THE ISOTOPIC COMPOSITION OF INTERSTELLARHELIUM MEASURED BY IN-VACUO ETCHING OF A
METAL FOIL EXPOSED ON THE MIR SPACE STATION
R. Wieler1 and H. Busemann2. 1Earth Sciences, ETH Zürich,Switzerland. wieler@erdw.ethz.ch. 2School of Earth, Atmospheric and Environmental Sciences, University of Manchester, UK
Introduction: Among the few successful sample return missions, the Swiss-Russian COLLISA (Collection of InterstellarAtoms) collaboration [1,2] has received relatively little attention.
Here we discuss the isotopic composition of interstellar neutral
He trapped on a CuBe foil exposed on the MIR Space station,obtained by stepped in vacuo etching of the thin BeO surface layer of the foil and online mass spectrometric analyses [3].
Trapping and analysis techniques: 50 cm2 of foil exposed
during some 60 h to the flux of interstellar neutral atoms impinging with a velocity of >60 km/s (corresponding to an implantation
depth in BeO of some 2-3 nm) were analysed in ten steps of etching by HF, five of which yielded both 3He and 4He above detection limits [3]. This technique resulted in much lower blanks thanstepwise heating extractions [2], since only the BeO layer wasdissolved. Tritiogenic 3He in the bulk foil [2] did thus hardly
compromise the etching data.
Results and Discussion: Summing 3He and 4He of the gasrich steps and correcting for blanks yields a 3He/4He ratio of
1.18*10-4. Corrected for the variable trapping efficiency of thetwo He isotopes, this yields a 3He/4He ratio for the Local Interstellar Medium (LIC) of (1.62 ± 0.29)*10-4. This is the most precise value for interstellar He obtained so far, but an earlier value
obtained on another MIR foil piece [2] and a ratio measured insitu with pick-up ions [4] both agree with our value within theirlarger uncertainties. The main source of uncertainty of our value
is the correction for variable trapping efficiency of 3He and 4He
while the uncertainty of the ratio deduced by [4] was governed byion counting statistics.
The 3He abundance relative to 4He in the LIC is very close to
the value of 1.66*10-4 inferred for the protosolar cloud [cf. 3],
showing that no significant increase of 3He took place since solar
system formation. In contrast, a small but notable increase of 3He
relative to its primordial value 13.7 Ga ago of ≤(1.1±0.2)*10-4 [5,cf. 3] occurred up to solar system formation. This observation
sets constraints on galactic chemical evolution models. As predicted by standard stellar models [6], some stars should be net
producers of 3He, but much of this 3He may be destroyed again
due to “extra mixing” [7]. Our data show that indeed most,though not all, low-mass stars undergo such extra mixing. At adistance of ~8kpc from the center of the Galaxy, the 3He abundance has not substantially changed since about 4.56 Ga ago.
Acknowledgements: We appreciate the enormous efforts ofour colleagues from the Russian Academy of Sciences and theUniversity of Bern which made these analyses possible.
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