Positron Lifetime Component Research Articles

Positron lifetimes of polycrystalline metals: A positron source correction study

Positron lifetime measurements have been made on a series of polycrystalline metals with atomic numbers varying from 13 to 82, using positron sources deposited on Ni, Al, and Kapton support foils. Bulk positron lifetimes for Al, Ni, Zr, In, and Pb are reported. The results support previous assignments of monovacancy lifetimes for Ag, Ta, and Pb. A method for obtaining the extrinsic positron lifetime components due to the radionuclide source is developed, and the experimentally determined source lifetime components are studied as a function of the sample atomic number. The Z dependence of the source foil component intensities were fitted using known expressions for positron backscatter and multiple scattering within the source sample geometry, neglecting backscatter from the foil. The foil material positron absorption coefficients were obtained and found to be comparable with the results of previous transmission experiments.

Neutron Irradiated Copper: Is the Main Positron Lifetime Component due to Stacking Fault Tetrahedra?

Neutron Irradiated Copper: Is the Main Positron Lifetime Component due to Stacking Fault Tetrahedra?

Pulsed slow-positron beam for polymer films

A pulsed slow-positron beam using 22 Na as the positron source has been constructed for characterizing thin polymer films, coating polymer materials and their interfaces. The construction is very simple and the time resolution achieved is about 500 ps, which is sufficient for measuring long-lived positron lifetime components in thin polymer films. The positron lifetime in a 45 nm thick epoxy resin thin film held on a single-crystal silicon substrate has been measured using this pulsed slow-positron beam. Formation of o-positronium ( ortho-Ps) is not observed when the incident positron energy is more than about 1 keV. The surface effect of the glass-transition temperature of a 0.3 mm thick polystyrene (PS) film has been also studied. It has been found that the glass-transition temperature of the sub-surface region of PS is lower than that of the PS bulk.

Positron annihilation studies of the effect of gamma irradiation dose in polymers

Positron annihilation lifetime (PAL) technique has been applied to study the effect of gamma irradiation dose on the structure of low-density polyethylene and polystyrene. The positron annihilation lifetime measurements were performed with a conventional fast–fast coincidence system. Gamma irradiation was carried out at room temperature with a 60Co gamma source up to a dose of 1000 kGy. The delayed lifetime spectra of irradiated and unirradiated samples were decomposed into three lifetime components. The lifetime parameters give indication of the free-volume size and concentration. It was found that the crosslinking efficiency in the polymer increases significantly with increasing irradiation dose. This appears as a decrease in the intensity of the long-lived positron lifetime components attributed to ortho-positronium formation. Measurements of the Doppler broadening energy spectra of the low-density polyethylene (LDPE) samples have also been performed using a variable mono-energetic positron beam. The trend of the Doppler broadening lineshape S-parameter as a function of the radiation dose supports the lifetime results.

Characterization of free volumes in amorphous and nanostructured Pr2Fe14B using positron lifetime spectroscopy

Amorphous Pr2Fe14B was produced by the melt-spinning technique and subsequent nanocrystallization was carried out by isothermal annealing. The free volumes in amorphous and in nanocrystalline Pr2Fe14B have been characterized using positron lifetime spectroscopy. The two primary lifetime components were ascribed to vacancy-type defects in the intergranular layers or the interfaces of the crystallites and to large free volumes of the size of several missing atoms, e.g., in triple junctions. The increase of the intensity ratio I1/I2 of the positron lifetime components with increasing crystallite size confirms the picture that the positrons are annihilated in the free volumes of grain boundaries and that of triple junctions.

Positron annihilation and ion-conductivity studies of PEO–NH 4ClO 4 complexes

We present positron annihilation lifetime spectroscopy (PALS) results on the polymer electrolyte [PEO](1− x)–[NH 4ClO 4]( x), for different x, denoting weight fraction and their correlation to other properties. Variation of composition through change in x causes the ion conductivity and crystallinity to vary. We find that the changes are well reflected in variation of the positron annihilation lifetime components and their intensities. Although three distinct positron lifetime components have been identified, most of the earlier work considered only the longest component arising out of the noncrystalline regions in their explanation of ion-conductivity variation with temperature. Our room temperature study shows a new correlation. The average and bulk lifetimes, calculated from the two shorter lifetime components, is less at compositions with higher ion conductivity, suggesting some contribution of the crystalline regions to conductivity.

Gallium vacancy and the residual acceptor in undoped GaSb studied by positron lifetime spectroscopy and photoluminescence

Positron lifetime, photoluminescence (PL), and Hall measurements were performed to study undoped p-type gallium antimonide materials. A 314 ps positron lifetime component was attributed to Ga vacancy (VGa) related defect. Isochronal annealing studies showed at 300 °C annealing, the 314 ps positron lifetime component and the two observed PL signals (777 and 797 meV) disappeared, which gave clear and strong evidence for their correlation. However, the hole concentration (∼2×1017 cm−3) was observed to be independent of the annealing temperature. Although the residual acceptor is generally related to the VGa defect, at least for cases with annealing temperatures above 300 °C, VGa is not the acceptor responsible for the p-type conduction.

Hole carrier localization and Pr substitution in YBCO systems by positron experiment

Pr-substituted YBCO system has been studied by positron lifetime experiment in the range from 0.0 to 1.0. The present results show that the value of short lifetime τ 1 in PrBa 2Cu 3O 7− δ is nearly the same as in the isostructural YBa 2Cu 3O 7− δ and the defect-related positron lifetime component τ 2 decreases as a function of Pr-substitution x. The increase of local electron density n e reflects the electronic intense localization. These experimental results are interpreted in terms of the localization of hole carriers in the Cu–O chains on Pr substitution due to disorder in the Ba and Pr planes by the partly occupation of Ba 2+ ion site with Pr 3+ ion. These will destroy the intrinsic conductivity of the Cu–O chains and so impresses the superconductivity above x=0.6 Pr-substitution concentration.

Unconventional features of oxide HTSC from positron annihilation measurements

Our recent measurements of positron lifetime (related to electron density at the annihilating sites) components or τ i and Doppler broadening of positron annihilation radiation lineshape or S-parameter (denoting the fraction of low-momentum electrons among the annihilating electrons) on Bi-2212 and (Bi,Pb)-2223 pellets and Y-123 single crystals firstly confirm step-like changes across T c. Our second and new observation is that S versus T graph in the superconducting transition region shows a double minimum for 2223 and 123 and a single minimum for 2212. These steps and minima, not seen for superconductors like Nb 3Sn, are linked to such features of HTSC electronic structure as are not found in conventional superconductors.

Atomic defects in hexagonal tungsten carbide studied by positron annihilation

Vacancies on the two sublattices of hexagonal tungsten carbide were identified by means of positron lifetime studies after irradiation with electron of various energies. In the as-prepared state two predominant positron lifetime components occur indicating positron annihilation in the free state with a lifetime ${\ensuremath{\tau}}_{f}=124\ifmmode\pm\else\textpm\fi{}10$ ps according to the simple trapping model and positron trapping and annihilation in metal vacancies $({\ensuremath{\tau}}_{{V}_{W}}=175\ifmmode\pm\else\textpm\fi{}20$ ps) of low concentration. By means of low-energy electron irradiation at 1 MeV with an electron dose of $1.8\ifmmode\times\else\texttimes\fi{}{10}^{23} {\mathrm{m}}^{\ensuremath{-}2}$ exclusively carbon vacancies are introduced giving rise to saturation trapping and annihilation of positrons at carbon vacancies with a low lifetime of ${\ensuremath{\tau}}_{{V}_{C}}=136\ifmmode\pm\else\textpm\fi{}3$ ps. The isochronal annealing of irradiation induced vacancies reveals two annealing stages at 800 and 1200 K, which are ascribed to the annealing of carbon vacancies and the dissolving of small carbon vacancy clusters.

Helium implanted ZrHf as studied by time differential perturbed angular correlation and positron lifetime measurements

Time differential perturbed angular correlation (TDPAC) at the 482 keV state of 181Ta and positron lifetime measurements were performed on homogeneously helium implanted ZrHf samples. TDPAC on the as-implanted sample shows the association of 0.12±0.02 fraction of probe nuclei with stacking faults experiencing a local fcc environment. The dissociation of this defect complex from probe nuclei is observed following the annealing treatment at 423 K. TDPAC measurements lead to the conclusion that Hf impurities do not bind He-vacancy complexes or helium bubbles in ZrHf. A recovery stage associated with helium migration is identified from the variation of the relative width of the Lorentzian distribution of the quadrupole frequency in TDPAC measurements. This correlates well with the variation of the helium associated positron lifetime component. The nucleation and growth stages of He bubbles in the sample have been identified by positron lifetime measurements as a function of the annealing temperature on the irradiated sample. Based on the analysis of positron lifetime measurements, helium bubble parameters such as bubble radius and bubble concentration have been extracted in the growth stage, leading to a quantitative understanding of the bubble growth in ZrHf.

Positron annihilation measurements of Pr-containing superconductor compounds

Positron annihilation lifetime and Doppler broadening energy spectra have been measured in Pr-containing YBa2Cu3O7−δ as a function of temperature between 18 and 295 K. The defect-related positron lifetime component τ2 was independent of temperature for the PrBa2Cu3O7−δ compound and was very close to the value τ1 associated with bulk or defect-free material. These results indicate that the structure of this compound is nearly free of defects which trap positrons and that there is no electronic structure change during cooling. There was a slight temperature dependence below Tc for the Y0.5Pr0.5Ba2Cu3O7−δ compound. It was also found that a change in the positron Doppler line-shape parameter S occurred at the superconducting transition temperature for 0.5 Pr-containing compound but not for the 1.0 Pr-containing compound. These results show that the average electron momentum at the annihilation sites increases as temperature is lowered across the superconducting transition range for the 0.5 Pr-containing compound but not for the PrBa2Cu3O7−δ compound.

Phase boundary structure of γ′-particles in Cu-10 at.% Be

The interphase boundaries between matrix and γ′-precipitates and the particle geometry are investigated by positron annihilation and small angle X-ray scattering. Full coherence for at least two pairs of boundary planes and an absence of coarsening are found. Particle dimensions of 80, 260 and 600 Å occur in the matrix directions 〈113〉, 〈110〉 and 〈332〉. Positron affinities of the precipitate material are calculated. A second positron lifetime component with τ = 137 ps is ascribed to trapping at structural misfit dislocations (associated with ledges) with Burgers vectors of a/12 〈112〉 containing no jogs. A gradual change of concentration caused by elastic interaction seems to occur at the matrix—particle interface in a region of about 10 Å thickness reducing the elastic strain.

Ortho-positronium lifetime studies of free volume in polycarbonates of different structures: influence of hole size distributions

Certain anomalies were obsd. in computer fitting of data from positron annihilations in polymers. These suggest that some reported ortho-positronium lifetimes and intensities in these polymers could be artifacts of the computer-fitting procedure. To evaluate this hypothesis, a computer simulation of exptl. data was developed, which can then be used to test the accuracy of the fitting program. The input to this simulation consists of the lifetimes and intensities of any no. of positron populations (including para-positronium and free positron decays), plus the spectrometer resoln. function, a contribution from annihilation in the positron source, and random background. The simulation uses the computer's random no. generator to make the output spectrum resemble an actual exptl. curve. The output spectrum is then used as input to the usual fitting program POSFIT, which dets. the best-fitting values of lifetime and intensity for three positron lifetime components. When the shortest lifetime, t1, was fixed at the theor. value of 120 ps, the values of the other lifetimes, t2 and t3, were very close to the values in the simulated input. [on SciFinder (R)]

Preparation and positron lifetime spectroscopy of nanocrystalline metals

Nanocrystalline metals prepared by evaporation or sputtering with crytallite condensation and compaction were studied by means of positron lifetime spectroscopy in order to obtain information on structural free volumes in the crystallite interfaces. In a number of bcc- and fcc-type nanocrystalline metals two dominant positron lifetime components were observed, which are attributed to positron trapping and annihilation in interfacial free volumes of the mean size of about one lattice vacancy or of about 10 – 15 lattice vacancies, and which are considered to represent two maxima of a size distribution of free volumes. Positron lifetime measurements on nanocrystalline Pd under quasi-hydrostatic pressure up to 4 GPa indicate a high compressibility of the vacancy-type structural free volumes and, therefore, support the evidence for a soft interfacial structure as derived from previous studies of mechanical and thermal properties.

Positron lifetime studies of defects in MBE-grown silicon

A timed positron beam has been used to study defects in MBE-grown silicon layers previously studied by conventional slow-positron techniques and electron microscopy. An expected void-related positron lifetime component has been observed, but at a much lower intensity than expected from the Doppler-broadening results. The implication is therefore that the momentum of the annihilation positron-electron pair at the void is considerably lower than previously assumed. In the lifetime spectra, there is evidence of a trapped positron state in the overlayers with a lifetime similar to that of a positron freely diffusing in silicon. The nature of this defect is, at present, unclear.

Positron annihilation study of the mechanochemical reaction between Zn + Se and Zn + S

Positron lifetime spectra were recorded and evaluated in mixtures of Zn + S and Zn + Se ground for various periods. The intensity of the long-lived positron lifetime component is shown to increase with grinding time until a sudden decrease takes place at a specific grinding time thereby indicating the onset of the effective chemical reaction. The suitability of positron annihilation for investigating mechanochemical reactions is clearly demonstrated.

Positron lifetimes in lithium disilicate glass with different degrees of crystallization

Positron annihilation has been studied in samples of lithium disilicate glass heat treated to produce different degrees of internal crystallization. It has been found that the intensity of the longer positron lifetime component (I2) decreases with percentage crystallinity. This decrease can be correlated linearly with the reduction in free volume on crystallization in this system. A decrease in the lifetime, tau 2, with crystallinity has also been observed.

Positrons in metals with voids, vacancies and surfaces

The behaviour of positrons near monovacancies and voids in a metal is discussed in terms of three contributions to the positron work function for the metal, namely the positron zero-point energy, the positron-electron correlation energy and the surface dipole barrier. The image potential when a positron comes near to a metal surface leads to a deep potential ‘trough’ just outside an exterior metal surface or just inside a void surface. Calculations indicate positron bound states localized at metal surfaces for most metals which should be manifested in a long lived positron lifetime component. The behaviour of positronium near metal surfaces is also discussed.

Effect of Intermolecular Forces on the Annihilation of Positrons in a Nematic Liquid Crystal

Lifetime spectra have been measured for positrons annihilating in the liquid-crystal $p$-azoxyanisole. Four lifetime components appear when positrons annihilate in either the nematic mesophase or in the isotropic liquid phase, but in solid $p$-azoxyanisole the longest-lived component is absent. No abrupt changes in either lifetimes or intensities are observed across the nematic-isotropic transition; however, the solid-nematic transition produces changes in all annihilation parameters which are unattributable to the accompanying 0.1% density change. Annihilation mechanisms are proposed to account for the observed positron lifetime components and their dependence on phase. The degree of molecular orientation appears to have little direct influence on annihilation behavior, but the nature and strength of the intermolecular forces in different phases can greatly affect the positron lifetimes and intensities.