The effect of the Galactic gas distribution on the expected cosmic rays spectrum

Cosmic ray (CR) nuclei are accelerated particles which move randomly through the interstellar medium (ISM), where they suffer scattering, reacceleration and energy loss processes before reaching Earth. Spallation processes also take place forming secondary nuclei by fragmentation of heavier ones. Due to the impossibility of observing directly their original direction, the determination of possible sources where these particles originated requires the use of codes to simulate the propagation of CR within the Galaxy. This consists of a spiral disk with a thickness of 2h ∼ 200 pc, where CR are created, and a halo with a height H, where they diffuse. From the existing data, mostly Be/Be and B/C, it is deduced that CR go through a mean density of ∼ 0.3 cm ([4,5]). Since the total density of the ISM is considered as 1 cm, (< nHI >∼ 0.5 cm ; [1,2,7,11]), a large effective halo and/or a local bubble or cavity of low density are required. This result is very dependent on the actual disk ISM density, largely uncertain. [6] have very recently obtained a new map of the diffuse gas distribution (Fig. 1), from which < nHI >∼ 0.2 cm , a factor of 2 lower than previous estimates.