Mathematics – Logic
Scientific paper
Mar 1989
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989gecoa..53..709s&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 53, Issue 3, pp.709-716
Mathematics
Logic
5
Scientific paper
10 Be and 26 Al are produced by cosmic-ray-induced spallation of atmospheric constituents, and also as has recently been shown, by spallation in the rocks exposed to cosmic rays. We now present experimental data showing that these two isotopes can also be produced by radiation from uranium and thorium decay chain radionuclides contained within the host rocks. We have measured the 10 Be and 26 Al concentrations in a number of shielded uranium (spontaneous fission half-life ~10 26 years) and thorium (no spontaneous-fission-decay mode) ores using accelerator mass spectrometry. The concentrations of 10 Be and 26 Al range from 2.8·10 6 to 29.6·10 6 and 1.1 · 10 7 to 1.43· 10 atoms g - , respectively. The high concentrations of 10 Be and 26 Al imply unacceptably-high branching ratios (~ 10 -4 to 10 -1 ) for their production from spontaneous fission and coupled with the presence of 10 Be and 26 A1 in the Th ores, eliminate spontaneous fission as the major source of either isotope. The large 26 A1 to 10 Be ratio found in all the samples eliminates atmospheric production. The most likely sources are the alpha reactions, 7 Li ( , p ) 10 Be and 23 Na ( , n ) 26 Al , and neutron reactions, 9 Be ( n , ) 10 Be , 10 B ( n , p ) 10 Be and 13 C ( n , ) 10 Be . Our work provides an upper limit on the contribution of in situ radioactivity to samples containing low levels of 10 Be and 26 Al. We have calculated the equilibrium concentrations of 10 Be and 26 Al in a wide variety of geologic materials including granites, ultramafic rocks, basalts, shales, pelagic sediments, ferromanganese nodules, tektites and Libyan Desert Glass. The calculated concentrations of 10 Be and 26 Al range from <1 to 4100 and 9 to 10.3 · 10 6 atoms g -1 , respectively. Implications of these calculations are discussed. In general, the calculated amounts of radiogenically produced 10 Be in geological materials are quite small and frequently negligible compared to cosmogenic production, while that of 26 Al are significant and sometimes the dominant source.
Middleton Richard
Sharma Pankaj
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