It would be difficult to see the effects of the ions because those effects will be overwhelmed by the lower energy cosmic rays and by the preionization in the ambient medium. Neutral regions render the Fermi mechanism inoperable because ionized plasma is necessary to sustain the diffusion of particles in magnetohydrodynamic turbulence. Such diffusion is essential for Fermi acceleration, since particles necessarily must cross the shock many times. Hence the presence of dense neutral regions near some remnants may play a crucial role in limiting the acceleration process. Someone asked if the model accounts for the composition of nucleons.
H is underabundant in the CR distribution.
Also, the mechanism selectively accelerates the higher Z atoms, so the CR distribution becomes skewed. There is a correlation between the dust composition and the CR composition.