Propagation of high-energy particles inside solid matter: cosmic-ray-induced spallation in iron meteorites

Abstract

A model is presented to evaluate the variations with depth of high-energy particle fluxes as they propagate inside spherical solid bodies. This model can be used to calculate the production rates of spallogenic nuclides inside meteorites of various sizes and compositions. The exponential attenuation of the flux, the gradual energy loss due to the Coulombian drag and the production of secondary particles in the spallation reactions are all taken into account in a self consistent way. It is shown that, in the energy range considered (300 MeV to 30 GeV), the particle fluxes are fairly insensitive to the actual angular distribution of the emitted secondary particles, as long as the assumed distribution remains within realistic limits. The propagation equations resulting from these interactions are solved on a spherical mesh consisting in a set of lines of various directions inside the sphere. The computed neutron and proton fluxes are presented in the case of an iron sphere irradiated by galactic cosmic ray (GCR) protons: these results show the critical importance of secondary particles and especially neutrons as depth increases within the sphere. The fluxes are used to calculate the production rates of some cosmogenic nuclides inside iron meteorites. The use of such production rates to gain information on exposure geometry of these meteorites and on the history of the GCR fluxes is discussed through a comparison with experimental data for the iron meteorite Grant. It is shown that the distribution of the different estimated ages within a meteorite could be used to distinguish between a spectral variation and an intensity variation of the GCR flux.