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By using two-dimensional particle-in-cell simulations, we demonstrate high-flux dense positrons generation by irradiating an ultra-intense laser pulse onto a tapered hollow target. By using a laser with an intensity of 4 ×1023W/cm², it is shown that the Breit-Wheeler process dominates the positron production during the laser-target interaction and a positron beam with a total number >1015 is obtained, which is increased by five orders of magnitude than in the previous work at the same laser intensity. Due to the focusing effect of the transverse electric fields formed in the hollow cone wall, the divergence angle of the positron beam effectively decreases to ~15° with an effective temperature of ~674 MeV. When the laser intensity is doubled, both the positron flux (>1016) and temperature (963 MeV) increase, while the divergence angle gets smaller (~13°). The obtained high-flux low-divergence positron beam may have diverse applications in science, medicine, and engineering. [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]

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]