Physics
Scientific paper
Jan 1959
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1959phrv..113..674s&link_type=abstract
Physical Review, vol. 113, Issue 2, pp. 674-678
Physics
10
Scientific paper
A high-sensitivity mass spectrometer has been used to measure the helium, argon, and neon produced in iron by 0.16-, 0.43-, and 3.0-Bev protons. The spectrometer has a sensitivity so that 10-11 standard cc of helium could be detected above the contamination level. The He4 cross sections are 120 mb, 450 mb, and 1300 mb at 0.16, 0.43, and 3.0 Bev, respectively, while the He3/He4 cross section ratios are, respectively, 0.09, 0.10, and 0.18. At 0.43 Bev, cross sections of 1.0 mb, 3.3 mb, 8 mb, and 4.1 mb were found for the argon isotopes 36, 37, 38, and 39, respectively. The cross section for neon-21 is 0.1 mb at 0.43 Bev. The results are discussed in relation to evaporation theory and the rare gas content of iron meteorites. The He3 yields are all higher than previously measured tritium values. At 3 Bev the He3/T ratio is 2.4. It is suggested that in the case of iron in evaporation theory the Coulomb barrier is not as important relatively as previously thought. Alternatively, a large fraction of the He3 and tritium may be produced during the nuclear cascade which precedes the evaporation from the excited nuclei. The cross sections measured bear directly on the cosmic-ray-produced rare gases in meteorites. From the cross section of directly produced He3 relative to T, previous measurements of He3-T exposure ages of iron meteorites must be reduced by a factor of about 3. From the argon isotope cross sections it is seen that 80% of the Ar36 in meteorites is the result of β decay of cosmic-ray-produced Cl36 and thus Ar36-Cl36 should be a reliable method for measuring exposure ages of meteorites.
Schaeffer Oliver A.
Zähringer Josef
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