On the depth dependence of spallation reactions in a spherical thick diorite target homogeneously irradiated by 600 MeV protons: Simulation of production of cosmogenic nuclides in small meteorites

Abstract

An artificial meteorite made out of diorite with a radius of 5 cm was irradiated isotropically with 600 MeV protons in order to simulate the production of cosmogenic nuclides in small meteorites by galactic cosmic ray protons. The primary proton dose of 4.82 × 10 15 cm −2 is equivalent to a some 50 Ma of cosmic ray exposure. The depth dependent production of a wide range of radionuclides from target elements O, Mg, Al, Si, Ti, Fe, Co, Ni, Cu, Ba, and Lu was measured. Furthermore, the production of He and Ne isotopes from Al, Mg, Si, as well as from glass and meteoritic material was determined. Thick-target data are compared with thin-target production rates measured simultaneously, thus allowing to separate the contribution of secondary protons and neutrons. The results demonstrate that, in contrast to present presumptions, even in small meteorites secondary particles have to be considered and that the depth profiles show differences in production of up to 30% between surface and center. The experimental data are discussed with respect to cosmic irradiation conditions of real meteorites. Using Monte Carlo techniques depth dependent nucleon spectra were calculated. On the basis of these spectra and of thin-target excitation functions theoretical production rates were derived and compared with the experimental ones. This comparison shows that it is possible to reproduce the experimental depth profiles quite well by thin-target calculations provided reliable excitation functions are at hand. The thick-target measurements and the thin-target calculations provide a basis for an advanced modelling of the production of cosmogenic nuclides in small meteorites, which strictly differentiates between all contributing production modes.