Simulation of the interaction of galactic cosmic-ray protons with meteoroids: On the production of radionuclides in thick gabbro and iron targets irradiated isotropically with 1.6 GeV protons

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 synchrot ron at Laboratoire National Saturne (LNS)/CEN Saclay in order to simulate t he interaction in space of galactic cosmic-ray (GCR) protons with stony and iron meteoroids. Proton fluences of 1.32 x 10(14) cm(-2) and 2.45 x 10(14) cm(-2) were received by the gabbro and iron sphere, respectively, which co rresponds to cosmic-ray exposure ages of about 1.6 and 3.0 Ma. Both artific ial meteoroids contained large numbers of high-purity target foils of up to 28 elements at different depths. In these individual target foils, element ary production rates of radionuclides and rare gas isotopes were measured b y x- and gamma-spectrometry, by low-level counting, accelerator mass spectr ometry (AMS), and by conventional rare gas mass spectrometry. Also samples of the gabbro itself were analyzed. Up to now, for each of the experiments, similar to 500 target-product combinations were investigated of which the results for radionuclides are presented here. The experimental production r ates show a wide range of depth profiles reflecting the differences between low-, medium-, and high-energy products. The influence of the stony and ir on matrices on the production of secondary particles and on particle transp ort, in general, and consequently on the production rates is clearly exhibi ted by the phenomenology of the production rates as well as by a detailed t heoretical analysis. Theoretical production rates were calculated in an a p riori way by folding depth-dependent spectra of primary and secondary proto ns and secondary neutrons calculated by Monte Carlo techniques with the exc itation functions of the underlying nuclear reactions. Discrepancies of up to a factor of 2 between the experimental and a priori calculated depth pro files are attributed to the poor quality of the mostly theoretical neutron excitation functions. Improved neutron excitation functions were obtained b y least-squares deconvolution techniques from experimental thick-target pro duction rates of up to five thick-target experiments in which isotropic irr adiations were performed. A posteriori calculations using the adjusted neut ron cross sections describe the measured depth profiles of all these simula tion 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 as in lunar sample s and terrestrial materials.