Phenomenological models of Cosmic Ray transport in Galaxies

When examining the abundance of elements in the placid interstellar medium, a deep hollow between helium and carbon becomes apparent. Notably, the fragile light nuclei Lithium, Beryllium, and Boron (collectively known as LiBeB) are not formed, with the exception of Li7, during the process of Big Bang nucleosynthesis, nor do they arise as byproducts of stellar lifecycles. In contrast to the majority of elements, these species owe their existence to the most energetic particles in the Universe. Cosmic rays, originating in the most powerful Milky Way's particle accelerators, reach the Earth after traversing tangled and lengthy paths spanning millions of years. During their journey, these primary particles undergo transformations through collisions with interstellar matter. This process, known as spallation, alters their composition and introduces secondary light elements in the cosmic-ray beam. In light of this, the relatively large abundance of LiBeB in the cosmic radiation provides remarkable insights into the mechanisms of particle acceleration, as well as the micro-physics of confinement within galactic magnetic fields. These lecture notes are intended to equip readers with basic knowledge necessary for examining the chemical and isotopic composition, as well as the energy spectra, of cosmic rays, finally fostering a more profound comprehension of the complex high-energy astrophysical processes occurring within our Galaxy.