elements, but compatible with measurements of the helium isotopic ratio at lower energies.