Relationship between Enhanced Positronium Formation and Enhanced Positron Trapping in Polymers at Low Temperature

J. Kansya and T. Suzukib aInstitute of Material Science, Silesian University, Bankowa 12, 40-007 Katowice, Poland bHigh Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan

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Received: 5 06 2006;

Positron annihilation lifetime spectroscopy is used for investigation of low-density polyethylene and ethylene-methyl methacrylate copolymers of 1.45, 3.0, and 5.4 mole % of methyl methacrylate. The lifetime spectra are collected at 30 K, one by one, as a function of elapsed time. In the computer analysis a new theoretical model is developed, which enables separating the annihilation from positron free state, its trapped state and bound state in positronium. The positron trapping rate μ and the enhanced positronium formation rate κ are determined. The calculated values of μ and κ turned out to be linearly correlated. This correlation presumably originates from an influence of trapped electrons on the trapping of positrons. The dependences of κ on measurement time are determined for low-density polyethylene and ethylene-methyl methacrylate of different methyl methacrylate content. A theoretical model describing quantitatively the dependences is proposed. The model considers the processes of electron-ion recombination, electron trapping, and electron scavenging by dipolar carbonyl groups supplied by methyl methacrylate additives.

DOI: 10.12693/APhysPolA.110.593 PACS numbers: 78.70.Bj, 71.60.+z, 61.80.Fe