Positron mobility in polyethylene in the 60?400 K temperature range

Brusa, R. S.; Naia, M. Duarte; Margoni, D.; Zecca, Antonio

doi:10.1007/bf01538768

Cited by 33 publications

(13 citation statements)

References 23 publications

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“…Positron diffusion constants in polymers, obtained from positron mobility measurements, vary over a wide range. 19,20 The lowest value was found for highly polar Kapton D ϩ (Kapton)ϭ2.5 nm 2 /ns and probably the highest values are found in nonpolar polymers such as PE, where D ϩ (PE)ϭ1.288ϫ10 5 nm 2 /ns at 399 K. Our fitted value of D ϩ is a reasonable value for the polar PMMA specimen. Nagai et al 21 have shown that the oxygen in poly͑ethylene-vinyl acetate͒, E/VA, acts as a trap for positrons, which causes a strong reduction of the positron diffusion coefficient with respect to PE.…”

Section: And For the Linear Parameters S P-ps S ϩ S O-psmentioning

confidence: 55%

Positronium formation in poly(methyl methacrylate)

2003

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Positron annihilation age momentum correlation ͑AMOC͒ experiments were performed in linear poly͑methyl methacrylate͒, PMMA, at 420 K and 100 K. The lifetime and Doppler broadened line-shape S(t) data were analyzed with Byakov and Stepanov's blob model for the formation of positronium. It is shown that the analysis is consistent with a delayed formation of positronium and also offers an alternative explanation for the observed broadening of the annihilation radiation line shape at short times ͑a few tens of picuseconds͒ after positron injection, the so called young age broadening. DOI: 10.1103/PhysRevB.68.132202 PACS number͑s͒: 61.80.Fe, 36.10.Dr, 78.70.Bj, 82.35.Lr The interpretation of positron annihilation data and of positronium formation in molecular liquids and solids, particularly in polymers, present some challenging riddles. Common lifetime spectra ͑LT͒ in polymers are analyzed as a composition of three exponential decay modes or components, indicated by their lifetimes 1,2,3 and their intensities I 1,2,3 . The shortest component is interpreted as due to the annihilation of parapositronium (p-Ps), the second component is due to annihilation of free positrons, and the longest one is the orthopositronium (o-Ps) component, where the intensity of the p-Ps component should be equal to one-third of the intensity of the o-Ps component. Such an interpretation is satisfactory if one is only interested in the o-Ps component and in the relation between o-Ps and the mean size of the free-volume sites. Detailed analysis may lead to a number of discrepancies, given as follows.͑1͒ The lifetime of p-Ps, which is expected to be not very different from the lifetime in vacuum, i.e., 124 ps, has values as low as 100 ps or as high as 200 ps and its fitted intensity ratio to the long component is much higher than 1:3. Most authors dismiss these discrepancies as due to a complicated mixing of p-Ps with other unidentified states.͑2͒ The intensity of o-Ps, which has been believed to be related to the density of free-volume sites, is now known to depend on the chemical composition, the radiation 1,2 and mechanical 3 history of the sample, the electric field, 4 the polarity, 5 the content of electron-paramagnetic-resonanceactive species, 6 and on bleaching by light. 7 ͑3͒ In many cases the longest component splits up into two components. A number of possible explanations have been suggested, such as ͑i͒ the existence of two o-Ps states or of bound positron-ion states, ͑ii͒ the influence of slow trapping of the positronium into the free volume, and ͑iii͒ the existence of a broad distribution of the o-Ps lifetime. 8 ͑4͒ The available AMOC ͑age momentum correlation͒ data on poly͑methyl methacrylate͒ ͑PMMA͒ 9 and PE 10 have shown clearly that it is impossible to reproduce the timedependent line-shape parameter S(t) with the standard 3-or 4-component model.In order to solve the foregoing problems it is necessary to establish a detailed model of positronium formation, particularly its evolution in time at the sub-nanosecond scale an...

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Section: And For the Linear Parameters S P-ps S ϩ S O-psmentioning

confidence: 55%

Positronium formation in poly(methyl methacrylate)

2003

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“…If a free positron leaves the blob, it diffuses through the material covering a relatively long distance (the estimated diffusion length in polyethylene is estimated to be 80 nm [12]). It annihilates from the free or trapped states or forms a positronium after meeting a weakly trapped electron.…”

Section: Theoretical Model Of the Pal Spectrummentioning

confidence: 99%

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

Kansy¹,

Suzuki

2006

Acta Phys. Pol. A

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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.

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“…Brusa et al indicated that the diffusion length of positrons was very large in polyethylene [14]. It was ca.…”

Section: Delayed Ps Formation By Free Positrons and Weakly Localized mentioning

confidence: 99%

Positronium Formation at Low Temperatures in Polymers and Other Molecular Solids

Hirade¹

2005

Acta Phys. Pol. A

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Positronium (Ps) formation by weakly localized long-lived electrons (trapped electrons or anions) and positrons at low temperatures successfully explained the Ps formation enhancement at low temperatures observed in polymers and molecular solids. This Ps formation process is different from Ore process and spur process. The phenomena expected by this Ps formation mechanism, such as an effect due to visible light exposure, a density dependence of weakly localized electrons and the delayed Ps formation were successfully observed as the experimental evidence of this Ps formation process. Some possibilities for application of this new Ps formation were also given. One is a new idea for estimation of spin polarity of positrons. Another is the possibility of application to study the local molecular motion by observing the decay of Ps formation intensity.

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Positron mobility in polyethylene in the 60?400 K temperature range

Cited by 33 publications

References 23 publications

Positronium formation in poly(methyl methacrylate)

Positronium formation in poly(methyl methacrylate)

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

Positronium Formation at Low Temperatures in Polymers and Other Molecular Solids

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