Observation of latent heavy-ion tracks in polyimide by means of transmission electron microscopy

The structure of latent tracks produced by high-energy heavy ions in an organic material has been revealed for the first time by transmission electron microscopy. The experiments involved 9.3 MeV/A Au-197 and 11.1 MeV/A Pb-208 ion beams and similar to 50-nm thick polyimide foils (ion stopping power similar to 110 MeV mg(-1) cm(2)). Two-week old tracks are open channels, indicating the importance of violent energy equilibration events. Four-month old tracks engulf small amounts of material. Hence, damage sites exhibit chemical activity on a timescale of months, with possible absorption of material from the atmosphere. The change of material density at the track boundary is sharp. The track width distribution is broad. This may have resulted from the inhomogeneity of the polymer density on a molecular scale. The mean track diameter is 8.1 +/- 1.7 nm. There is no evidence for any severe structural alteration outside the track core. (C) 1999 Elsevier Science B.V. All rights reserved.