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The precision of relative positron lifetime measurements is now sufficient to observe directly changes due to lattice thermal expansion. This information is necessary for the determination of accurate vacancy formation energies from positron annihilation data obtained over a higher temperature range. Measurements have been made of the change in positron lifetime in the metals Au, Ag, Cu, and Al over the temperature range 100–300 K where there is no contribution from positron trapping at vacancies. The results are compared with theoretical calculations which take account of positron annihilations with both valence and core electrons.

High precision measurements of positron lifetimes in indium have been carried out at five temperatures in the range 83K to 405K. Constraint free analyses of the results clearly establish the multicomponent nature of the lifetime spectra and the fitness of the 'no escape' models of positron defect trapping phenomena. The implications for positron studies of vacancy formation energy are discussed.

Both the time spectra and the angular distributions of the annihilation gamma rays have been measured for positrons annihilating in Teflon and polyethylene in an attempt to clarify which annihilation mechanisms account for the various lifetimes in these materials. Some authors have suggested that, in addition to the longest lifetime component, one of the shorter lived components also arises from the pickoff annihilation of orthopositronium. If this were the case, substantially more positronium would be formed than is indicated by the intensity of the longest lived component alone, and should be detectable through a measurement of the intensity of the low momentum portion of the momentum distribution obtained from the angular correlation data. In the present work, four lifetimes are apparent in the time spectra although there is considerable uncertainty in resolving the two shortest components. The angular distribution data, in both cases, yielded a low momentum component whose intensity indicates that only the longest lifetime component arises from pickoff annihilation of orthopositronium. The three shorter lifetime components are discussed.

The fractionk of positrons annihilating in source-supporting foils (mica or polyester film) was measured as a function of the foil thickness and of the atomic number of the surrounding material in which positrons are absorbed. It was found thatk attains considerable values when the positron absorber has a high atomic number; at very lowZ the fractionk is smaller for mica than for the polyester film. On the basis of the obtained results a procedure is indicated for correcting the time spectra. This procedure when applied to analyse time annihilation spectra in some metals, has shown that metals are characterized by a single exponential decay, in agreement with the results of recent investigations.

The absolute lifetimes of positrons decaying in aluminum and mica have been found to be 1.7 ± 0.4 × 10−10 sec. and 2.4 ± 0.4 × 10−10 sec., in accord with the determination by B ell and Graham using a different technique.

The photon spectra from positron annihilation in hydrogen for different angles were calculated between 100 and 300 MeV. A particular beam handling system for the positrons to improve the photon resolution was also proposed.

Previous experiments with positron annihilation in single crystals of quartz have revealed an interesting series of narrow peaks on the momentum distribution of the annihilation photons. Our experiments repeat these observations and also confirm that no very long lifetime exists. We outline a simple theoretical model based upon the necessity of treating the positronium electron and the remaining electrons in the crystal as indistinguishable. The model yields results compatible with both momentum and lifetime data.

The Green function of a positron in an electron gas is calculated numerically, in a simple approximation which allows for possible binding between the positron and an electron, using statistical Green function techniques in the low-temperature limit. The results suggest that at normal electron densities the positron appears in unbound, relatively free quasiparticle states. For rs approximately=8-60 these quasiparticles can be described approximately as a mixture of a free elementary positron and a bound quasipositronium (Ps*)- electron hole-pair. The model is applied to a positron annihilation calculation.

The time required for the thermalization of positrons in ionic crystals is estimated. It is shown that the proton-thermalization time in an ionic crystal is much shorter than the shortest positron lifetime with respect to annihilation, so positron annihilation can be used to study the energy spectrum of valence electrons in ionic crystals.

The annihilation of positrons in methane gas at room temperature has been measured at pressures below 2 atm. The annihilation rate for the free positron component is proportional to the gas density and corresponds to an effective number of electrons per molecule taking part in annihilation, Zeff = 139.6 ± 1.0 (standard error). The quenching of orthopositronium corresponds to 1Zeff = 0.57 ± 0.07. Resonance annihilation is discussed in a general way.

We have calculated the lifetime of a positron in solid argon as a function of two-quantum decay and the angular correlation of the resulting $\ensuremath{\gamma}$ pair. The shape of the angular distribution obtained is in close agreement with experiment, and the area under it gives the measured total rate to within 15%. The calculations are based on a general theory that was developed for annihilation in metals but is also applicable with slight modifications to simple nonionic insulators like the inert-gas solids where a single decay mode is observed. The point is simply that we can treat all electrons as core electrons, and the expressions previously derived for core annihilation in metals can readily be adapted to simple nonionic insulators.

Backscatter coefficients have been measured for positron spectra emitted by radioactive sources and incident on 20 elements ranging from Li to U. The technique, based on recording the annihilation-$\ensuremath{\gamma}$-ray spectrum in a Ge (Li) detector, eliminates uncertainties encountered in methods where the positrons themselves are detected. The coefficient $R$ for backscatter into $2\ensuremath{\pi}$ is found to vary with atomic number $Z$ according to the relationship $R=0.342 \mathrm{ln}Z\ensuremath{-}0.146$.

Positron generation at the end of the accelerator, positron acceleration after the positron converter and also the annihilation photons are studied using a 30 MeV electron linear accelerator. In the positron generation, it is found that the thickness of 1.5~2.0 mm is most efficient for a tantalum converter in the present energy region. In the case of positron acceleration, the positron energy can be varied from 8 to 22 MeV. A ratio of the positron current within an energy spread of 3% to the total electron current is estimated to be of the order of 10-7. The spectra of the annihilation photons are obtained by a NaI (T1) crystal spectrometer.

Figures are presented which enable the target parameters of a positron annihilation in flight system to be chosen to obtain the maximum photon yield consistent with the required photon energy spread in the annihilation “spike”.

The empirical equation for the backscattering coefficient of monoenergetic positrons impinging normally on thick targets is found. This equation is expressed by the η(Z, T) function of the target atomic number for 6 ⩽ Z ⩽ 79 in the energy region from 0.1 to 20 MeV. The values of η(Z, T) are found by computer calculation with the use of the “segment model”, which was verified by comparison with the experimental data for electrons. The calculated backscattering coefficients of electrons and positrons show a significant difference, especially in the high-energy region and for high-Z materials.

Substantial extensions of the shorter of two positron lifetimes in p-type silicon upon proton irradiations have been observed, showing that some induced defects can act as traps for positrons. The mean electron density of the defect has been found to be about 30 per cent less than that in the perfect part of the crystal. The result demonstrates that the positron-lifetime measurement is a useful tool for studying defects in semiconductors.

The annihilation scheme of positrons in liquid helium at 4.2\ifmmode^\circ\else\textdegree\fi{}K was studied. Three components in the decay were observed with mean lives: ${\ensuremath{\tau}}_{1}l5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ sec, ${\ensuremath{\tau}}_{2}=(2.6\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ sec, and ${\ensuremath{\tau}}_{3}=(1.2\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ sec. The abundance of the ${\ensuremath{\tau}}_{3}$ component is (16\ifmmode\pm\else\textpm\fi{}2)% of all positrons which annihilate in the liquid. Three-quantum annihilation occurs in about 15% of all positron annihilations. This leads to the conclusion that the three components are due to parapositronium, free positron annihilation, and orthopositronium.

Positron is the antipartie le of electron and its existence was predicted “by Dirac’s relativistic theory of electron. Positrons were experimentally discovered “by Anderson in cloud chamber photographs of cosmic rays. In a material medium, positrons do not live long because they can either annihilate with electrons in the medium, or can form a metastahle system of a positronium atom which is short-lived. In the first type of decay, called prompt or free annihilation, a positron, after thermalisation, annihilates with an electron in “the medium either througji a singlet collision (spins antiparallel) or through a triplet collision (spins parallel). Selection rules governing the annihilation show that a singlet collision results in emission of two photons in exactly opposite direction and each one having an energv of about 511 (= m0 c2) KeV. This annihilation has a life time ~10-10 s. On the other hand, a triplet collision gives rise to an annihilation into three photons with a lifetime ~10-7 secs. In this case the total rest energy (= 2 m0 c2) of the electron-positron pair is shared between the three photons which are emitted in one plane. The ratio of singlet (2 photon) to triplet (3-photon) annihilation rates was calculated by Ore and Powell2 to “be $${\lambda _s}/{\lambda _t} = 1115$$ (1)

Positron lifetime measurements were carried out in four different samples of silicon, namelyn-type (P-doped) 75 Ωcm,n-type (Sb-doped) 0.018 Ωcm,p-type (B-doped) 60 Ωcm andp-type (B-doped) 0.02 Ωcm. The measurements were made at room temperature and at 77K. A positron lifetime of τ1=(230±2) psec was found for all samples, independant of dopant or temperature.

The annihilation rate of positrons in helium gas is proportional to an effective electron number Z which is calculated using the positron scattering wave function obtained by Houston and Moiseiwitsch in 1967. The value Z = 4·61 is obtained and this may be compared with the experimental value Z similar,equals 3·9 obtained by Falk et al. in 1965.

According to our measurements, the long lifetime τ 2 of positrons in water shows a sudden transition at about 4°C. The transition in the peak-rate of angular correlation of annihilation photons is situated at 0°C.

The positron annihilation mechanism in single crystals is proposed which allow to interpret the temperature effect observed before.

The positron lifetimes in an electron gas are calculated using the bound state concept of Bergersen. In order to explain the results of two-γ angular distribution, a fast exchange between the electron in the bound state and the free electrons outside it, is assumed. The calculated annihilation rates are found in good agreement with the experimental and theoretical results.

The observed changes of positron lifetimes in Na and Ga on melting are at variance with the predictions of the free electron gas model. They appear to be determined by the increase in vacancy concentration on melting.

Measurements of the temperature dependence of positron lifetimes in some metals show a marked increase in lifetimes below the melting point. A simple theoretical treatment assuming “trapping” of positrons by defects reproduces the essential features of these measurements.

The angular correlation of positron annihilation radiation was measured in Se and for comparison in Al, Si, and Ge. The curve calculated from atomic orbitals with constant positron wave function agrees well in the case of Se.

A dependence of the direct annihilation rate of positrons in helium gas on electric field has been observed. The results were compared to those predicted according to several current theories of the interaction between a positron and helium atom.

Positron decay curves in sensitized emulsion grains and in AgBr crystals have been measured. The results seem to show that annihilation process is not affected by sensitizing specks.

The angular correlation between the two photons created by annihilating positron-electron pairs in lattices with a basis is derived in various approximations. The effects of temperature as introduced through the momentum distribution of the positrons and through lattice vibrations are examined. Self-consistent electron wave functions with exchange are calculated for the LiH crystal in the cell approximation. The resulting x-ray structure factors are in close agreement with the recent extensive experimental data of Calder et al. The positron wave function is obtained in the same approximation and the momentum distribution of annihilating positron-electron pairs is calculated. The apparent discrepancy noted by Stewart and March between their angular-correlation data and the electron density distribution in the LiH crystal consistent with the x-ray structure factors can be traced to the phase relations between the wave functions within the unit cell of a lattice with a basis.