N. Djourelov

Positron annihilation in polypropylene studied by lifetime and coincidence Doppler-broadening spectroscopy

Abstract The momentum density distributions (MDDs) of electrons taking part in the annihilation processes in polypropylene (PP) have been measured by coincidence Doppler-broadening spectroscopy. MDDs at the beginning of measurements to those at the saturation level of Ps formation have been compared in order to follow the possible changes in concentration of carbonyl groups (CG). A high initial CG concentration in PP has been observed, while for antioxidant-containing PP no significant presence of CG has been detected, and no changes have been observed during positron irradiation.

Positron Annihilation in Polymers with Highly Developed Specific Surface

PAL spectroscopy and a technique for low-temperature gas sorption together with Brunauer-Emett-Teller (BET) theory (1) were used for the studies of elementary free volumes (EFV) in super-cross-linked polystyrene networks (2,3). The ranges of effective sizes observed by PAL and BET overlap partly confirming and complementing each other very effectively. I. Samples Polystyrene (PS) networks were prepared by the cross-linking of linear PS swollen in a solvent. Cross-agents of different length p-xylylene dichloride (X) and monochloro-dimethylether (E) were used After removing a solvent, the samples had extremely high specific surface up to 1600 m 2 /g

Development on a pulsed slow-positron beam: Moderator and bunching signal waveform

A pulsed slow-positron beam has been constructed for applications to polymer films. A new setup of the moderator system and a new design for the bunching signal waveform are introduced. The results show that such a kind of moderator appears to be good one, and that a new waveform for the bunching is very practical. The current pulsed slow-positron beam can be used to characterize polymer films or porous thin films.

Carbon-implanted ultra-high molecular-weight polyethylene studied by a pulsed slow-positron beam

The pulsed slow-positron beam (PSPB) technique was applied to study the structural changes in ultra-high molecular-weight polyethylene (UHMWPE) after carbon implantation, and the unimplanted material as well. A simple macroscopic model was introduced to explain the PSPB results. In the case of carbon-implanted UHMWPE, a three-layer model was used, and a very good agreement between the experimental and calculated results was demonstrated. A low-porosity layer, the region of which was in satisfactory agreement with the carbon ions stopping region, calculated by the TRIM code, was detected. An increase in the free volume and a decrease in the hole sizes were observed for a subsurface layer.

Carbon-Implanted Polyethylene Characterized by a Pulsed Slow-Positron Beam

The pulsed slow-positron beam technique was applied to study the structural changes in ultra-high molecular-weight polyethylene (PE) after carbon implantation at three implantation doses. The influence of subsequent gamma-irradiation on the structure of the carbon-implanted PE was studied. A three-layer model was used in order for the experimental results to be explained. An increase in the number of free-volume holes and a decrease in their sizes were observed for the subsurface layer. A considerable inhibition of the Ps formation in a layer next to the subsurface layer was detected. The position of the former, at the lowest carbon concentration, was in good agreement with the position of the carbon ions stopping region, calculated by the TRIM code. In the case of higher carbon concentrations significant discrepancy of the positions was observed.

Application of Coincidence Doppler-Broadening Spectroscopy to Different Carbon Phases

Coincidence Doppler-broadening spectroscopy was applied to C60 and C70 fullerenes and other carbon phases, such as nanotubes and graphites. Remarkable differences were observed in the Doppler-broadening of the positron annihilation -rays in these materials, which reflect the different densities distribution of the momentum for electrons annihilated with positrons. This would be caused by the different composition of the and electrons in annihilation with positrons. This result shows the sensitivity of the coincidence Doppler-broadening method to determine the density distribution of electrons sampled by positrons.