Solar cosmic-ray-produced radionuclides in meteorites

Abstract

Model calculations were performed for the production of cosmogenic nuclides by solar protons in meteorites. Depth-dependent proton spectra are derived via energy loss calculations for a simple irradiation geometry (vertical incidence on a half plane). In a second step, solar proton fluxes in arbitrarily shaped meteorites can be derived from thesespectra by numerical methods or, for ellipsoidal geometry, by closed form expressions. Based on these proton spectra, depth profiles of cosmogenic radionuclides in meteorites were evaluated by a system of excitation functions for the production of radionuclides (44⩽A⩽59)from target elements (22⩽Z⩽28). Moreover, production rates for 26Al and 22Na were calculated. The new excitation functions were also used to derive production ratesfor 26Al, 53Mn and 59Ni in cosmic dust. Our data differ considerably from earlier predictions and indicate that the interactions of solar protons in meteorites should be of greater importance than supposed earlier. Exemplarily, 53Mn is used for a detailed discussion of the role of various target elements, the influence of meteoritic shape, and the contributions by solar and galactic cosmic rays. Theoretical production rates for 53Mn are compared with depth profiles measured in St. Severin and Madhipura. Evidence is supported that for a successful interpretation of cosmic ray interaction especially in small meteorites and/or in surface samples with minor ablation it is necessary to consider interactions of both solar and galactic cosmic rays.