American Journal of Scientific ResearchISSN 1450-223X Issue 52 (2012), pp. 33-41© EuroJournals Publishing, Inc. 2012http://www.eurojournals.com/ajsr.htm------------------------------------------- Free Volume Properties of Beta- Irradiated High Density PolyEthylene (HDPE) Studied by Positron Method         Khalid S. Jassim Department of Physics, College of Education/Ibn-HayyanUniversity of Babylon, P.O.Box 4, Hilla-Babel, IraqE-mail: Khalid_ik74@yahoo.com Ali A. AbdullahDepartment of Physics, College of ScienceBaghdad University, Baghdad, IraqA. A. AL-BayatiDepartment of Physics, College of ScienceBaghdad University, Baghdad, Iraq   Abstact:Positron Annihilation Lifetime (PAL) technique has been employed to investigate the effect of low doses of ?-irradiation (in air and vacuum) on the type of polymers namely: HDPE. A conventional fast-slow coincidence system was used for measuring the positron lifetime spectra. The time resolution of the system was 422±20 ps (the full width at half maximum (FWHM)) at 22Na energy window. The lifetime spectra were resolvedinto three components by using PFPOSFIT program. The irradiation performed in air (in the presence of oxygen) and under vacuum with ?-rays by using (90Sr/90Y) source at room temperature for ten different total dose, ranging from (0.3-12.3) Gy . The o-Ps lifetime parameters were used to determine the microstructure properties, namely: free volume hole size (Vh) and free volume hole fraction (Fh). The parameters were measured as a function of  beta-irradiation dose. The results of  beta-irradiation show that, the influence of  Beta-irradiation on o-Ps lifetime, hence the Vh and Fh values for samples irradiated in air are less than their values for samples irradiated under vacuum, due to the effect of peroxy and hydroperoxideradicals formed in the presence of oxygen. The results show that cross-linking process is the predominant process as a result of ?-irradiation PS. Keywords: Positron, Free volume, Oxygen effect, Polymer.Link of paper: http://www.eurojournals.com/AJSR_52_04.pdf