  We study the acceleration of heavy nuclei at SNR shocks when the process of
ionization is taken into account. Heavy atoms ($Z_N >$ few) in the interstellar
medium which start the diffusive shock acceleration (DSA) are never fully
ionized at the moment of injection. The ionization occurs during the
acceleration process, when atoms already move relativistically. For typical
environment around SNRs the photo-ionization due to the background galactic
radiation dominates over Coulomb collisions. The main consequence of ionization
is the reduction of the maximum energy which ions can achieve with respect to
the standard result of the DSA. In fact the photo-ionization has a timescale
comparable to the beginning of the Sedov-Taylor phase, hence the maximum energy
is no more proportional to the nuclear charge, as predicted by standard DSA,
but rather to the effective ions' charge during the acceleration process, which
is smaller than the total nuclear charge $Z_N$. This result can have a direct
consequence in the prediction of the knee structure of the cosmic ray spectrum.
Moreover the acceleration of ultra-heavy elements beyond the Iron's maximum
energy is very hard to achieve making unlikely their possible contribution to
the cosmic ray spectrum in the transition region between Galactic and
extragalactic component.
