Charge equilibration and irradiation damage threshold for MeV ions in polyimide

Polyimide foils were irradiated with MeV ions below charge state equilibrium and etched in sodium hypochlorite solution to measure the material depth below which ion tracks occurred. It was observed that no etchable ion tracks were formed in the first few nanometres of the foil as the electronic stopping power was below the damage threshold. The damage threshold was crossed at a certain depth in the foil and intermittent tracks began to form. The measurement of depth for crossing the damage threshold in polyimide enabled verification of the previously theoretical models for the evolution of nonequilibrium charge variation of electronic stopping power during charge state equilibration of energetic ions and explicitly demonstrated that the equilibration process plays a role in ion-induced nanostructure formation.