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Abstract: An SiO x C y H z /mixed cellulose ester (MCE) composite membrane was fabricated by plasma deposition of hexamethyldisiloxane (HMDSO) monomer on the surface of an MCE substrate. The purpose is to apply the membrane for the oxygen enrichment of an O2/N2 mixture. The variation in the fine structure of the plasma-polymerized layers (SiO x C y H z ) prepared at different plasma conditions was identified using slow-positron beams. Although the SiO x C y H z layers displayed similar FTIR spectra, they evidently showed differences based on the data from Doppler broadening energy spectroscopy (DBES) and positron annihilation lifetime spectroscopy (PALS). These data indicated distinct differences in the physical structure despite the similarities in the chemical structure of the films. The deposited layer (mixture of HMDSO species and MCE species) had more hydrocarbon group, resulting in a looser structure of the deposited layer at the initial period of plasma deposition. However, the MCE species content of the deposited layer decreased with the deposition time, and the deposited layer became denser. From PALS, the o-Ps lifetime (τ 3, corresponding to the free-volume size) decreased along the plasma deposition layer growth direction. These results corresponded with those obtained from XPS very well. The plasma-deposited membrane was applied for O2/N2 separation. The O2 permeability decreased and the concentration of O2 in the permeate increased with the deposition time and as the plasma power increased. [Copyright &y& Elsevier]

Abstract: 60Co γ-rays and positron irradiation effect on positronium (Ps) formation in the bulk and near surface region of polypropylene (PP) was studied. It is revealed that the reduction of Ps formation probability in PP bulk as a function of elapsed time may not be due to enhanced concentration of oxygen groups, and should not be ascribed to a simple accumulation of free radicals. Results indicate that Ps in PP is not formed through only one branch. That branch sensitive to the irradiation as well as the existence of sample surface seems related to relatively mobile positrons. [Copyright &y& Elsevier]

A pulsed slow-positron beam using 22Na as the positron source has been constructed for characterizing thin polymer films, coating polymer materials and their interfaces. The construction is very simple and the time resolution achieved is about 500 ps, which is sufficient for measuring long-lived positron lifetime components in thin polymer films. The positron lifetime in a 45 nm thick epoxy resin thin film held on a single-crystal silicon substrate has been measured using this pulsed slow-positron beam. Formation of o-positronium (ortho-Ps) is not observed when the incident positron energy is more than about 1 keV. The surface effect of the glass-transition temperature of a 0.3 mm thick polystyrene (PS) film has been also studied. It has been found that the glass-transition temperature of the sub-surface region of PS is lower than that of the PS bulk. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

The intense reactor-based slow positron beam (POSH) at the Delft research reactor has been coupled to a Two-Dimensional Angular Correlation of Annihilation Radiation (2D-ACAR) setup. The design is discussed with a new target chamber for the 2D-ACAR setup based on Monte Carlo simulations of the positron trajectories, beam energy distribution and beam transmission in an increasing magnetic field gradient. Numerical simulations and experiment show that when the slow positron beam with a FWHM of 11.6 mm travels in an increasing axial magnetic field created by a strong NdFeB permanent magnet, the intensity loss is negligible above ∼6 keV and a focusing factor of 5 in diameter is achieved. Monte Carlo simulations and Doppler broadening experiments in the target region show that in this configuration the 2D-ACAR setup can be used to perform depth sensitive studies of defects in thin films with a high resolution. The positron implantation energy can be varied from 0 to 25 keV before entering the non-uniform magnetic field. 2D-ACAR depth-profiling results in He-irradiated Si obtained with the new setup are presented. [Copyright &y& Elsevier]

A pulse stretcher providing a 5.2 keV quasi-continuous beam has been developed for use with a pulsed slow-positron beam generated by an electron linear accelerator operated with a repetition frequency of 50 Hz. Pulses of positrons, of width 1.2 µs, were trapped in a Penning–Malmberg trap and then gradually released downstream, stretching the pulse width to nearly 20 ms. [ABSTRACT FROM AUTHOR]