Mathematics – Logic
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994e%26psl.125..341n&link_type=abstract
Earth and Planetary Science Letters (ISSN 0012-821X), vol. 125, no. 1-4, p. 341-355
Mathematics
Logic
28
Aluminum 26, Cosmic Rays, Meteoritic Composition, Neon Isotopes, Quartz, Radiation Effects, Radioactive Isotopes, Spallation, Geomorphology, Glaciology, Nevada
Scientific paper
The study of cosmic-ray-produced radioactive and stable nuclides on the surface of the Earth can provide relevant geomorphological and glaciological information. At present, the cosmic ray production rates of stable Ne-21 are not well known. This study attempts to remedy the situation by determining the production rate ratio of Ne-21 and Al-26, P-21/P-26, in quartz. Al-26 concentrations and P-26 rates have previously been investigated for quartz separates of Sierra Nevada rocks which were brought to the surface by glacial scouring during the Tioga period at the end of the last ice age. We used splits of the same samples for our studies and found that Ne in these rocks represents a mixture of several components: trapped Ne, nucleogenic Ne-21 and Ne-22 produced by (alpha, n) reactions in oxygen and fluorine, respectively, as well as cosmic-ray-produced Ne, which is the component of interest in this study. The trapped component was substantially lost in one sample (W86-12) by crushing and by a density separation of the grain sizes 38-90 microns and 90-125 microns, permitting the resolution of the in situ produced Ne-21 into cosmic-ray spallation and (alpha, n) produced components and the determination of a lower limit to P-21/P-26. In a second sample (W86-8) one split contained small enough amounts of nucleogenic Ne-21 to permit the determination of a reasonable upper limit to P-21/P-26. The two ratio determinations are consistent within error limits and the value adopted, 0.65 +/- 0.11 (2-sigma), agrees with ratios observed in extraterrestrial matter. Apparently, P-21/P-26 is thus not very sensitive to the neutron spectrum. However, the observed production rate ratio is substantially larger than theoretical estimates for Si targets, reflecting poorly known neutron excitation functions. The above P-21/P-26 value, coupled to the observed Al-26 production rate, corresponds to a Ne-21 production rate of P-21 = 21 atoms/g/a in quartz or to P-21 = 45 atoms/g Si/a (at sea level and high latitudes). This rate is based on an adopted exposure age of 11,000 yr for our quartz samples.
Graf Th.
Kim Jinyoung Serena
Kohl C. P.
Marti Kurt
Niedermann Samuel
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