Radiation Physics and Chemistry 68 (2003) 457–461 Positron annihilation studies of the effect of gamma irradiation dose in polymers I.Y. Al-Qaradawi a, *, D.T. Britton b , E.E. Abdel-Hady c , D.A. Abdulmalik a , M.A. Al-Shobaki a , E. Minani b a Department of Physics, College of Science, University of Qatar, P.O. Box 3304, Doha, Qatar b Department of Physics, University of Cape Town, Rondebosch 7701, South Africa c Department of Physics, El-Minia University, El-Minia, Egypt Abstract Positron annihilation lifetime (PAL) technique has been applied to study the effect of gamma irradiation dose on the structure of low-density polyethylene and polystyrene. The positron annihilation lifetime measurements were performed with a conventional fast–fast coincidence system. Gamma irradiation was carried out at room temperature with a 60 Co gamma source up to a dose of 1000 kGy. The delayed lifetime spectra of irradiated and unirradiated samples were decomposed into three lifetime components. The lifetime parameters give indication of the free-volume size and concentration. It was found that the crosslinking efficiency in the polymer increases significantly with increasing irradiation dose. This appears as a decrease in the intensity of the long-lived positron lifetime components attributed to ortho-positronium formation. Measurements of the Doppler broadening energy spectra of the low-density polyethylene (LDPE) samples have also been performed using a variable mono-energetic positron beam. The trend of the Doppler broadening lineshape S-parameter as a function of the radiation dose supports the lifetime results. r 2003 Elsevier Ltd. All rights reserved. Keywords: Polymers; Polyethylene; Polystyrene; Gamma irradiation; Cross-linking 1. Introduction A unique property of polymeric materials that has been well established is the existence of free-volume holes (Ferry, 1980). The free-volume hole theory (see for example Schrader and Jean, 1988) is used to explain the properties of polymers by relating free-volume para- meters to molecular properties. In the early days of polymer research there was no quantitative description of free volume, and no direct method of measuring it, as a result of its small (few angstroms) size and its existence for a very short time B10 13 s. Positron lifetime spectroscopy has been one of the techniques used to study polymers in the last two decades. It gives unique information about free-volume sizes and concentration in the polymer structure (Suzuki et al., 1992, 1995). The usefulness of positron annihila- tion studies in characterizing the physical properties of polymers is now indisputable (Pethrick, 1997; Jean, 1990). Polymers are not particularly simple or ideal amorphous materials, but positron annihilation mea- surements were found to be very sensitive to free- volume-based structural changes in their unique properties such as; glass transition temperatures, nano- scale voids, and free-volume size and distribution (Liu et al., 1995; Dale et al., 1987). Positron annihilation lifetime spectroscopy (PALS) in particular allows the study of more subtle secondary transitions, which are otherwise undetectable (Uedono et al., 1996). For this reason PALS compliments existing macroscopic polymer characterization techniques (Schmidt and Maurer, 2000). ARTICLE IN PRESS *Corresponding author. Tel.: +974-581-3023; fax: +974- 469-0779. E-mail address: ilham@qu.edu.qa (I.Y. Al-Qaradawi). 0969-806X/03/$-see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0969-806X(03)00205-6