On the production of cosmogenic nuclides in meteoroids by galactic protons

Abstract

Abstract— A purely physical model is presented describing the depth‐ and size‐dependence of the production of cosmogenic nuclides in meteoroids with radii up to 85 cm and in planetary surfaces by galactic cosmic ray protons. The model is based on Monte Carlo calculations of the intra‐ and internuclear cascades, by which depth‐ and size‐dependent spectra of primary and secondary protons and of secondary neutrons are derived, and on experimental and theoretical thin‐target cross sections of the underlying nuclear reactions. Model calculations are presented for production rates of 53Mn, 26Al, 22Ne, and 21Ne in H‐ and L‐chondrites and of 53Mn and 26Al in lunar surface material and compared with experimental data. From the analysis of 53Mn and 26Al in the Apollo 15 lunar drill core and in the L‐chondrite Knyahinya GCR p‐spectra and integral particle fluxes at 1 A.U. and in the meteoroid orbits averaged over the last 10 Ma are derived. An analysis of experimental depth profiles in four H‐ and L‐chondrites demonstrates, that the new model is well capable of describing depth‐ and size‐dependences of production rates of cosmogenic nuclides. Moreover, it is possible to determine exposure ages for these meteorites on the basis of the theoretical 21Ne production rates. The model calculations further explain the depth‐ and size‐dependence of 22Ne/21Ne‐ratios and the dependences on these ratios of 21Ne, 26Al and 53Mn production rates. The future requirements for model calculations of cosmogenic nuclide production rates in extraterrestrial matter are outlined.