Simulation of the interaction of Galactic cosmic-ray protons with meteoroids

Thick spherical targets made of gabbro (R=25 cm) and of steel (R=10 cm) were irradiated isotropically with 1.6 GeV protons at the Saturne synchrotron at Laboratoire National Saturne (LNS)/CEN Saclay in order to simulate the interaction in space of Galactic cosmic-ray (GCR) protons with stony and iron meteoroids. Proton fluences of 1.32×10(sup 14) cm(sup -2) and 2.45×10(sup 14) cm(sup -2) were received by the gabbro and iron sphere, respectively, which corresponds to cosmic-ray exposure ages of about 1.6 and 3.0 Ma. Both artificial meteoroids contained large numbers of high-purity target foils of up to 28 elements at different depths. In these individual target foils, elementary production rates of radionuclides and rare gas isotopes were measured by X- and gamma-spectrometry, by low-level counting, accelerator mass spectrometry (AMS), and by conventional rare gas mass spectrometry. Also samples of the gabbro itself were analyzed. Up to now, for each of the experiments, approximatly 500 target-product combinations were investigated of which the results for radionuclides are presented. The experimental production rates show a wide range of depth profiles reflecting the differences between low-, medium-, and high-energy products. The influence of the stony and iron matrices on the production of secondary particles and on particle transport, in general, and consequently on the production rates is clearly exhibited by the phenomenology of the production rates as well as by a detailed theoretical analysis. Theoretical production rates were calculated in an a priori way by folding depth-dependent spectra of primary and secondary protons and secondary neutrons calculated by Monte Carlo techniques with the excitation functions of the underlying nuclear reactions. Discrepancies of up to a factor of 2 between the experimental and a priori calculated depth profiles are attributed to the poor quality of the mostly theoretical neutron excitation functions. Improved neutron excitation functions were obtained by least-squares deconvolution techniques from experimental thick-target production rates of up to five thick-target experiments in which isotropic irradiations were performed. A posteriori calculations using the adjusted neutron cross sections describe the measured depth profiles of all these simulation experiments within 9%. The thus validated model calculations provide a basis for reliable physical model calculations of the production rates of cosmogenic nuclides in stony and iron meteorites as well a in lunar samples and terrestrial materials.