Ratio between internal (dashed) and kinetic (solid) energy of the shell to initial energy E (dotted) after the interaction. In the non-relativistic case with η ≪ 1, all the energy of the GRB outflow is transformed into internal energy (heat) of the baryonic shell. In the ultra-relativistic case with η ≫ 1, the energy of the GRB outflow is transformed mostly into kinetic energy of the shell.