Impact of transport modeling on the Fe60 abundance inside Galactic cosmic ray sources

Abstract

The ACE-CRIS Collaboration has recently released the measurement of radioactive $^{60}\mathrm{Fe}$ nuclei abundance in Galactic cosmic rays, in the energy range $\ensuremath{\sim}195--500\text{ }\text{ }\mathrm{MeV}$ per nucleon. We model cosmic ray propagation and derive from this measurement the $^{60}\mathrm{Fe}/^{56}\mathrm{Fe}$ ratio that is expected in the sources of Galactic cosmic rays. We describe cosmic ray origin and transport within the framework of the disk/halo diffusion model, namely a scenario in which the matter and the cosmic ray sources in our Galaxy are confined to a thin disk, while cosmic ray propagation occurs in a much larger halo with negligible matter density. We solve the cosmic ray transport equation accounting for spallation reactions, decay and ionization losses as well as advection. We find that the $^{60}\mathrm{Fe}/^{56}\mathrm{Fe}$ ratio at the source must be very close to the value detected in the local cosmic ray spectrum at Earth, due to the fact that spallation reactions are more effective for $^{56}\mathrm{Fe}$ than for $^{60}\mathrm{Fe}$. Such a result could help identify the sources of Galactic cosmic rays.