Calculated beam intensities for low energy positron diffraction

The intensity of the specular beam for low energy positron diffraction (LEPD) from the Cu(111) surface is calculated in the energy range 20–150 eV. Close agreement is found with the first experimental data for LEPD published recently. This result indicates that LEPD may be exploited as a technique for determining surface structures in the near future. It is suggested that LEPD may have advantages in surface structure analysis in comparison with low energy electron diffraction (LEED).     

Demonstration of positron re-emission microscopy using an immersion objective

Hulett et al. have suggested forming a magnified image of the positrons re-emitted from a surface having a negative positron affinity as a new kind of microscope, the positron re-emission microscope (PRM). We have built an immersion objective (cathode lens) for the PRM and obtained the first images using a Ni film back-illuminated with 5 keV positrons from a brightness enhanced slow positron beam. The magnification at the detector plane is 330x and the resolution is better than 1m. With the addition of a projector lens the magnification and resolution should be sufficient to observe defect structures and large molecules on the surface.     

Monte-Carlo calculations of keV electron and positron slowing down in solids

A Monte-Carlo simulation technique based on the screened Rutherford differential cross section for the elastic scattering and Gryzinski's semiempirical expression for the inelastic core and valence electron excitation is used to describe electrons and positrons slowing down in solids. The theoretical results are compared with the experimental backscattering, absorption and transmission results for aluminum, silicon, copper, and gold thin film and semi-infinite targets and good agreement is observed. The simulated stopping profiles are fitted with a simple analytic expression. The profiles are Laplace-transformed to give a useful data base for analyzing phenomena associated with slow positron re-emission from solids.     

Theory versus experiment in low-energy positron diffraction by Cu{111}

Four experimental spectra measured in a low-energy positron diffraction (LEPD) experiment on Cu{111} are satisfactorily matched by intensity calculations. The calculations were carried out with a computer program developed for LEED (low-energy electron diffraction) but using a potential consisting of negative Coulomb contribution, no exchange term and the correlation correction used normally for electrons. The present experimental data are not refined enough to show that positrons do not feel an exchange potential.     

Characterization of a SiC/SiC composite by X-ray diffraction, atomic force microscopy and positron spectroscopies

A SiC/SiC composite is characterized by X-ray diffraction, atomic force microscopy and various positron spectroscopies (slow positron implantation, positron lifetime and re-emission). It is found that besides its main constituent 3C-SiC the composite still must contain some graphite. In order to better interpret the experimental findings of the composite, a pyrolytic graphite sample was also investigated by slow positron implantation and positron lifetime spectroscopies. In addition, theoretical calculations of positron properties of graphite are presented.     

Some investigations of moderators for slow positron beams

Measurements were made on a low-energy positron beam apparatus in an attempt to increase the efficiency of the slow positron yield from radioisotopes. A study was made to sweep thermalized positrons to the surface of a silicon wafer with an applied electric field at 298 and 140 K. Temperature studies were also made on more conventional Pt and Pt+MgO powder moderators and the results are discussed. The role of the MgO powder has been clarified, though fundamental questions remain. The positron apparatus beam and relevant information regarding sources, temperature and magnetic fields are discussed in sufficient detail so that such a slow positron beam utilizing a “conventional” slow positron moderator could be easily duplicated for use in solid state studies.     

Positron-atom and positron-molecule collisions

The scattering of positrons by atoms and molecules is reviewed. The recent development of monoenergetic slow positron beams and their applications to the measurement of cross sections for positrons interacting in various gases are discussed briefly. The theoretical development of positron collisions in gases is covered from the beginning. Both elastic and the inelastic processes have been included. Comparisons are made between the measured values and theoretical predictions. The deficiencies of the theoretical models and experimental apparatus are also pointed out.     

Reflection high-energy positron diffraction pattern from a Si(1 1 1)-(7 × 7) surface

We have investigated the feature of reflection high-energy positron diffraction (RHEPD) pattern from a Si(1 1 1)-(7 × 7) surface. The RHEPD pattern observed in the total reflection condition is quite different from the conventional reflection high-energy electron diffraction (RHEED) pattern. This fact is attributed to the different penetration depths of positrons and electrons. We show that the intensity distribution of RHEPD pattern is reproduced considering the dimer-adatom-stacking fault (DAS) model with optimized atomic positions and scattering potentials of adatoms and rest atoms.     

Positrons — an alternative probe to electron scattering

Interaction of positrons with atoms and molecules differs from electron interaction due to the difference in polarity of the charge. This makes positrons an alternative tool to study atomic and molecular structure. Recent measurements of the total cross-sections for positron scattering at low energies on He, Ar, H₂, N₂, C₆H₆, C₆H₁₂, C₆H₇N carried out at Trento University [Karwasz et al., Acta Phys. Pol. 127, 666 (2005)] are discussed and compared to electron scattering results. All measured total cross-sections exhibit an increase with decreasing positron energy in the limit of zero energy; H₂, N₂, Ar, show regions of constant cross-section which are a few eV-wide, characteristic of scattering on a hard-sphere potential. Helium shows two resonant structures much below the positronium formation threshold. They may be attributed to virtual positronium formation. In conclusion, positron scattering is complementary to electron scattering. The total cross-sections do not show Ramsuaer minima but constant values, and new resonances appear.     

A novel design for a variable energy positron lifetime spectrometer

We present computer simulations of a new design of a variable energy positron lifetime beam that uses for a start signal the secondary electron emission from a 25-nm thick carbon foil (C-foil) located in front of the sample. A needle of ∼30 μm diameter is positioned on-axis right behind the foil, creating a radial electric field that deflects the secondary electrons radially outward so as to miss the sample and to hit the micro-channel plate (MCP) detector placed down beam. The MCP signal provides the start signal for the positron lifetime spectrometer. A grid can be further introduced between the sample holder and the MCP to yield a cleaner signal by preventing the positrons with large transmitted scattering angle from hitting the MCP. The cylindrical symmetry of this design reduces the experimental complexity and offers good timing resolution. We show that the design is robust against the transmitted energy and angle of the secondary electrons and positrons.     

On approximations involved in the theory of positron transport in gases in electric and magnetic fields

A multi term theory for solving Boltzmann’s equation is briefly reviewed and used to test various concepts and approximate expressions for the determination of the positron transport properties in neutral molecular gases in crossed electric and magnetic fields. Among many important approximations which have found their way into contemporary positron studies, the following are particularly discussed: (1) is the approximation of using the cross sections for the electron scattering to describe the positron behavior satisfactory, (2) how accurate is two term approximation for solving Boltzmann’s equation in the context of positron studies, and (3) what is the domain of applicability of Langevin elementary transport theory and Tonks’ theorem for positrons in electric and magnetic fields. We highlight the limitations, range of applicability and inadequacies of such assumptions for positrons in H₂ and N₂. It is pointed out that there is no real alternative to the accurate multi term theory and/or Monte Carlo simulations if high precision is required. It is demonstrated that if the demands for accuracy associated with some of these approximations are relaxed, results may not be even qualitatively correct.     

A positron buncher-cooler

The positron buncher-cooler is a concept for a new compact device, which serves to produce a high quality pulsed positron beam for use in atomic physics experiments. It is predicted that pulses of at least 8000 positrons (from a 50 mCi source) with an energy spread of 25 meV and a bunch length of <50 ns may be produced at a repetition rate of 10 Hz with a duty cycle of 90%.     

Geminga contribution to the cosmic-ray positron excess according to the gamma-ray observations

We attempt to interpret the cosmic-ray positron excess by injection from the nearby pulsar Geminga, assuming a two-zone diffusion scenario and an injection spectrum with a low energy cutoff. Since the high energy positrons and electrons from Geminga can induce γ rays via inverse Compton scattering, we take into account the extended γ-ray observations around Geminga from HAWC for ∼10 TeV and from Fermi-LAT for ${ \mathcal O }(10)$ GeV. According to the extended γ-ray observation claimed by an analysis of Fermi-LAT data, we find that Geminga could explain the positron excess for a 30% energy conversion efficiency into positrons and electrons. However, based on the constraint on the extended γ rays given by another Fermi-LAT analysis, positrons from Geminga would be insufficient to account for the positron excess. Further robust analysis of Fermi-LAT data for the extended γ rays would be crucial to determine whether Geminga can explain the positron excess or not.     

Intense low energy positron beams

Intense positron beams are under development or being considered at several laboratories. Already today a few accelerator based high intensity, low brightness e⁺ beams exist producing of the order of 10⁸–10⁹ e⁺/s. Several laboratories are aiming at high intensity, high brightness e⁺ beams with intensities greater than 10⁹ e⁺/s and current densities of the order of 10¹³–10¹⁴ e⁺ s^–1 cm^–2. Intense e⁺ beams can be realized in two ways (or in a combination thereof) either through a development of more efficient ⁺ moderators or by increasing the available activity of ⁺ particles. In this review we shall mainly concentrate on the latter approach. In atomic physics the main trust for these developments is to be able to measure differential and high energy cross-sections in e⁺ collisions with atoms and molecules. Within solid state physics high intensity, high brightness e⁺ beams are in demand in areas such as the re-emission e⁺ microscope, two-dimensional angular correlation of annihilation radiation, low energy e⁺ diffraction and other fields. Intense e⁺ beams are also important for the development of positronium beams, as well as exotic experiments such as Bose condensation and Ps liquid studies.     

Theoretical study on the positron annihilation in Rocksalt structured magnesium oxide

Based on the atomic superposition approximation(ATSUP) and first-principles pseudopotential plane-wave methods,the bulk and Mg mono-vacancy positron lifetime of magnesium oxide were calculated using Arponen-Pajamme and Boron’ski-Nieminen positron-annihilation-rate interpolation formula respectively.The calculated values are in good agreement with experimental values and the first-principles method gives more convincing results.The positron annihilation density spectra analysis reveals that positrons mainly annihilate with valence electrons of oxygen atoms when the magnesium-vacancy appears within magnesium oxide.     

Symmetry and Condensed Matter Physics – A Computational Approach,by M. El-Batanouny and F. Wooten

The behaviour of positrons below the inelastic collision threshold in rare gases can usually be completely accounted for in terms of the annihilation and elastic scattering cross sections for collisions between a positron and a single gas atom. Determining these cross sections has been the subject of many theoretical and experimental investigations. However the basic premise that the positron interacts with the gas atoms, one at a time, has been recently questioned by the yet unexplained results of some low-temperature experiments. This article discusses the annihilation and elastic scattering cross sections as they relate to the conventional picture of positrons in He, Ne, Ar, Kr and Xe gas; as well as the possibility of bound states for positrons in the gases at low temperatures.     

Can the observed structures in positron spectra be caused by internal conversion?

The energy distribution of positrons emitted in quasimolecular collisions of Uranium on Uranium and Uranium on Curium has been measured by several groups. Peak structures in the positron spectra were observed. We discuss the possibility that these structures originate from internal conversion processes following nuclear Coulomb excitation or transfer reactions. Consequences for the nuclear photon spectra and theδ-ray distribution are pointed out and experimental procedures for an unambiguous determination of the significance of conversion processes are discussed.     

On the positron in a metal

The self-consistent calculations of the spatial distributions of electrons and potentials in vacancies of metals with a localized positron, the spatial distributions of positrons localized in surface states, and the binding energies of positrons and their lifetimes have been performed in terms of the Kohn-Sham method and the stabilized jellium model. The presence of a localized positron in a vacancy leads to the effect that the vacancy is weakly distinguishable for electron waves: the positron weakens the potential field in the vicinity of the vacancy and leads to a phase shift of the scattering electron wave functions. The calculation of the phase shifts of the wave functions for quasi-free positrons scattered by unperturbed vacancies and the representation of a system of vacancies as a “superlattice” in a metal have made it possible to find the shift of the positron work function and the vacancy contribution to the positron effective mass.     

The production oflow energy positrons and positronium

A summary is given of the techniques of positron and positronium production which may be of relevance to the production of antihydrogen atoms at low kinetic energies. Topics covered include positron beams from radioactive isotopes and via pair production at electron accelerators, methods for trapping and accumulating positrons and prospects for the use of slow positrons to create antihydrogen at CERN.     

Recent positron-atom cross section measurements and calculations

We review recent cross section results for low-energy positron scattering from atomic targets. A comparison of the latest measurements and calculations for positron collisions with the noble gases and a brief update of the newest studies on other atoms is presented. In particular, we provide an overview of the cross sections for elastic scattering, positronium formation, direct and total ionisation, as well as total scattering, at energies typically between about 0.1 and a few hundred eV. We discuss the differences in the current experimental data sets and compare those results to the available theoretical models. Recommended data sets for the total cross section are also reported for each noble gas. A summary of the recent developments in the scattering from other atoms, such as atomic hydrogen, the alkali and alkaline-earth metals, and two-electron systems is finally provided.