AbstractThe principle of operation of an apparatus developed to study gas permeation through thin films is described, and the measurement method is discussed. Use is made of diffusion theory to obtain an expression for the permeability coefficient as a function of the rate of increase of the pressure in the receiving volume. The activation energy for permeation of helium through PET is determined. The permeability coefficient for helium is found to increase significantly with the range of the implanted protons although the incident charge has been kept constant. The hypothesis of structural modifications of the proton implanted PET seems to be confirmed by small‐angle X‐ray scattering experiments on the irradiated samp
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