Astronomy and Astrophysics – Astrophysics
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.247l&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 247
Astronomy and Astrophysics
Astrophysics
Scientific paper
We report here the measurement of ^26Al by thermal ionization mass spectrometry in iron meteorites. Nuclides produced by the interaction of galactic cosmic rays with meteoritic bodies are widely used to reconstruct the parental history of meteorites and also to address the problem of constancy of cosmic rays in the past. In iron meteorites the half life of ^26Al is much shorter than the exposure age and saturation is reached. Its concentration is then directly related to the preatmospheric shielding of the analyzed sample. It can be also used together, with other short-lived nuclides, to calculate the terrestrial residence time for found meteorites. Natural contents of ^26Al in iron meteorites are very small (a few dpm per kg) and have been measured earlier by counting techniques and AMS. For thermal ionization the difficulty resides mostly in the measurement of the ^26Al/^27Al ratio. ^27Al may be contained in the sample and also is introduced by the chemical separation. ^27Al beams of 10^-11 A are readily obtained with a few ng of aluminium and are measured on a standard faraday cup. ^26Al was measured on a low background electron multiplier operated in the ion counter mode. ^27Al content was measured by isotope dilution using a ^26Al spike. The ^26Al ion beam can be interfered by traces of ^26Mg. Usually the ^26Mg background could be brought lower than 10^-9 relative to ^27Al. This is sufficient for the present experiment. No organic interference was present at the same level. The abundance sensitivity stemming from the ^27Al beam on mass 26 is 3 10^-9. Results: Samples sizes for this study range from 100 to 300 mg of iron. Ratios are measured with a precision of about 1% thereby leading to a final ^26Al content with an accuracy around 2%. Two meteorites were investigated so far: Grant and Canyon Diablo. Grant is one of the best documented meteorites with regard to spallation effects. The result on Grant is an agreement with literature AMS data (Graf et al., 1987). Our result on Canyon Diablo is in agreement with literature counting data (Kohman et al., 1967) but large differences may exist from sample to sample due to differential shielding. ^26Al spallation yield from sulfur is about two orders of magnitude higher than that from iron but this contribution is less than 2% due to the low sulfur content of the metal itself. Sample ^25Mg/^27Al ^26Al/^27Al [26Al] dpm/kg Spike <2 10^-1l 6098,0+-10,2 10^-9 Grant B+15 4,0+-0,9 10^-9 175,6+-4,2 10^-9 2,40+-0,08 Canyon Diablo 3,6+-0,9 10^-11 59,9+-1,4 10^-9 0,94+-0,03 As a conclusion, ^26Al is measurable in iron meteorites using standard modern thermal ionization mass spectrometers (a Finnigan 262 is used here). The precision attainable compares favourably with AMS. Higher ^26Al/^27Al ratios can certainly be measured with this method by the use of energy filtering devices, which were not neccessary here. Whether such measurements can be done in silicates depends also on their contents in ^27Al. References: Graf, T., Vogt S., Bonani G., Herpers U., Signer P., Suter M., Wieler R., and Wolfli W. (1987) Nucl. Instr. and Meth. in Phys. Res. B29, 262-265. Kohman, T.P. and Bender M.L. (1967) In High Energy Nuclear Reactions in Astrophysics. (ed. B.S.P. Shen), pp 169-245. Benjamin, New York.
Allègre Claude J.
Birck Jean-Louis
Langellier C.
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