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Single crystals of CaF2, cleaved along the (1 1 1) plane, were irradiated by ³⁵Cl, ³⁹K, ⁵²Cr, ⁵⁸Ni, ⁷⁹Br and ¹⁹⁷Au ions with energies from 2 to 48 MeV, charge states from 1+ to 11+ and ion fluences from 2e10 to 1e13 ions/cm². The surface properties of the irradiated samples were studied using Atomic Force Microscopy in contact mode. Nanoscale surface structures were observed for some surfaces irradiated by Cl, K, Cr, Ni, Br and Au ions. Thresholds for nanostructure formation were found in terms of projectile ion velocity and electronic stopping power (S_e). Clear surface modification was observed for projectiles with specific energies above approximately 0.04 MeV/u. The threshold in S_e for the generation of distinct triangular structures was observed to be velocity-dependent. The average formation efficiency, referring to the ratio of the number of distinct nanostructures observed on the surface to the number of incident ions, as well as the average height of nanostructures increases with S_e.

Polyimide films of thickness 7.5 µm are irradiated by a wide range of ions (¹H to ¹⁹⁷Au) with energies between 1.05 and 48 MeV. Irradiated samples are then chemically etched in sodium hypochlorite solution to investigate nanopore formation due to ion track etching. A threshold in terms of electronic stopping power, S_et, needs to be surpassed to preferentially etch the ion tracks. Close to S_et, intermittent tracks are formed where only part of the track is etchable. The fraction of these etchable parts increases as we move away from S_et, toward higher stopping powers, eventually yielding continuous etchable tracks. Both intermittent and continuous track formation thresholds are observed to be velocity-dependent, yielding a decrease of the thresholds in the present work compared to previously reported thresholds for swift heavy ions. This finding leads the authors to suggest that electrostatic ion accelerators with terminal voltages of several MV are applicable for the production of ion track membranes up to ≈10–20 µm in thickness. Suitable ions for nanopores in 7.5 µm polyimide films include 42 MeV ⁵⁹Co and 48 MeV ¹⁹⁷Au. The growth mechanism for the pores during etching is discussed, relating it to the properties of the original ion track.

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.