Effect of degree of crystallinity of a plastic detector on the characteristics of heavy-ion tracks

For a polymeric nuclear track detector of a given chemical nature, it is interesting to study how the variation of a physical parameter, the crystallinity, affects the characteristics of developed tracks. Polyethylene terephthalate, obtained in several forms; completely amorphous or with different degrees of crystallinity was studied. The plastic films were irradiated with fission fragments of 252Cf or with Ar and Kr ions of different energies. For 1 MeV/N ions, the developed tracks are always found longer in a crystallized sample than in amorphous ones. The Ar and Kr ions of greater energy are not detected in amorphous films while they are detected in crystalline ones; the track lengths are then comparable with Northcliffe's calculated values. Moreover, under 252Cf fission fragments bombardment, different densities of tracks between amorphous and crystalline samples are observed; this can be explained by the fact that the critical registration threshold dE/dx is found much higher in the amorphous form. In a limited range of energy, for a specific polymer, the critical dE/dx is found the same for ions of different mass. In conclusion, and without stating anything about the length of the latent track, it has been shown that the different lengths of developed tracks observed respectively in amorphous and crystalline films arise from a different threshold for registration between the two types of plastic. (author)