Direct Annihilation Research Articles

The annihilation of superdislocations in Ni3Al during deformation at room temperature

Abstract Transmission electron microscopy evidence is given of mechanisms. of annihilation that take place in the course of the room-temperature plastic deformation of Ni3Al. Under a load orientation that favours the operation of a single-slip system, the short dipoles that are present in foils sectioned parallel to the slip plane are identified as elongated closed superdislocation loops instead of dipoles terminating at free surfaces. The fact that dipoles are often aligned in rows such that one extremity of a given dipole corresponds with the closest extremity of the next dipole in the screw orientation is explained in terms of the cross-slip properties of Ni3Al-based alloys. The presence of two distinct categories of dipole rows is interpreted in terms of two different approach mechanisms. The formation of antiphase-boundary tubes-by direct annihilation of screw segments is questioned.

One-photon annihilation of thermal positrons with bound atomic electrons

Direct one-photon annihilation rate of positrons with a bound atomic electron is evaluated in the nonrelativistic limit. The K- and L-shell contributions are estimated including the screening and effective Coulomb repulsion effects. The annihilation rate of thermal positrons is calculated for various temperatures. The total number of one-photon annihilation events in the interstellar medium is discussed. These results provide the directional and structural information for cosmic gamma-ray sources.

A model for the creep of oriented high‐modulus fibers

AbstractRecently it has been shown that the creep of oriented high‐modulus fibers satisfies a logarithmic time law over very long time intervals. Moreover, the modulus of these fibers increases due to the creep. This was interpreted by the annihilation of conformation defects during creep. In structures with strong intermolecular interactions as in aramides, however, the direct annihilation of defects is difficult to explain. Moreover a direct annihilation leads to a false time law if no further assumption about the properties of the defects are made. In this paper a model for the transport of conformation defects in oriented polymer chains imbedded into an oriented fibrillary structure with strong intermolecular interactions is presented. This modified transport model leads directly to the experimentally observed time law without any further assumptions about the defect properties.

Complete spectra of the delayed luminescence from aromatic compounds in liquid solutions: On the observability of direct radiative triplet-triplet annihilation

With many aromatic compounds in liquid solution, the population of the metastable triplet state T 1 leads to delayed S n →S 0 fluorescence (DF n, n⩾ 1) of the pyrene type, which results from the generation excited singlet states S n by nonradiative triplet-triplet annihilation (TTA) T 1 + T 1 ⇝ S n + S 0. Knox and Swenberg have postulated that, apart from the DF1, there should be an extremely weak direct radiative TTA: T 1 + T 1 → S 0 + S 0 + hν rad. The spectrum of the direct radiative TTA is likely to be si the convolution of the phosphorescence spectrum with itself. Due to this similarity, part of the high-energy delayed luminescence from solutions of pyrene and phenazine can be assigned to direct radiative TTA. The probability of direct radiative TTA relative to the generation of S n by nonradiative TTA is typically ≲ 5 × 10 −7. With anthracene, 9-methylanthracene and 1,2-benzanthracene no similarity has been found between the spectrum of the observed high-energy luminescence and the calculated spectrum of radiative TTA. Some special aspects of the delayed fluorescence are treated: nonradiative TTA as a special case of Dexter energy transfer, the temperature dependence of the delayed excimer fluorescence from pyrene, nonequilibrium hot-band delayed fluorescence from S 1 of anthracene, and the problem of the indirect determination of lifetimes of upper excited singlet states.

Gamma ray lines from dark matter annihilation

If direct annihilation of dark matter particles into a pair of photons occurs in the galactic halo, a narrow γ-ray line can be discovered at future γ-ray detectors sensitive to the GeV region. The signals predicted by different dark matter candidates are analyzed.

Direct positron annihilation and positronium formation in thermal plasmas

In the present evaluation of the rate of direct positron annihilation with electrons in the nonrelativistic limit, general analytic expressions are given for the radiative recombination of positrons to form positronium. Formulae are derived for the radiative capture to bound states of atomic hydrogen, and the connection between the two problems is demonstrated. Annihilation from excited states of positronium is considered, and it is estimated that 90 percent of the annihilations occur from the ground 1s state for both ortho and para positronium following radiative capture and cascade. A convenient form is given for the photodissociation cross section of positronium.

The efficiency of direct annihilation of resistivity silicon primary radiation defects in high-resistivity silicon

Abstract The change of charge-carrier concentration and lifetime in high-resistivity float-zone n- and p-Si with equilibrium concentrations of electrons n0=(2 to 30)× 1012 cm−3 and holes p0=(5 to 30)× 1011 cm−3 when irradiating by 60Co γ-rays (T= 330 K) is studied. There is a drastic increase of the initial charge- carrier removal rate and the coefficient for the radiation variation of their lifetime. The results obtained are explained on the assumption of the dependence of the annihilation probability of Frenkel pairs on the charge state of vacancy (V) and interstitial Si atom (I).The energetic position of the levels separating the states I0/I++ and V−/V0 which are at Ec −(0.43−0.45) eV and Ec −(0.47−0.54) eV, respectively, is estimated.

Influence of irradiation temperature on radiation defect accumulation in silicon

The processes of radiation defect formation in n-type silicon are studied at various irradiation temperatures (Tirr = 20 to 400°C) during 640 MeV proton or 10 MeV electron bombardment. The results are obtained from the analysis of the temperature dependences of charge carrier concentration in crystals irradiated at various temperatures and investigating samples (which are not irradiated but subjected to similar thermal treatment). It is established that production efficiencies for main radiation defects (A-, E-centres, divacancies, complexes of carbon–oxygen-divacancy) are complicated functions of Tirr, however the form of these functions appear to be different for the two types of irradiation used. The results obtained are interpreted taking into account the fact that during cascade defect formation (in the case of proton irradiation) the effect of Tirr is due to the probabilities of secondary processes (dissociation and formation of complexes) changing with temperature, while in the case of diffusive mechanism (electron irradiation) the irradiation temperature Tirr has also an influence on the direct annihilation of Frenkel pairs generated by irradiation. [Russian Text Ignored].

Exclusive non-leptonic decays ofD-,D s - andB-mesons

Recent experimental results onD-,Ds- andB-decays are interpreted on the basis of the valence quark model. For nonleptonic decays generally good results are obtained using factorization with little direct annihilation. Final state interaction causes corrections, however: small bare amplitudes can be fed by stronger amplitudes through channel mixing. This effect can simulate or enhance weak annihilation processes in particular if high lying resonances contribute. A comparison of\(D^0 \to \bar K^0 \phi \) withDs→ϱπ decays will clarify this issue. The effective QCD coefficients obtained in the analysis are discussed and used to estimate the contribution of two-body decays to the total widths. The result reflects already the lifetime differences betweenD0 andD+ mesons. Predictions for numerousB-decay branching rations allow for further tests and for a determination of so far unknown decay constants. A first bound on |Vub| from nonleptonic decays is found from the experimental limit on\(\bar B^0 \to \pi ^ + \pi ^ - \).

Laboratory simulation of direct positron annihilation in a neutral-hydrogen galactic environment.

The linewidth for thermalized positrons directly annihilating with bound electrons in low-density molecular hydrogen gas has been found to be 1.56 \ifmmode\pm\else\textpm\fi{} 0.09 keV, by use of a Ge detector with a low-energy pulsed positron beam. This linewidth measurement together with previous experimental results makes it possible to determine precisely the composite spectrum due to positrons annihilating in a neutral-hydrogen galactic medium.

Recombination radiation and electroreflection of excitons in ZnxCd1−xSe solid solutions

AbstractThe simultaneous investigation of electroreflection (ER) and secondary radiation (SR) spectra of ZnxCd1−xSe solid solutions under off‐resonance (resonance) laser excitation at 5 to 300 K reveals the fine structure of the fundamental absorption edge which arises from the direct and indirect creation and annihilation of excitons of series Γ6–Γ8 or Γ7–Γ9 und Γ7–Γ7. The shape of the SR exciton line at low temperatures is determined by the exciton trapping processes at random potential fluctuations of composition. This statistic random potential manifests itself in a blue shift of the SR exciton line with the N2‐laser excitation. The behaviour of the SR exciton line at 77 K is strongly governed by the exciton annihilation processes with the simultaneous emission or absorption of LO phonons. A possible effect of extended and self‐trapped exciton states in SR spectra at high temperature is considered.

Lifetimes of Free and Bound Excitons in High-Purity GaAs

An analysis has been carried out for obtaining the free- and bound-exciton lifetimes from optical phase shifts measured on crystals with surface loss. It is found that the bound-exciton lifetime can be obtained directly from the measured phase shifts associated with the free- and bound-exciton lines, regardless of the surface condition. The optical phase shift of the free-exciton line from a surface-free crystal should be larger than that measured on the same crystal but with a lossy surface. Experiments to measure the phase shifts of free excitons and excitons bound to neutral and ionized donors (${D}^{0}$ and ${D}^{+}$) and neutral acceptors (${A}^{0}$) have been carried out and the various lifetimes have been determined from the above analysis. It was found that ${\ensuremath{\tau}}_{{D}^{0}}=1.07\ifmmode\pm\else\textpm\fi{}0.1$ nsec, ${\ensuremath{\tau}}_{{D}^{+}}=0.8\ifmmode\pm\else\textpm\fi{}0.08$ nsec, and ${\ensuremath{\tau}}_{{A}^{0}}=1.6\ifmmode\pm\else\textpm\fi{}0.6$ nsec at 1.6 \ifmmode^\circ\else\textdegree\fi{}K. The free-exciton lifetime decreases with increasing impurity concentration in the material (the longest lifetime obtained being 2.9\ifmmode\pm\else\textpm\fi{}1.3 nsec), indicating that the decay of free excitons is not governed by the direct annihilation of the electron-hole pairs.

Annihilation of positrons in helium, neon, and argon

The electric field dependences of the direct annihilation rates for positrons in helium, neon, and argon are calculated. This is done by using a systematic description of the scattering process. The momentum transfer and direct annihilation cross sections are calculated within the framework of the polarized-orbital method which has worked well for electron–atom scattering. The cross sections are then used in the appropriate diffusion equation to determine the experimentally observable annihilation rates appropriate to the exponential decay region of the spectrum. The resulting annihilation rates are found to be extremely sensitive to the low-energy behavior of the cross sections. Best agreement between calculated and experimental annihilation rates is obtained for helium where fairly rigorous calculations of the cross sections are available. Good agreement between theory and experiment for neon and argon is obtained only by making judicious choices for the components of the distortion included in the calculations. It is thus concluded that the positron–atom scattering process is considerably more sensitive to the details of the mutual distortion interaction than is observed in the corresponding electron–atom collision process.

Annihilation of Positrons in Argon at High Densities

The annihilation of positrons in argon as a function of argon density at densities from 10 to 65 amagats has been investigated at room temperature. The direct annihilation rate, the reciprocal of shoulder width, and the quenching rate of $o$-positronium are found to be linearly dependent on argon density in the density range investigated within the range of the experimental error. The value of ${Z}_{\mathrm{eff}}$ of the direct annihilation rate is found to be 26.2 for an argon density up to 50 amagats.

Annihilation of Positrons in Argon II. Theoretical

The results of measurements of the electric field dependence of the direct annihilation rate of positrons in argon are used to test the validity of simple models representing the interaction between a positron and an argon atom. Despite the fact that such models yield results in agreement with electron scattering data, they are inadequate in the case of positrons. By comparing this data with recent measurements of the temperature dependence of the lifetime of positrons in argon, a value of $(33\ifmmode\pm\else\textpm\fi{}5)\ensuremath{\pi}a_{0}^{}{}_{}{}^{2}$ is obtained for the scattering cross section of thermal positrons in argon.

Annihilation of Positrons in Argon I. Experimental

The annihilation of positrons in argon has been investigated as a function both of argon density and applied electric field. The direct annihilation rate decreases with increasing field as observed in earlier work. These electric field results are compared with simple theoretical models of the positron-argon atom interaction in the following paper. A small nonlinear dependence on density of the direct annihilation rate became apparent at densities larger than about 10 amagats. The linear portion of the direct rate was characterized by ${Z}_{\mathrm{eff}}=27.3\ifmmode\pm\else\textpm\fi{}1.3$. From the density dependence of the orthopositronium lifetime, the free orthopositronium annihilation rate (${\ensuremath{\lambda}}_{1}$) and linear quenching rate in argon (${\ensuremath{\lambda}}_{q}$) were found to be${{\ensuremath{\lambda}}_{1},=(7.53\ifmmode\pm\else\textpm\fi{}0.18)\ifmmode\times\else\texttimes\fi{}{10}^{6} invsec,}{{\ensuremath{\lambda}}_{q},=(0.24\ifmmode\pm\else\textpm\fi{}0.02)\ifmmode\times\else\texttimes\fi{}{10}^{6} invsec {\mathrm{amagat}}^{\ensuremath{-}1}.}$ The role that impurities play in these measurements is also discussed.

Electric field dependence of the annihilation rate of positrons in helium

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.

Neutrino-Electron Interactions and Stellar Evolution

A large part of the energy lost from interiors of evolving stars at temperatures greater than about 5 × 108°K is carried away by neutrinos. Three neutrino-producing reactions which have been incorporated in various evolutionary models are the photoneutrino process: direct annihilation of electron-positron pairs: and the plasma process:

EXPERIMENTAL STUDIES OF THE ANNIHILATION OF POSITRONS IN THE NOBLE GASES

The direct annihilation rate of positrons in He, A, and Kr has been studied. Deviation from an exponential for the initial portion of the time distribution of these events is observed and is interpreted as a velocity dependence of the direct annihilation rate. The product of the gas pressure and the width of the non-exponential portion of the time distribution has a value of 270 nsec atm and is approximately independent of the pressure. An estimate of the fraction of positrons forming positronium in argon yields a value of 0.37 ± 0.03. Using this result, a check of the statistical ratio of ortho- to para-positronium formation indicates that 69 ± 7% of the positronium is formed in the ortho state.

Annealing of Radiation Defects in Semiconductors

Radiation induced defects studied through changes in conductivity and Hall coefficient have been observed to anneal in a number of different temperature ranges. Only those processes occurring above 80°K and involving defects created by electron irradiation have been considered in this paper. It has been found that the first annealing process in n-type germanium occurs at about 50°C and is structure sensitive, apparently to the original chemical donor impurity. Higher temperature annealing processes, observed at about 200°C and previously interpreted as due to direct annihilation of vacancies and interstitials must also be sensitive to other crystal defects. In p-type germanium there is a process of rearrangement of a defect center at about 200°K, exhibiting first order kinetics, but with a time constant which is strongly dependent upon the charge state of the defect. At about 120°C the defects in p type apparently anneal out completely, in striking contrast to the n-type case. Less extensive silicon measurements, showing lifetime recovery between 200 and 400°C again indicate through their kinetics the importance of other impurities or defects in the annealing process.