  We describe the formation of swift heavy ion tracks in polyethylene (PE) by
combining the Monte Carlo code TREKIS, which models electronic excitation in
nanometric proximity of the ion trajectory, with the molecular dynamics
simulating a response of the atomic system to the perturbation. The model
predicts circular tracks in amorphous PE but elliptical ones in crystalline PE
caused by preferential propagation of excitation along polymer chains during
the cooling stage. The obtained track sizes and shapes agree well with the
high-resolution microscopy of tracks in PE. The velocity effect in PE is shown:
the track parameters differ for ions with the same energy losses but different
velocities.
