this budget to hydrogen and helium (in number), respectively. We do not indulge here in a more detailed discussion of the determination of these parameters and of their error from independent observations, since they are largely degenerate with the normalization of the diffusion coefficient (see, [*e.g.*]{}, [@MaurinEtAl2010]).

For the nuclear production and spallation cross sections, we use as reference the sets of tables from the [](https://galprop.stanford.edu/) package, while allowing for normalization, energy shift and low-energy slope changes according to the NSS method described in [@DeromeEtAl2019].

### Diffusion in real space and momentum space, and convection {#sssec:kdiff}

An important physical ingredient to all propagation models is the diffusion coefficient, which describes the scattering of CRs off magnetic turbulence. We assume that it can be taken as a scalar function, homogeneous and isotropic all over the magnetic slab. This is likely a good approximation in the context of B/C analyses because the CR flux is locally isotropic and the magnetic field configuration of the Milky Way exhibits relative fluctuations $\delta B/B\sim 1$ over all relevant spatial scales $\lambda$ (or wavenumbers $k=2\pi/\lambda$) [@CasseEtAl2002; @SunEtAl2008a; @JanssonEtAl2012a]. Since at the energies of interest, the CR flux is expected to be contributed to by many sources 