Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.548m&link_type=abstract
Meteoritics, vol. 30, no. 5, page 548
Other
1
Cross Sections, Isotopes, Cosmogenic, Models, Calculations, Radionuclides, Simulation, Experiments
Scientific paper
Cosmogenic nuclide production rates in meteoroids depend on size and bulk chemical composition of the meteoroid, on the shielding depth and the chemical composition of a sample in it, on spectral distribution, composition and intensity of solar and galactic cosmic radiation, and on the possibly complex exposure history. Except for bulk and sample chemical compositions, all parameters are unknown and must be reconstructed. In order to interpret cosmogenic nuclide abundances in meteorites with respect to their exposure histories, to reconstruct the preatmospheric shapes of the meteoroids and to draw conclusions about long-term spectral distributions and intensities of the cosmic radiation, reliable model calculations of producton rates must be available. The lack of knowledge about the parameters which influence the production rates causes ambiguity of empirical and physical model calculations, if exclusively meteorite data are taken into account. Physical models of cosmogenic nuclide production in meteoroids without free parameters can be established on the basis of thick-target experiments by which the cosmic ray exposure of meteoroids in space is simulated as close as possible under completely controlled conditions. During recent years, we have performed five such experiments to simulate the exposure of meteoroids to galactic protons [1-6]. Here, we report new results on the latest one of these experiments, in which an artificial iron meteoroid made of steel with a radius of 10 cm was isotropically irradiated by 1.6 GeV protons [4,5]. Measurements and evaluation are now completed for shortand mediumlived radionuclides. Results for long-lived nuclides by AMS and of stable rare gas isotopes are partially available with additional measurements still going on. The results obtained up to now for radionuclide production are presented and discussed with respect to some aspects of the production of cosmogenic nuclides in iron meteoroids and of the influence of bulk chemical composition on production rates. As in our earlier experiments, the irradiation of the artificial iron meteoroid was found to be isotropic in general. Improved measuring techniques, however, allowed to discover small local anisotropies of the irradiation due to interactions of primary particles with the axis and bearings of the experimental setup. Also, local chemical inhomogeneities from the individual targets with high mass numbers influenced the measured production rates at some locations inside the iron sphere. The observed effects are discussed with respect to influences of deviations from spherical shape and from bulk composition on production rates in meteoroids. Since the neutron spectra in the iron sphere irradiated with 1.6 GeV protons differ significantly from those in the stony meteoroid mockups irradiated in our earlier simulation experiments with 1.6 GeV and 600 MeV, the experimental production rates obtained from all five simulation experiments can be used to extract information on the otherwise not measureable cross sections of neutron-induced reactions. A least-squares unfolding method is used to model simultanously the production rates measured in all our simulation experiments and to establish a consistent set of excitation functions of the underlying neutron-induced reactions as an improved basis for physical model calculations of cosmogenic nuclide production rates in meteoroids. Acknowledgement: This work was partially supported by the Deutsche Forschungsgemeinschaft and the Swiss National Foundation. References: [1] Michel R. et al. (1986) Nucl. Instr. Meth. Phys. Res., B16, 61-82. [2] Michel R. et al. (1989) Nucl. Instr. Meth. Phys. Res., B42, 76-100. [3] Michel R. et al. (1993) J. Radioanal. Nucl. Chem., 169, 13-25. [4] Michel R. et al. (1993) Meteoritics, 28, 399-400. [5] Michel R. et al. (1994) in Nuclear Data for Science and Technology (J. K. Dickens, ed.), 377-379, Am. Nucl. Soc., La Grange Park. [6] Luepke M. (1993) Thesis, Univ. Hannover.
Dittrich-Hannen Beate
Herpers Ulrich
Kubik Peter W.
Lange Hans-Jürgen
Leya Ingo
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