On the Production of He, Ne, and AR Isotopes from Mg, Al, Si, Ca, Fe, and NI in an Artificially Irradiated Meteoroid

Details On the Production of He, Ne, and AR Isotopes from Mg, Al, Si, Ca, Fe, and NI in an Artificially Irradiated Meteoroid On the Production of He, Ne, and AR Isotopes from Mg, Al, Si, Ca, Fe, and NI in an Artificially Irradiated Meteoroid

Jul 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27..315w&link_type=abstract

Meteoritics, vol. 27, no. 3, volume 27, page 315

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Scientific paper

The production of He, Ne, and Ar isotopes from their main target elements was investigated in an experiment (1) by irradiating a 50-cm-diameter gabbro sphere isotropically with 1.6 GeV protons. The model meteoroid contained, among a large number of other targets, pure element foils of Mg, Al, Si, Fe, and Ni at 10 different depths and wollastonite targets at 3 different depths in central bores. After the irradiation, radionuclide production in these targets was measured by gamma spectrometry. Stable He, Ne, and Ar isotopes were measured in statically operated mass spectrometers. Here, we report the results for stable He, Ne, and Ar isotopes and for ^22Na. The production depth profiles vary widely, ranging from profiles with near-surface production 15% higher than in the center (^22Na from Fe) to such profiles with production in the center 45% higher than near the surface (^20Ne from Mg). The isotope ratios ^3He/^4He and ^3He/^21Ne in Mg, Al, Si and ^22Ne/^21Ne in Mg all decrease significantly with increasing shielding. The production rates of He, Ne, and ^22Na from Mg, Al, and Si in the 1600-MeV simulation experiment are 1.5 to 3 times higher than in the model meteoroid of similar size but irradiated earlier with 600 MeV protons (2). This increase is attributed to the increase of the production of secondary neutrons with primary energies rising from 600 to 1600 MeV. This effect also causes the depth dependences of isotope ratios observed in the 1600-MeV simulation that was not seen in the 600-MeV experiment. Model calculations of the production of He, Ne, and Ar isotopes and of ^22Na were performed for the artificial meteorites of the 600- and 1600 MeV-exposures as well as for real meteoroids. Production rates were calculated from depth-dependent p- and n- spectra, which were derived by Monte Carlo techniques using the HERMES code system (3), and from cross sections for the relevant nuclear reactions as described earlier (4). The cross section database for p-induced reactions was extended by new data. Particularly, new thin target cross sections for the production of He and Ne isotopes from Mg, Al, Si, Fe, and Ni at energies of 800, 1200, 1600, and 2600 MeV measured by our group were included. For the production of He isotopes by n- and p-induced reactions identical cross sections were assumed. The new calculations for real meteorites confirm the recently published production rates of ^21Ne in stony meteoroids (4) and newly also describe the depth profiles of ^22Ne/^21Ne ratios. Comparison of production in the model meteoroid and real meteoroids shows that the 1600-MeV irradiation simulates the cosmic exposure to galactic protons within about 20%. The systematic deviations are attributed to differences between the monoenergetic irradiation and the exposure to the GCR spectrum and to the resulting differences between the secondary particle fields. Acknowledgement: This work was supported by the Deutsche Forschungsgemeinschaft and by the Swiss National Science Foundation. References: (1) Michel R. et al. (1991) Meteoritics (abstract) 26, 372a. (3) Cloth P. et al. (1988) Juel-2203. (2) Michel R. et al. (1989) Nucl. Instr. Meth. Phys. Res. B42, 76-100. (4) Michel R. et al. (1991) Meteoritics 26, 221-242.

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