Isotopic Composition and Abundance of Interstellar Neutral Helium Based on Direct Measurements

One of the important problems in astrophysics is the determination of the abundances of the helium isotopes ³He and ⁴He in various regions of the universe, since those abundances can provide evidence of the intensities of various possible processes of the production and decay of light elements and can thereby reflect their history. In this paper we describe the procedure and results of the first determination by a direct method of the abundances of helium isotopes in the local interstellar medium surrounding the solar system. The experiment was carried out on the piloted MIR station by the prolonged exposure in open space of specimens of metallic foil with their subsequent return to earth and detailed laboratory mass-spectrometric analysis. As a result, we were able to obtain estimates of the ⁴He density (about 7.5·10^-3 cm^-3) and the ³He/⁴He isotopic ratio (about 1.7·10^-4) for the local interst ellar medium.