Some effects on polymers of low-energy implanted positrons

The positron annihilation spectroscopy is a well established non-destructive tool to characterize materials and defects. Low-energy positron beams are suitable for thin-layer materials because a defined penetration depth of positrons can be obtained. This thesis is focused on two issues: 1. The median penetration depth of positrons z1/2(E), related to the positron implantation profile is assumed to be a power-law z1/2(E)= (α/ρ)En. Here ρ is the sample density and the constants α and n are empirical parameters which have been under some debate. For polymers, from positron lifetime experiments, the values α = 2.8(±0.2) µg cm−2 keV−n and n = 1.71(±0.05) have been suggested. However, these experiments were performed on several non-detached spin-coated polymers. Therefore, for the first time, positron annihilation experiments are performed on self-supporting nanometric polymer films. Also a novel method to obtain the values for α and n is proposed. 2. When bombarded by low-energy positrons, a phenomenon that appears in some metal oxides and in polymeric materials is the emission of positronium (Ps) from the sample surface. In Doppler broadening of annihilation radiation (DBAR) experiments, it has been shown that in a longitudinal setup, the para-Ps (p-Ps) contribution to the photo-peak is detected as a narrow fly-away peak at the low-energy side of the 511-keV-line. In this work, it is proven that the detected photo-peak from DBAR experiments can also be affected by the contribution of the p-Ps emission at the high-energy side (blue-shift) or at the central part of the photo-peak. Two different specimen-detector geometries are proposed: The sample located (1) at 45° and (2) perpendicular with respect to the positron beam axis and the detector located beside the sample position, but perpendicular to the positron beam line. Poly(methyl-methacrylate) (PMMA) and Kapton are analyzed. From the results, and with the values of the parameters α and n discussed in the item 1., the thermal and epithermal positron diffusion length, the efficiency for the emission of Ps by picking up an electron from the surface, and in the case of PMMA, the bulk Ps fraction and the diffusion length of p-Ps and ortho-positronium are obtained.

Author dc:creator

• Palacio Gomez, Carlos

Contributors dc:contributor

• Segers, D

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

und

Identifier

https://biblio.ugent.be/publication/469997
http://doi.org/1854/13574
https://biblio.ugent.be/publication/469997/file/4334855

OAI identifier oai:identifier

oai:archive.ugent.be:469997