Positron annihilation in solid and liquid metals

The angular correlation of the gamma rays resulting from the annihilation of positrons in 15 solid and liquid metals and semiconductors has been studied. Experiments have been done on each material at room temperature and at temperatures above and below the melting point.

The elements investigated fall into three categories according to the way the angular correlation distribution changes as the melting point is reached. To within the experimental angular resolution (0.5 milliradians) no change in the angular correlation distribution is observed for Li, Se, Na, and Tl upon heating from room temperature to beyond the melting point. The elements Sb, Bi, Ga, Hg, Sn and Te exhibit changes in their angular correlation distributions only upon being melted, whereas for the metals Al, Cd, In, Pb and Zn changes occur when the specimen is heated from room temperature to temperatures below the melting point.

Changes in the angular correlation distribution upon heating or melting are generally manifested as (a) a narrowing of the central part of the curve, (b) a rounding-off of the parts of the curve near the Fermi cut-off angle and (c) a change in the area of the broad background curve as compared with the area under the central peak.     

A theory of phase transitions in solid krypton and xenon monolayers on graphite

A statistical mechanical model of solid Kr and Xe monolayers on graphite is described. A phase transition in a Kr monolayer is interpreted as a transition from a solid localised in the surface potential wells of the graphite, to an unlocalised solid having a higher density. Comparison with experimental data enables the graphite well depth to be estimated. Explicit calculations are presented for four different interatomic potential models. The density of the unlocalised phase, the isosteric heat, and the gas-solid transition are given as a function of temperature and pressure for both Kr and Xe. It is shown that Xe first condenses into an unlocalised phase, and its ability to undergo a transition into localisation is examined. It is found that this transition is unlikely at p ? 10^?3 Torr where volumetric measurements have been made, but may occur at lower pressures. The gas-solid transition data is re-interpreted in terms of a transition to the unlocalised phase. A combination of the solid-solid and gas-solid transition data is shown to be a good test of interatomic potential models. The comparison is made with particular reference to the magnitude of the Sinanoglu-Pitzer potential.     

Environmental perturbations on foreign atoms and molecules in solid argon,krypton and xenon

Perturbations which are responsible for the shifts of electronic and vibrational spectra of species trapped in a solid are considered in terms of the intermolecular potential which describes interactions between these species and neighbouring atoms. It is shown that in certain instances London's theory can give an adequate approximation to the dispersion energy between an electronically excited species and a rare gas atom. The experimental shifts in the electronic absorption spectra of Hg, NH and C₂ at lattice sites in rare gas crystals at 4·2°k are explained quantitatively on the basis of a Lennard-Jones (6-12) or (6-8-12) potential between the trapped species and the rare gas atoms. The theory does not adequately explain the shifts in those cases where strong angular dependent forces differing appreciably in the two electronic states are present, data on trapped NH₂ free radicals being presented as a case in point. The interaction of sodium atoms with argon at 4·2°k is very complex, the data being consistent with multiple trapping sites for the atoms, a large repulsive interaction between the excited state of sodium and the rare gas, and apparent removal of the three-fold orbital degeneracy in the excited state by the environmental perturbation. The three-fold orbital degeneracy in the ³P₁ state of mercury was found also to be removed. The blue shift of 1281 cm^-1 for the ³P₁ ← ¹S₀ transition of mercury in solid argon at 4·2°k corresponds almost exactly with the position of one of the two prominent features in the spectrum of mercury in argon gas at comparable densities and illustrates the similarity of structure in the two physical states. The interesting perturbations on the vibrational states of NH and C₂ suggest a close similarity to the effect which causes environmental variations of coupling constants for hyperfine interactions in trapped hydrogen atoms.     

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 annihilation in alpha- and beta-cyclodextrin

We have measured positron annihilation lifetimes in α- and β-cyclodextrin samples both in air and in vacuum. The existence of different hollow spaces in these clathrates offers a good test of the “free volume” model of positronium decay in molecular solids. Supported in part by the Research Corporation     

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 alpha- and beta-cyclodextrin

A new type of misfit dislocation multiplication is deduced from high-voltage electron micrographs of thin Ge layers on GaAs substrates. Two misfit dislocations with the same Burgers vectors on different glide planes cross and annihilate at the intersection point resulting in the formation of two angular dislocations. The tip of one of these dislocations may reach the growth surface by glide breaking into two separate dislocation segments. These segments may glide to form additional misfit dislocations, which may undergo the same multiplication process.     

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 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 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 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 solid,liquid and undercooled melts of Co80Pd20

Positron annihilation measurements have been performed in solid, liquid and undercooled Co₈₀Pd₂₀ alloy using electromagnetic levitation as containerless processing method. The formation enthalpy for a single vacancy is H_1v = (1.7 ± 0.1) eV. In the melt, the thermal expansion continues linear in the undercooled phase and is larger than that of the solid alloy. The mean free volume in the liquid phase is slightly larger than the volume of a single vacancy. At the Curie temperatures of both solid and liquid phase, the S-parameter indicates no effect on the atomic structure. Different H₂ concentrations in the processing gas atmosphere have no detectable influence on the data.     

Nucleation in superheated argon,krypton and xenon liquids

The superheating of condensed noble gases (argon, krypton, xenon) was investigated by the techniques of continuous heating of liquid specimen in a glass capillary. Experiments covered the pressure range 1–35 bar at the nucleation rates ≈10⁵ cm^-3 s^-1. The superheating data obtained here were compared with the results of calculations based on the Volmer-Becker-Döring-Zeldovich-Frenkel homogeneous nucleation theory. A good agreement between theory and experiment as well as the thermodynamic similarity of the studied substances with respect to the fluctuation nucleation was found.     

Small-scale electric discharge krypton and xenon chloride laser

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 53, No. 2, pp. 337–339, August, 1990.