ANOMALOUS DIFFUSION OF PB ATOMS IN POLYETHYLENE IMPLANTED WITH F+ ANDAS+ IONS TO DIFFERENT DOSES

The diffusion of Pb atoms from a 0.5M aqueous solution of lead acetate into polyethylene (PE) implanted with 150 keV F+ and As+ ions to diff erent doses was studied at room and boiling temperatures. The standard Rutherford backscattering (RBS) technique was used for the determinat ion of concentration depth profiles of incorporated atoms. Pb diffusio n in the pristine polymer proceeds according to the common Fickian law with diffusion coefficients of 1.5 x 10(-16) cm2 s-1 and D = 5.3 x 10 (-14) cm2 s-1 at room and boiling temperatures, respectively. Anomalou s diffusion of Pb atoms was observed into the PE implanted to ion dose s below 5 X 10(14) cm-2. In these cases a pronounced diffusion-capturi ng mechanism takes place with the Pb atoms being captured on trapping centres produced by electronic energy losses of implanted ions. For im planted doses above 5 x 10(14) cm-2, nearly Fickian diffusion is again observed with diffusion coefficients close to those of pristine mater ial. Strongly reduced Pb incorporation in specimens implanted to highe r doses is explained by structural changes of the polymer surface laye r.