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The Helmholtz-Center Dresden - Rossendorf operates a superconducting electron linear accelerator as a driver for secondary radiation sources, which include two IR-FEL, a broadband high-field THz radiation source, high-energy X-rays, neutrons and positrons. The accelerator runs in continuous-wave (CW) mode and in a 24/7 regime serving an international user community. Electron-bremsstrahlung is being converted into an intense beam of positrons by means of pair production. After moderation to thermal energies, positrons are re-accelerated to form a mono-energetic positron beam with variable kinetic energies ranging from 0.5 to 18 keV for depth profiling of atomic defects and porosities on nm-scales in thin films. High timing resolutions ( < 100 ps) at high average rates (105 s-1) and adjustable beam repetition rates allow performing high-throughput experiments of positron annihilation lifetimes. The accelerator-based positron source is complemented by a several radioisotope-driven setups for conventional annihilation lifetime measurements (defect characterizations) and Doppler-broadening spectroscopy, which is sensitive to the defect’s chemical surroundings. In my presentation, I will highlight some of the unique features of the experimental facilities and I will show various experimental results obtained with positrons in defect characterizations of materials and porosimetry due to their sensitivity on open-volume defects ranging from sub-nm to µm scales.

The Helmholtz-Center Dresden - Rossendorf operates a superconducting electron linear accelerator as a driver for secondary radiation sources, which include two IR-FEL, a broadband high-field THz radiation source, high-energy X-rays, neutrons and positrons. The accelerator runs in continuous-wave (CW) mode and in a 24/7 regime serving an international user community. Electron-bremsstrahlung is being converted into an intense beam of positrons by means of pair production. After moderation to thermal energies, positrons are re-accelerated to form a mono-energetic positron beam with variable kinetic energies ranging from 0.5 to 18 keV for depth profiling of atomic defects and porosities on nm-scales in thin films. High timing resolutions ( < 100 ps) at high average rates (105 s-1) and adjustable beam repetition rates allow performing high-throughput experiments of positron annihilation lifetimes. The accelerator-based positron source is complemented by a several radioisotope-driven setups for conventional annihilation lifetime measurements (defect characterizations) and Doppler-broadening spectroscopy, which is sensitive to the defect’s chemical surroundings. In my presentation, I will highlight some of the unique features of the experimental facilities and I will show various experimental results obtained with positrons in defect characterizations of materials and porosimetry due to their sensitivity on open-volume defects ranging from sub-nm to µm scales.

The application of a magnetic field to the melt in the transverse direction of the wheel rotation during the solidification process induces magnetic anisotropy in amorphous magnetic ribbons. This procedure is here applied to a FeB amorphous alloy in order to check their influence on local electronic and nanoscale structure by comparing two samples (as quenched without field and field quenched). The existence of magnetic domains was showed via the Bitter technique. Mössbauer spectra analysis confirms an increase in the spin density (from 28 to 48%) in the applied field direction for the field quenched ribbon. Positron lifetime spectra analysis determines that at room temperature there is no influence on the nanoscale structure, but the annealing at 300 °C provokes the beginning of the nanocrystallization process in the field quenched sample while the samples as quenched without field remain fully amorphous., Peer Reviewed, Postprint (published version)

First experiments and gained knowledge using and improving a multi-detector setup for Positron Lifetime Spectroscopy are presented.

First experiments and gained knowledge using and improving a multi-detector setup for Positron Lifetime Spectroscopy are presented.

A positron lifetime spectrometer for defect studies in bulk, with time resolution of 185 +-7 ps, is described. Its reliability, tested on 4N Fe samples in various states, is demonstrated. For comparison, the same samples are studied also by Slow Positron Implantation Spectroscopy.

Positron lifetime technique and photoluminescence (PL) were employed to study the vacancy type defects in p-type Zn-doped and undoped GaSb samples. In the positron lifetime study, Ga vacancy related defect was identified in these materials and it was found to anneal out at temperature of about 350 deg C. For the PL measurement on the as-grown undoped sample performed at 10 K, a transition peak having a photon energy of about 777 meV was observed. This transition peak was observed to disappear after a 400 deg C annealing. Our results is consistent wide the general belief that the 777 meV transition is related to the V(sub Ga)Ga(sub sb) defect, which is the proposed residual acceptor of GaSb., Pub in: Materials Research Society Symposium Proceedings, Volume 692. This article is from ADA405047 Progress in Semiconductor Materials for Optoelectronic Applications Symposium held in Boston, Massachusetts on November 26-29, 2001.

SCOPUS: cp.k, info:eu-repo/semantics/published

SCOPUS: cp.k, info:eu-repo/semantics/published

SCOPUS: cp.k, info:eu-repo/semantics/published

SCOPUS: cp.k, info:eu-repo/semantics/published

Microstructural changes in polyetherether-ketone (PEEK) films have been investigated in the temperature range of 23 deg C to -196 deg C by using positron lifetime spectroscopy. It has been determined that the total free volume decreases by about 46 percent in amorphous PEEK samples and about 36 percent in semicrystalline PEEK samples when they are cooled from room temperature to the temperature of liquid nitrogen. If this trend in reduction of free volume with decreasing temperature continues, as expected, PEEK should be able to withstand cooling to the temperature of liquid hydrogen without any detrimental effect on its diffusivity for liquid hydrogen.