D. Segers

Free volume in nanostructured Ni

Abstract The free volume in nanostructured Ni, synthesized by different techniques, was measured by electron microscopy and positron lifetime spectroscopy. Even in the fully dense samples 1–2 nm sized pores as well as smaller nanovoids were observed indicating that these nanovoids are an intrinsic property of the microstructure of nanostructured materials.

Hydrogen exchange with voids in tungsten observed with TDS and PA

Abstract Hydrogen trapping and dissociation phenomena in single crystalline tungsten with voids have been studied by thermal desorption spectrometry (TDS) and positron annihilation lifetime measurements (PA). The voids were created by MeV electron or proton irradiation and hydrogen decoration of the voids was performed by 2 keV hydrogen ion bombardment. A reduction of the positron lifetime in voids from 500 to 350 ps was observed during the hydrogen bombardment, indicating that hydrogen is transported to voids situated at depths > 10 μm. This implies the saturable character of the voids for hydrogen. The reduction of the positron lifetime and the activation energies derived for the release of hydrogen from voids (1.0–1.4 eV) are explained by the presence of highly pressurised molecular hydrogen inside the saturated voids. The results for tungsten are compared with results obtained by PA for molybdenum.

Annealing behaviour of reactor pressure-vessel steels studied by positron-annihilation spectroscopy, Mössbauer spectroscopy and transmission electron microscopy

The annealing behaviour of commonly used reactor pressure-vessel steels was studied using positron-annihilation spectroscopy (PAS) (lifetime and Doppler broadening techniques), transmission Mossbauer spectroscopy (MS), integral low-energy electron MS (ILEEMS) and transmission electron microscopy (TEM). The results of these methods applied to different types of steels are discussed. Some significant differences in the experimental results are observed between the Russian (3 types) and Western Europe (5 types) steels and between individual specimens within these two groups. All specimens were annealed in vacuum and studied after this thermal treatment. It was confirmed that the heat affected zone (HAZ) is the most sensitive region for thermal and neutron irradiation-induced embrittlement in the reactor. Positron-annihilation lifetime measurements on HAZ specimens annealed at successively higher temperatures show the drastic increase in the vacancy-type defect formation between 525°C and 600°C. Therefore these specimens were selected for further detailed studies by TEM.

Evolution of the positron annihilation lifetime for ageing in β phase Cu-Al-Ni-(Ti)-(Mn) shape memory alloys

During the last years, the positron annihilation technique has proven to be a sensitive tool for the characterization of phase transitions, and in particular of the martensitic transformation in Cu-based alloys. The differences in structure between the high temperature phase (cubic) and the martensite (monoclinic) produce distinctive positron annihilation characteristics which allow the determination of the transformation temperature range. In the present article the influence of these parameters on the positron annihilation characteristics of three Cu-Al-Ni shape memory alloys is discussed. Two of the chosen compositions contain Ti and Mn, which have been added to the ternary Cu-Al-Ni alloy in order to improve its mechanical properties. The effect of those elements has also been evaluated.

Positron annihilation study of nanocrystalline iron.

Abstract Positron lifetime measurements on different nanocrystalline iron samples show that the microstructure, with respect to the inherent free volumes in the samples, can be determined. Three distinct defect types can be detected: i) a vacancy-like defect in the interface, ii) a microvoid at the intersection of interfaces and iii) larger free volumes (missing grains) where the annihilation of ortho-positronium is observed. The effect of the crystallite size and preparation conditions of the samples are discussed.

Temperature characteristics of positron trapping at defects in electron-irradiated silicon

Positron-annihilation lifetime and Doppler-broadening measurements are used to investigate defects in silicon irradiated at 373 K with 6 MeV electrons to a dose of 1×l019e/cm2. In the unirradiated silicon sample (p type) a temperature-independent behaviour of the bulk-lifetime is observed in the temperature interval 110–500 K with a constant value of 220±1 ps. The slight effect observed on the S-parameter evolution is explained taking into account the thermal expansion of the lattice. The lifetime results obtained at 80 K and at 300 K after isochronal annealing as well as the behaviour of the intensity of the second lifetime componentI2 during lifetime measurements below the irradiation temperature in the irradiated silicon sample (n type), clearly indicate the temperature dependent characteristics of the positron trapping cross section σt(T) ∝Tn withn= −1.905±0.016. From isochronal annealing results, an annealing stage is observed in which di-vacancies agglomerate into quadri-vacancies. The mean positron lifetime in those quadri-vacancies is 350 ps.

The analysis of non-exponential decay in relation to positronium thermalization

A model function for the lifetime analysis of slowly thermalising ortho-positronium is presented. Simulated spectra are analysed for a wide range of parameters and of statistical contents. A comparison with standard two-component analysis shows the power of the new model for specific problems. Experimental support in alumina fine particles is presented.

Measurement of the activation enthalpy for the recovery of low-temperature deformed indium as determined by positron annihilation

Abstract The positron annihilation lineshape was measured in low-temperature ( T = 77 K) deformed indium in the temperature range between 80 and 400 K. A recovery stage was found at 110 K. After isothermal annealing of the deformed indium at 100, 103, 105, 108 and 110 K, an activation enthalpy of 0.36 ± 0.01 eV was obtained.

BEAM HANDLING WITH A PENNING TRAP OF A LINAC-BASED SLOW POSITRON BEAM.

Abstract A Penning trap was constructed at the Ghent LINAC-based slow positron beam. It allows us to store the positrons without considerable loss in intensity and to create a quasi-continuous beam so that positron Doppler broadening measurements are possible. Starting from the stored positrons it is also possible to create a pulsed beam with pulse width of the order of 100 ns so that time-of-flight measurements are possible.

Trapping of positronium in a size-dependent spherical square well potential

Abstract The real trapping potential energy U ( r ) of positronium in vacancy-type holes and self-sustaining bubbles in liquids is replaced by a spherical square well potential of finite energy depth U m and the scaling procedure proposed by Yu et al. is applied. Available data for the ortho -positronium lifetime in molecular crystals and in liquids are re-examined and a unique relation is found between the potential energy depth U m and the calculated radius R of the trapping site. The general form of a surface-enhanced real potential energy U ( r ) which is compatible with the behaviour of U m is proposed.