Monte Carlo Calculations of Cosmic Rays in the Galaxy: Effect of the Source Distribution on the Age and Matter Distributions

We have carried out a set of Monte Carlo calculations of the transport of cosmic-ray nuclei in the Galaxy. These calculations are used to examine the effect of the cosmic-ray source distribution on the matter path length distribution traversed by cosmic rays in the Galaxy. A comparison of cosmic-ray data with calculations using a simple leaky-box propagation model has earlier suggested that this matter distribution is close to an exponential, with a possible depletion of short path lengths at low energies. Such an effect could be caused by either a nonuniform source distribution or nonuniform propagation conditions. The Monte Carlo calculations are used to study the matter path length distribution for realistic uniform propagation conditions and various types of nonuniform source distribution. It is found that to produce the amount of path length depletion at low energies required to fit the cosmic-ray data it is necessary to consider extremely nonuniform types of source distribution. This suggests that the observations may be better fitted with various types of nonuniform propagation models rather than with nonuniform source distributions.