P. Hautojärvi

The effect of microvoid size on positron annihilation characteristics and residual resistivity in metals

Abstract The annihilation characteristics of a positron trapped in spherical clusters of 1., 50 vacancies in Al and Mo are calculated. The results are found to depend sensitively on the void size up to a radius of about 5 A, beyond which the size dependence disappears. The residual resistivity is shown to be nearly independent of the microvoid size during initial stages of vacancy clustering. The sizes of the microvoids in electron irradiated Mo as a function of annealing temperatures are evaluated from recent experimental positron lifetime results.

Screening of light impurities in simple metals

Abstract The Kohn-Sham density functional method is used to calculate self-consistent electron densities around H, He and Li impurities in jellium host corresponding to Li, Al and Mg metals. The differences in the density profiles for interstitial and substitutional impurities are investigated. Residual resistivities, relaxation energies and charge transfer in dilute alloys are evaluated.

Binding between positron and dislocation in aluminum

The interaction of positions and dislocations has been studied in aluminum by analyzing the effect of deformation on the angular correlation of the 2γ-annihilation radiation. The binding energy of trapped positrons is estimated by a rough model to be 0.57 ± 0.10 eV. The average decrease in valence electron density seen by positrons at dislocations is 16%. These figures are an order of magnitude larger than one expects on the basis of the electric properties of dislocations predicted by the isotropic theory of elasticity.

A positron study of age-hardenable Al-Zn-Mg alloys

Abstract Age-hardenable Al-Zn-Mg alloys have been studied by positron lifetime, Doppler broadening and angular correlation measurements. Zinc-rich Guinier-Preston zones in the Al-Zn alloy are observed to trap positrons. In the quenched alloys containing magnesium, positrons are localized at stable vacancy-type defects, which are evidently bound to Mg atoms. Some indications of positron trapping by incoherent precipitate-matrix interfaces are also shown.

Crystallization of Li2OSiO2 glass studied by positron annihilation

Abstract The crystallization of 34 Li 2 O65 SiO 2 1 P 2 O 5 glass has been studied by positron annihilation. The changes in the positron lifetime spectra have been correlated to the magnitude of the amorphous X-ray scattering. A linear relationship has been found for heat-treatment temperatures less than 700°C, above which non-linear phenomena in the lifetime spectra set in. The volume crystallinity reaches a saturation value, which varies from 70% at the heat-treatment temperature of 535°C to 80% at 700°C because of the phase separation. The dimensionality of the crystal growth in this glass is found to be n = 1.5 ± 0.1. The effect of the phase separation on the rate and dimensionality of the crystallization is discussed. The results show that positron annihilation is a sensitive and accurate method to follow crystallization in vitreous materials.

Positron as a probe for the degree of crystallization in glass

Abstract Positron lifetime and X-ray measurements have been performed in a heat-treated 34 Li 2 O65 SiO 2 1P 2 O 5 glass. The results show that changes in the lifetime spectrum can be converted into the crystallinity of glass. The degree of crystallization reaches a saturation value, which is approximately 66 per cent at 535°C.

Self-consistent electron densities in metal vacancies

Abstract The Hartree-Fock method in the density functional formalism is applied to calculate the self-consistent electron densities in metal vacancies. Comparison shows that the mere Thomas-Fermi method describes fairly well the electron density over the whole metallic range. The calculated annihilation characteristics of positions trapped at vacancies agree well with experimental results. The formation energies of vacancies are found to be sensitive to the lattice model.

Successful implementation of fast preamplifiers in a positron lifetime spectrometer

A method to improve the long-term stability of fast coincidence apparatuses, like a positron lifetime spectrometer, is reported. Ageing of the photomultiplier tubes (PMT), i.e., nonreversible degradation of the gain, can be slowed down by lowering the supply voltages over the PMTs and by compensating the lower gain with fast preamplifiers set at the anodes. The timing characteristics of the PMTs can be preserved by using voltage dividers with which the voltage in the input optics of the PMTs remains high enough for good photoelectron collection efficiency (for XP2020 above 300 V). With this setup, the anode current and the rate of gain degradation can be reduced at least by a factor of 20 with no loss in the time resolution of the spectrometer.

Residual resistivity of vacancies in simple metals

Abstract The residual resistivities of vacancies in simple metals are estimated by using the Kohn-Sham method to calculate a self-consistent scattering potential for free electrons. The results are compared to other calculations and experimental resistivities.

Pressure dependence of orthopositronium lifetime and bubble parameters in liquid helium

Abstract Orthopositronium lifetime in liquid He-4 at 4.2 K is found to decrease from 100 nsec to 50 nsec under pressures 1–60 atm. The radius and potential well-depth of the positronium bubble are estimated to vary from 14 A to 7 A and from 0.25 eV to 0.55 eV.