High-Sensitivity Mass Spectrometric Measurement of Stable Helium and Argon Isotopes Produced by High-Energy Protons in Iron

Abstract

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 ${10}^{\ensuremath{-}11}$ standard cc of helium could be detected above the contamination level. The ${\mathrm{He}}^{4}$ cross sections are 120 mb, 450 mb, and 1300 mb at 0.16, 0.43, and 3.0 Bev, respectively, while the ${\mathrm{He}}^{3}$/${\mathrm{He}}^{4}$ 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 ${\mathrm{He}}^{3}$ yields are all higher than previously measured tritium values. At 3 Bev the ${\mathrm{He}}^{3}$/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 ${\mathrm{He}}^{3}$ 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 ${\mathrm{He}}^{3}$ relative to T, previous measurements of ${\mathrm{He}}^{3}$-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 ${\mathrm{Ar}}^{36}$ in meteorites is the result of $\ensuremath{\beta}$ decay of cosmic-ray-produced ${\mathrm{Cl}}^{36}$ and thus ${\mathrm{Ar}}^{36}$-${\mathrm{Cl}}^{36}$ should be a reliable method for measuring exposure ages of meteorites.