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We present experiments on glucose-gelatin compounds using positron annihilation lifetime spectroscopy (PALS) in order to study the behavior of the free volume dependent on H2O loading, drying, and uniaxial pressure. A semiempirical quantum mechanical model was applied in order to correlate the lifetime of orthopositronium in nanoscaled voids to the void size. This allowed us to determine the absolute value of the mean void radius in the biopolymer samples. In addition, the variation of the total free volume of the differently treated samples is quantified and illustrated by a log-normal distribution function. Most interesting results have been obtained after saturation loading with H2O that leads to the formation of voids with a mean size of 84.3(1.9) Å(3) and to an increase of the total free volume by a factor of 2.5. This observation in the swelled sample is explained by the entropy elastic regime well above the glass transition temperature that greatly facilitates the formation of free volume. Differential scanning calorimetry (DSC) measurements were performed in order to determine the glass transition temperature and to support the interpretation of the results obtained by PALS.

Humidity control and water management in polymer electrolyte membranes for fuel cells are still of high importance to improve the fuel cells' efficiencies. In this study, poly (vinyl alcohol) (PVA)was crosslinked using 15 wt% sulfosuccinic acid (SSA) by a solution casting method and additionally thermally crosslinked at 100 °C. Positron annihilation lifetime spectroscopy was used to study the mean free volume size and the distribution at different humidity. A slight decrease in the free volume was found up to a relative humidity of 30% whereas it increases strongly for a relative humidity of more than 30%. The volume of the voids duplicates from 0.036 to 0.078 nm³ by changing the relative humidity from 30 to 80%. Thermogravimetric analyzer was used to determine the thermal stability of the membrane. From thermogravimetric analyzer data, it was found that the PVA with 15 wt% SSA membrane are chemically stable up to 200 °C. [ABSTRACT FROM AUTHOR]

Abstract: Recent results from depth dependent (C)DB measurements on thin metallic films using the high intense monoener-getic positron beam NEPOMUC are presented. Vapor-deposited films of gold and copper were characterized by depth dependent DB-spectroscopy. The positron diffusion length in thin films was determined by the Doppler broadening of the annihilation radiation and additionally the formation of positronium (Ps) at the surface. Furthermore, the depth dependent DB-and CDB-measurements were compared. During high temperature measurements, the annealing of a thin gold film was observed. [Copyright &y& Elsevier]

Abstract: The NEutron-induced POsitron source MUniCh (NEPOMUC) at the research reactor FRM II delivers a low-energy positron beam (E =15–1000eV) of high intensity in the range between 4×107 and 5×108 moderated positrons per second. At present four experimental facilities are in operation at NEPOMUC: a coincident Doppler-broadening spectrometer (CDBS) for defect spectroscopy and investigations of the chemical vicinity of defects, a positron annihilation-induced Auger-electron spectrometer (PAES) for surface studies and an apparatus for the production of the negatively charged positronium ion Ps−. Recently, the pulsed low-energy positron system (PLEPS) has been connected to the NEPOMUC beam line, and first positron lifetime spectra were recorded within short measurement times. A positron remoderation unit which is operated with a tungsten single crystal in back reflection geometry has been implemented in order to improve the beam brilliance. An overview of NEPOMUC''s status, experimental results and recent developments at the running spectrometers are presented. [Copyright &y& Elsevier]

Abstract: A spatially resolving coincidence Doppler spectrometer has been developed at the intense positron beam NEPOMUC at the nuclear research reactor FRMII in Garching near Munich. The positrons are focused onto the sample which is positioned in a vacuum chamber in order to achieve lateral resolution. The emitted annihilation radiation is detected by two facing germanium detectors in coincidence in order to enable background-free measurements. Since the measured spectra are not degraded by background, the high momentum region of the annihilation line can be precisely analysed. The positron beam structure is described and first measurements are presented in which thermally treated brass was investigated in order to determine the chemical environment of the positron at the annihilation site. [Copyright &y& Elsevier]

Abstract: In summer 2004 the NEutron-induced POsitron source MUniCh—NEPOMUC—of the new Munich research reactor FRM-II was set into operation at the nominal reactor power of 20MW. With this novel facility one of the worlds most intense low-energy positron beam is now available. Intensity and positron beam profile measurements were performed at 1 and 30eV, respectively. Within the first reactor cycle a yield of up to moderated positrons per second was achieved. In the present arrangement of NEPOMUC''s instrumentation the monoenergetic positron beam is magnetically guided to a coincident Doppler broadening (CDB) facility and to a positron-induced Auger electron spectroscopy (PAES) analysis chamber. [Copyright &y& Elsevier]

Abstract: The positronium formation is studied in the near surface region of Polymethylmethacrylate (PMMA), which was treated with isopropanol and acetone by energy dependent Doppler-broadening spectroscopy. In addition, the variation of Ps-formation was investigated at aluminium-coated PMMA. The coincident DB (CDB) spectra reveal the fraction of positrons, which annihilate in the aluminium. All measurements were performed with mono-energetic positrons up to an energy of 31keV at the CDB-spectrometer at the new positron beam facility NEPOMUC at FRM II. [Copyright &y& Elsevier]

Abstract: Positron Annihilation Spectroscopy (PAS) performed with continuous and pulsed positron beams allows to characterize the size of the intrinsic nano-voids in silica glass, their in depth modification after ion implantation and their decoration by implanted ions. Three complementary PAS techniques, lifetime spectroscopy (LS), Doppler broadening spectroscopy (DBS) and coincidence Doppler broadening spectroscopy (CDBS) will be illustrated by presenting, as a case study, measurements obtained on virgin and gold implanted silica glass. [ABSTRACT FROM AUTHOR]