  Heavy ions channeling through crystals with multi-GeV kinetic energies can
create electron-positron pairs. In the framework of the ion, the energy of
virtual photons arising from the periodic crystal potential may exceed the
threshold $2mc^2$. The repeated periodic collisions with the crystal ions yield
high pair production rates. When the virtual photon frequency matches a nuclear
transition in the ion, the production rate can be resonantly increased. In this
two-step excitation-pair conversion scheme, the excitation rates are coherently
enhanced, and they scale approximately quadratically with the number of crystal
sites along the channel.
