The University of Arizona

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



Thick spherical targets, one made of gabbro (R = 25 cm) and one made of iron (R = 10 cm), were irradiated isotropically with 1.6 GeV protons at Laboratoire National Saturne (LNS)/Saclay to simulate the interactions of galactic cosmic ray protons with meteoroids in space. At various depths, both artificial meteoroids contained a large number of high-purity, single-element target foils and chemical compounds of up to 28 target elements. In these individual target foils, the elemental production rates of radionuclides and noble gas isotopes were measured. Here, we report the results for the light noble gas isotopes 3, 4He, 20, 21, 22Ne, and 36, 38, 39Ar for the most cosmochemically relevant target elements as well as for some meteoritic material from Jilin, Farmington, and Cape York. From 3He analyses done several years apart, 3H diffusive losses during sample storage have been obtained, and direct as well as cumulative 3He production rates for O, Mg, Al, Si, Fe, Ni, and the meteoritic material are given. Losses by diffusion of tritium from metallic Mg and Fe are found to occur on time scales of months, while metallic Al, Si, and stone meteorites are much more retentive. The production rate ratios P(3H)/P(3He)d obtained in the simulation experiments are 0.73, 1.28, and 1.16 for O, Al, and Si, respectively. These rates are based on our best knowledge about the 3H and 3He production rates and should, therefore, replace data published earlier (Leya et al. 2000a). The earlier calculations for 4He, 20, 21, 22Ne, and 36, 38, 39Ar remain valid. The new modeled correlation 3Hecum/21Ne versus 22Ne/21Ne for chondrites exposed to cosmic rays with an energy spectrum characterized by a modulation parameter of Φ = 650 MeV is in fair agreement with the empirical relationship (Berne plot). However, for small meteorites and little shielding in larger ones, there are systematic differences that most likely are due to an underestimation of the spallogenic 22Ne/21Ne ratio by ~2%.


Cosmic rays;Cosmogenic nuclides;Berne plot;Tritium;Noble gases

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