DETERMINATION OF FREE RADICALS CONCENTRATION IN γ - IRRADIATED POLYMETHYLMETHACRYLATE USING POSITRON ANNIHILATION LIFETIME SPECTROSCOPY

Low dose γ-irradiation effects on Polymethylmethacrylate (PMMA) polymer are studied by means of positron annihilation lifetime spectroscopy. An empirical formula is suggested to relate the ortho- positronium (o-Ps) lifetime,т3 with the molecular weight of the polymer, Mn, and the concentration of the free radicals, [X], generated in PMMA polymer due to irradiation in air. It is shown that free radicals concentration increase with the absorbed dose, and it is measurable from the results of т3. The results showed that the positron lifetime parameters illustrate a significant behavior as the dose increase. In addition, a unique behaviors of the free positron annihilation parameters, т2 and I2, are reported. т2 is found to increase as the dose increased and the formation of (free radicals-positron) compound is thought to explain this result. The value of I2 is found to reach a nearly constant (saturated) value at a threshold dose, which is shown to support the lifetime results.