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

Abstract

— 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 × 1014 cm−2 and 2.45 × 1014 cm−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 γ‐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, ∼500 target‐product combinations were investigated of which the results for radionuclides are presented here. 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 as in lunar samples and terrestrial materials.

Simulation of the interaction of galactic cosmic ray protons with meteoroids: On the production of 3H and light noble gas isotopes in isotropically irradiated thick gabbro and iron targets

“…The chemical compositions are given in Table 1. More than 1400 and 850 individual targets were placed inside the gabbro and iron spheres, respectively (Leya et al 2000b), comprising up to 28 different elements ranging in mass from C to Au. In the case of O, Na, S, K, and Ca, single-element targets were deemed impractical.…”

“…Details of the experiments were presented by Michel et al 1996, Leya 1997, and Leya et al 2000b. Briefly, the artificial meteoroids were irradiated with 1.6 GeV protons at the Saturne synchrotron at LNS/Saclay.…”

“…The influence of the bulk chemical composition on the elemental production rates, the so called "matrix effect" (Begemann and Schultz 1988), has been discussed for meteorites by Masarik and Reedy (1994), Leya (1997), and Albrecht et al (2000) and, for the simulation experiments, by Leya (1997) and Leya et al (2000b). As emphasized by these authors, noticeable effects on production rates are limited to products only a few mass units away from the target masses (e.g., 58 Co from Ni and 54 Mn from Fe).…”

“…Also, for all three elements (Mg, Al, Si), these ratios are lower by ~15-30% upon irradiation in the iron sphere as compared to irradiation of the targets within gabbro. This can be explained by the dependence on chemical composition of the intranuclear cascades and of particle loss effects (Leya 1997;Leya et al 2000b). The flux density of secondary neutrons with E <40 MeV is slightly higher in the iron sphere than in the gabbro sphere.…”

“…For proton-induced reactions, the database is fairly complete, but for neutron-induced reactions, cross section data are still very scarce. To overcome this problem, we derived the neutron excitation functions by subtracting from the total (i.e., proton-plus neutron-induced) production rates, determined in all five simulation experiments, the calculated proton-induced component (see Leya 1997;Leya and Michel 1997;Leya et al 2000b). From these data, the neutron excitation functions were then calculated by an energydependent, least-squares adjustment procedure.…”