Positron annihilation studies of a conjugated polymer

Lineshapes of the Doppler broadened annihilation radiation (DBAR) of a conjugated polymer polydiacetylene, 4-butoxycarbonylmethylurethane (P-4BCMU), in its amorphous state, were measured over the temperature range of 30 to 190°C. A differential-scanning-calorimeter (DSC) was used to correlate the transition temperatures. The DBAR lineshapes show a distinct change during the structural transitions of the polymer. A two states trapping model is used to qualitatively describe the temperature dependence of the lineshape. The nature of formation of the trapped states of positrons above T_c, through the interplay of electronic structure/conformation change, is discussed.     

Positron annihilation in germanium

High precision long-slit and cross-slit geometry angular distributions of annihilation radiation from oriented germanium are presented. The momentum distributions from the long-slit measurements are compared with recent Compton profile data to test the importance of the positron wavefunction and positron-electron correlations. Evidence for Umklapp annihilation is discussed. No observable differences are found between distributions from heavily doped and intrinsic germanium samples. Differences in the angular distributions between 5 K and 300 K are attributed to positron thermal motion, and the results are analyzed by a new method to obtain an estimate of the positron effective mass. Paper A21 presented at 3rd Internat'l Conf. Positron Annihilation, Otaniemi, Finland (August 1973). Work supported by the National Science Foundation and the U.S. Army Research Office, Durham, North Carolina USA.     

Positron annihilation in copper

Electron momentum distribution in copper is studied through the angular correlation of positron annihilation radiation, by employing self-consistent linear combination of Gaussian orbitals band structure method. The two-photon momentum density is presented and analysed through three dimensional plots and angular momentum decomposed contributions. Calculation of two-dimensional angular correlation has been carried out in the independent particle approximation. The results for the analysis using Lock, Crisp and West theorem and the reciprocal form factors are also presented. The present theoretical results are compared with the experimental ones wherever possible, and the agreement in general, is good.     

Positron investigations of free-volume elements in polymer gas-separation membranes

This work is a part of an investigation aimed at converting the positron method into a tool for quantitative investigations of microdefects in polymers. The number (10¹⁹ cm⁻³) and effective radii (0.2–2.8 nm) of elementary free volumes in polymer gas-separation membranes polytrimethyl silylpropine (PTMSP) and porous polyphenylene oxide (PPO 200 and 70 m²/g) are estimated on the basis of positron data. The calculations are facilitated by using the mutually complementary programs PATFIT and CONTIN as well as by the possibility of estimating the diffusion coefficients of positronium in PPO samples having different specific surface areas. Fiz. Tverd. Tela (St. Petersburg) 40, 164–167 (January 1998)     

Positron annihilation in polycrystalline metals

The empirical relation θ _p ⁶ /I _p=aK (where θ _p is the limiting angle of the parabolic component in the angular distributions of annihilation photons in metals, I _p is the integrated contribution of this component, K=1, 2, 3, ... is an integer, and a is a constant independent of the type of metal) observed earlier has been tested on magnesium, aluminum, copper, zinc, lead, and bismuth samples. The validity of this relation has been substantiated. The value of the dimensionless constant a has been determined and was found to coincide within experimental error with the result obtained in previous measurements. It is shown that the value of K for the same metal but for different samples may be different. It is conjectured that this may be due to different defect concentrations in samples. Fiz. Tverd. Tela (St. Petersburg) 40, 600–602 (April 1998)     

Positron annihilation in Ge-S glasses

Positron lifetime and Doppler broadening measurements have been carried out on GeS_1.375 and GeS_1.5 glasses. A pronounced decrease of the mean lifetime was observed at crystallization of GeS_1.375 glass into the GeS structure. Also the Doppler-broadened curves are strongly sensitive to this solid-solid phase transition. Unlike the situation for many metals, the positron annihilation parameters were found non-sensitive to fast neutron irradiation doses as high as 10¹⁹ n/cm². No difference between positron annihilation parameters measured on GeS_1.375 and GeS_1.5 glasses was found. The state of the positron in the GeS glasses is discussed on the basis of these results. The idea is put forward, that the GeS-GeS₂ system provides possibilities of sample parameter variations that might prove useful in the understanding of the positron state in non-metallic solids.     

Positron annihilation in liquid mercury

A positron wave function has been calculated for a vacancy and applied to the angular correlation data of liquid mercury. Our results indicate that 19% of all annihilations occur at vacancy sites.     

Positron annihilation in nonmetallic solids

In positron annihilation investigations of nonmetallic solids, the standard deviation of the gaussian component of the angular correlation curve is elucidated as material constant. It is related to the apparent radius of the chemical unit of the substance in question.     

Positron annihilation in Ni-Co alloys

The binary system Ni-Co was studied by the positron-annihilation method. The value of the annihilation-spectrum peak was calculated from an approximate analytical formula and plotted against the cobalt concentration. In the plot two minima were observed corresponding to the ordered state and to the phase transition of the system Ni-Co, respectively.     

Positron annihilation in ionic crystals

Positron lifetime measurements were made on Ca²⁺ doped NaCl, Ba²⁺ doped NaCl and X-ray- and additively-coloured KCl crystals. In the NaCl(Ca) system a correlation which could be approximated by a power equation was found between the dopant concentration and the positron annihilation parameters. In the NaCl(Ba) system the decay parameters do not change significantly as expected and no significant difference in the decay parameters of the coloured and uncoloured crystals could be found. Paper C 4 presented at 3rd Internat'l Conf. Positron Annihilation, Otaniemi, Finland (August 1973).     

Positron annihilation in synthetic zeolites

Both lifetime and angular correlation of positron annihilation have been measured for a series of synthetic zeolites for which the void structures are known fairly well. All of the zeolites had long lifetime components and a narrow momentum component which are ascribable to o-Ps and p-Ps annihilations in the voids, respectively. The correlation between the width of the p-Ps narrow component and the size of the largest voids showed a remarkable agreement with a theoretical estimate based on the spherical potential well model. The measurement of p-Ps momentum thus appears to be prospective as a tool to determine the size of voids of materials. The lifetime of the long lifetime component, on the other hand, showed a poor correlation with the void size even from a qualitative viewpoint, suggesting that factors other than the simple void size effect are dominant in determining the o-Ps lifetimes. Discussion is made on the cause of the different dependences of o-Ps and p-Ps annihilation parameters on the void size. It has also been found that p-Ps fraction is always larger than one third of the o-Ps fraction in all the zeolites studied. A discussion is presented on this point, too.Formerly, RIISOM     

Positron annihilation in electron irradiated Cz-Si

Defects in electron irradiated Cz-Si were studied by the positron annihilation technique. In order to know effects of the thermal history of crystals on the introduction of defects, the specimen was quenched from 1390°C to room temperature before irradiation. A clear correlation between results obtained by the positron annihilation experiments and those by infrared spectroscopy was established. From the isochronal annealing experiment, it was found that oxygen clusters were introduced by quenching treatments.     

Positron annihilation in copper single crystals

A point geometry angular correlation measurement of the annihilation radiation from an annealed single crystal of copper is presented. The results are compared with results for an unannealed sample and theoretical predictions obtained from a simple picture of the electronic wave-functions combined with two models for the positron wavefunction.     

Positron annihilation studies in ZnO nanoparticles

We report results on positron annihilation spectroscopic (PAS) studies using lifetime and coincidence Doppler broadening techniques in zinc oxide (ZnO) nanoparticles (4 to 40 nm) synthesized by solid state pyrolytic reaction followed by annealing in the temperature range of 200 ^°C to 800 ^°C. Positron lifetime in the nanoparticles are observed to be higher than bulk lifetime in all the cases. Theoretical calculation of lifetime indicates the presence of either Zn or (Zn, O) vacancy clusters which migrate and anneal out at high temperature. Comparison of ratio spectra from coincidence Doppler broadening measurement and calculated electron momentum distribution indicates the presence of either Zn or (Zn, O) vacancies. In addition, photoluminescence (PL) measurements have been carried out to examine the role of defects on the intensity of emission in the visible region.     

Positron annihilation in gamma irradiated crystals

The angular correlation between the two gamma-quanta arising from positron annihilation in untreated and gamma-irradiated alkali halides is reported. For low level gamma irradiation the percentage of positrons annihilating with F-centre electrons is not very significant. When the F-centre concentration is increased to as much as 1.5 × 10¹⁷ per cm³, about 15 per cent of positrons annihilate with F-centre electrons. The angular correlation curve becomes progressively narrower with increase in F-centre concentration. Positrons annihilating in gamma-irradiated crystals reduce the F-centre considerably. The results of a systematic investigation on F-centre bleaching by positron annihilation are reported.     

Positron annihilation in small metal voids

The electron density profile around small voids of varying radii in Al is calculated in a fully self-consistent manner using the density-functional formalism of Hohenberg-Kohn-Sham. The results are then used to calculate positron lifetimes and angular correlation between annihilation photons as a function of the size of the void.     

Positron annihilation in boron-implanted n-type silicon

The ion damaged effect and subsequent isothermal annealing in boron-implanted Si was studied by positron annihilation lifetime measurements. The mean positron lifetime in preimplanted n-type Si is 243 psec. The variation of mean lifetime is detectable when the implanted boron dose is greater than 1.0x10¹⁵/cm². The saturated mean positron lifetime (247 psec) occurs when the implantation dose reaches 2.5x10¹⁵/cm². The mean electron density of the positron sensitive defects is estimated to be about 85% less than that in the perfect parts of the crystal. Isothermal annealing was held in every 5-minute step at 1000°C. In the first step, the positron lifetime in the implanted sample increases slightly and then decreases completely to its initial state in the 3rd step. Sheet resistance of the sample monitored by 4-point probe method has been found closely related to the positron lifetime.     

Positron annihilation in metals and alloys

This review article summarizes the present knowledge of positron-annihilation research in metallic substances. Including a short historical survey, the annihilation processes in metals and alloys are discussed and various physical conditions such as ordering processes, magnetic phenomena, liquid state, superconductivity and lattice imperfections are considered. Finally, the present state of theoretical investigation is given and the outlook for the new and very useful technique in metal physics is pointed out.     

Positron annihilation in neutron-irradiated p-type Ge

Abstract

Observations of sintering of fine particles of diamond, SiO₂ and Al₂O₃ and possibly Al in a neutron radiation field are reported. Possible mechanisms are discussed and it is concluded that the transport of interstitials to the surface is most probably the responsible mechanism. This may occur by diffusion, channeling or focusing, and it is suggested that measurement of radiation sintering at two temperatures will show if the effect is thermally activated diffusion and that examination of changes in stoichiometry of the sintered interface will show if focusing or channeling dominates as a transport mechanism. Computations are also presented which show that the solar wind on the lunar surface could sinter dust into the observed clumps in a reasonable length of time.