T. van Hoecke

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.

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.

Some aspects of the interactions of orthopositronium with perfect and defect surfaces of insulating materials

ETLA (Elastic Thermalisation Lifetime Analysis) is a new model function for the fitting of lifetime spectra of slowly thermalising orthopositronium in the free space between the grains of insulating oxide powders. From this model one can obtain three fundamental parameters; the mass of the cluster of atoms which scatters the o-Ps, the probability μ that the o-Ps is forced into two-quantum decay at each collision and the emission energyE(0) of the o-Ps from the powder surface. As a consequence of full thermalisation of the Ps it is shown that the o-Ps quenching constant λq(℞) follows atT1/2 dependency for MgO, Al2O3 and SiO2 and for temperatures between 77 and 500 K. In MgO powder at low temperatures the irradiation by the positrons induces surface paramagnetic defects which are identified as Mg+ sites. These surface defects do not affect the thermalisation, but they induce paramagnetic o-p conversion i.e., an increase in μ, and they are also responsible for an increase in the emission energyE(0) of the o-Ps.

Defects in neutron transmutation doped silicon studied by positron annihilation lifetime measurements

Neutron transmutation doping of n-type silicon was obtained by irradiating the samples with thermal neutrons in order to create P atoms. Positron lifetime measurements were carried out and the evolution of the induced defects was studied by thermal annealing of the samples. The annealing out of the divacancy was seen between 200 and 350 °C. Around 700 °C large vacancy clusters (containing about eight vacancies) are formed. All the defects are annealed above 800 °C.

Investigation of defects and stresses in SiFe steel surfaces

Abstract SiFe specimens prepared from GO steel with different surface treatments were studied from the point of view of structure, stresses and defect density using the conversion electron Mossbauer spectroscopy (CEMS) and the slow positron annihilation spectroscopy (SPAS). Amounts of the high defect density (HDD) phase and defect depth profiles were obtained for specimens after mechanical and/or chemical surface polishing.

Thermalisation of Positronium in Polymers

A new model is presented which describes the annihilation of positronium (Ps) in polymers. Ps is formed in the bulk and is trapped into the free volume holes with epithermal initial energy and it thermalises towards its ground state during its lifespan through elastic collisions with the inner walls, This thermalisation is characterised by an asymptotic Ps-decay constant lambda(po)(infinity), an energy ratio parameter xi and by a relaxation parameter, from which the free volume hole size R, the effective recoiling mass M, and the quenching probability per collision mu can be obtained, We have verified this model on a set of cross-linked PMMA samples.

Slow positron studies of anodized aluminium

Porous anodic layers were grown on aluminium alloys under different anodizing conditions. Positron annihilation energy spectra were measured as a function of the positron energy. Ortho-positronium formation in the porous layer was detected. The positron characteristics could be correlated to known parameters of the layers. This demonstrates that the positron annihilation method is a particularly useful technique to characterise the anodic layers.

A low cost time-energy correlation apparatus for positronium annihilation

A two parameter correlation system to measure the energy of a thermalizing ortho-positronium as function of its age was constructed. The Doppler broadening of the 2 γ annihilation line due the quenching ofo-Ps is measured in coincidence with the age of the positronium. We use only one high resolution nuclear ADC for the energy spectrum, together with a self build low resolution ADC for the timing information and an integrated logical unit. In this way, AMOC (Age Momentum Correlation) becomes possible with small extra expenses.

Positron Doppler broadening measurements with a LINAC-based slow positron beam

All the LINAC-based slow positron beams are pulsed. In order to perform Doppler broadening measurements one needs a continuous distribution of the slow positron intensity. This intensity has to be kept as constant as possible. A smearing out of the positron intensity with a Penning trap is described. The first depth profiling results are presented.