Unstable cosmic ray nuclei constrain low-diffusion zones in the Galactic disc

Observations of the vicinity of a variety of galactic gamma-ray sources have indicated a local suppression of diffusivity of cosmic rays (CRs) by up to three orders of magnitude. However, the impact of these low-diffusion zones on global properties of CR transport is, however, only poorly understood. Here, we argue that CR nuclear ratios, like the boron-to-carbon ratio and relative abundances of Beryllium isotopes are sensitive to the filling fraction of such low-diffusion zones and hence their measurements can be used to constrain the typical sizes and ages of such regions. We have performed a careful parameter study of a CR transport model that allows for different diffusion coefficients kappa(disc) and kappa(halo) in the galactic disc and halo, respectively. Making use of preliminary data from the AMS-02 experiment on the ratio of Beryllium isotopes, we find a 3.5 sigma preference for a suppression of the diffusion coefficient in the disc with a best-fitting value of kappa(disc)/kappa(halo) = 0.20(-0.06)(+0.10). We forecast that with upcoming data from the HELIX balloon experiment, the significance could increase to 6.8 sigma. Adopting a coarse-graining approach, we find that such a strong suppression could be realized if the filling fraction of low-diffusion zones in the disc was similar to 66 per cent. We conclude that the impact of regions of suppressed diffusion might be larger than usually assumed and ought to be taken into account in models of Galactic CR transport.