Variable-energy positron annihilation lifetime and Doppler broadening of the annihilation line study of carbon-implanted polyethylene

摘要

The variable-energy positron annihilation lifetime (VEPAL) and Doppler broadening of the annihilation line (VEDBAL) techniques were applied to study structural changes in ultra-high molecular weight polyethylene after implantation with 30-keV carbon ions with fluences of 5 x 10~(14), 5 x 10~(15), 5 x 10~(16) ions/cm~2. The influence of a subsequent γ-irradiation of 30-kGy dose on the structure of the samples was also studied. For the VEPAL study, a simple macroscopic model was used to model the behavior of the ortho-positronium intensity and lifetime versus the incident positron energy. An increase in the number of free-volume holes and a decrease in their sizes were observed for a subsurface layer. A considerable inhibition of positronium formation was found in a middle layer. The determined depth position and thickness of the middle layer was in satisfactory agreement with the carbon ions stopping range calculated by the TRIM (transport of ions in matter) code only for the lowest carbon-ion implantation fluence. A shifting of the middle layer towards larger depths and layer broadening for the higher carbon-ion implantation fluences were established. The VEDBAL information was not as rich as the VEPAL data. In some cases, a determination of the closer to the surface boundary of the middle layer was even impossible.