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Plasma wakefield acceleration of positron beams in the wake of a dense electron beam (in the blowout regime) is numerically analyzed. The acceleration is possible only if the energy content of the wakefield is not very high. This is in contrast to electron acceleration, for which the optimum performance requires driver currents and wave energies to be as high as possible. For positrons, the efficiency of plasma-to-witness energy exchange can amount to several tens percent, but high efficiencies require precise location of the positron beam and sophisticated beam shapes. Unlike an electron witness, the positron always gets an energy spread of about several percent caused by the transverse inhomogeneity of the accelerating field. [ABSTRACT FROM AUTHOR]

A 10 keV positron beam has been developed using coaxially symmetric electromagnetic lenses for reflection high-energy positron diffraction (RHEPD) experiments. The beam brightness is ∼107 e+/s/cm2/rad2/V which is comparable to that obtained using brightness enhancement technique. The beam parallel and normal coherence lengths are over 100 and 40 Å, respectively, which are long enough to observe diffraction patterns associated with large surface super-structures. RHEPD patterns from a Si(111)-(7×7) reconstructed surface have been successfully observed with a much better quality than previously reported. [ABSTRACT FROM AUTHOR]

Shows that in a hybrid slow positron beam, extraction of the positrons from the magnetic field to field-free space is a necessary operation. Theoretical and experimental demonstration which shows that the proposed beam extraction method works as predicted; Field termination and aperture design.

ZnO crystals were implanted with N+, O+, and Al+, and co-implanted with O+/N+ and Al+/N+ ions. Positron annihilation measurements indicate introduction of vacancy clusters upon implantation. In the N+-implanted and Al+/N+ co-implanted samples, these vacancy clusters are only partially annealed at 800 °C, as compared with their entire recovery in the O+- and Al+-implanted samples at 800–900 °C, suggesting a strong interaction between nitrogen and vacancy clusters. However, in the O+/N+ co-implanted sample, most vacancy clusters disappear at 800 °C. Probably oxygen scavenges nitrogen to enhance the annealing of the vacancy clusters. Upon further annealing at 1000–1100 °C, nitrogen also forms stable complexes with thermally generated vacancies. These nitrogen-related vacancy complexes need high-temperature annealing at 1200–1250 °C to be fully removed. [ABSTRACT FROM AUTHOR]

The possibility of a high intensity low energy positron beam production is discussed. The proposed Positron String Trap (PST) is based on the principles and technology of the Electron String Ion Source (ESIS) developed in JINR during the last decade. A linear version of ESIS has been used successfully for the production of intense highly charged ion beams of various elements. Now the Tubular Electron String Ion Source (TESIS) concept is under study and this opens really new promising possibilities in physics and technology. In this report, we discuss the application of the tubular-type trap for the storage of positrons cooled to the cryogenic temperatures of 0.05 meV. It is intended that the positron flux at the energy of 1–5 eV, produced by the external source, is injected into the Tubular Positron Trap which has a similar construction as the TESIS. Then the low energy positrons are captured in the PST Penning trap and are cooled down because of their synchrotron radiation in the strong (5–10 T) applied magnetic field. It is expected that the proposed PST should permit storing and cooling to cryogenic temperature of up to 5×109 positrons. The accumulated cooled positrons can be used further for various physics applications, for example, antihydrogen production. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Slow positron beams generally employ a radioactive β+ emitter with a crystal moderator in which the positrons thermalize. A small fraction of the thermalized positrons are desorbed at low (1–3 eV) energy, characteristic of the positron work function in the crystal. The energy width of the emitted positrons from W(110) and Ni(111) moderators can be narrowed significantly by cooling to liquid N2 temperatures. A practical configuration for a cooled moderator positron gun is presented which produces a narrow energy positron beam with good efficiency. [ABSTRACT FROM AUTHOR]

Investigates the solid state reaction of cobalt/silicon and cobalt/titanium/silicon by slow positron beam. Implantation of the variable-energy positrons into samples at different depths; Characterization of the positron energies; Sensitivity of S parameter to thin-film reaction and crystalline characteristics.

A narrow energy beam of slow positrons can be generated by moderating the beta-decay spectrum of a radioactive β[sup +] source. We report here the development and use of the first moderator in which the low-energy positrons are not extracted from the surface facing the β[sup +] emitter but from the opposite surface, i.e., the transmission moderator. An advantage is that such a configuration eliminates the problem of moderated positrons passing by the radioactive source. This is an essential problem when the source is physically large such as the present commerically available Na[sup 22] sources. This moderator is a 5000-Å self-supporting W (100) film. The growth and treatment of the film were found to provide a high-quality positron moderator. The slow to fast positron conversion efficiency, using a Na[sup 22] source, was found to be 4 × 10[sup -4]. [ABSTRACT FROM AUTHOR]

A pulsed, slow positron beam, with a diameter of 200 μm, was extracted into air through a thin SiN window of an atmospheric positron probe microanalyzer (PPMA), and used to measure the ortho-positronium lifetimes τ in polyvinyl alcohol and polycaprolactam sub-μm-thick films. By measuring the variation of τ as a function of relative humidity, the effect of water molecules on the hole sizes, deduced from τ, was examined for the films with consideration to the chain mobility. The results demonstrate the usefulness of the atmospheric PPMA to the in-situ characterization of nanoscopic holes in thin films under practical conditions. [ABSTRACT FROM AUTHOR]