MEASUREMENT OF ~3He/~4He IN THE LOCAL INTERSTELLAR MEDIUM: THE COLLISA EXPERIMENT ON MIR

摘要

An accurate measurement of the noble gas isotopic composition in the local interstellar medium (LISM) provides a constraint of primary importance for modeling Galactic evolution and, in some cases, for studying the production of light nuclei in the early universe. The foil collection technique offers a direct way to measure some local interstellar gas abundances. With this method thin metal foils are exposed to the flux of neutral interstellar particles. Particles with sufficient energy penetrate the foil and remain trapped within its atomic structure. With the COLLISA experiment we have used this technique to collect a sample of interstellar neutral matter with the aim of determining the helium isotopic ratio in the LISM. The foils were exposed on board the Russian space station Mir. After exposure in space, the foils were brought back to the Earth, and the amount of captured particles was determined by mass spectrometric analysis at the University of Bern. The analysis has allowed the detection of interstellar ~3He and ~4He and the determination of the LISM isotopic number ratio ~3He/~4He = (1.7 +- 0.8) x 10~(-4). This value is consistent with protosolar ratios obtained from meteorites and Jupiter's atmosphere, supporting the hypothesis that negligible changes of the abundance of ~3He occurred in the Galaxy during the past 4.5 Gyr.