Structure of χ-Borophene Studied by Total-Reflection High-Energy Positron Diffraction (TRHEPD).

We have investigated the structure of χ-borophene on Ag(111), a monolayer material of boron atoms, via total-reflection high-energy positron diffraction (TRHEPD). By comparing the experimental rocking-curves with ones for several structures calculated by using dynamical diffraction theory, we confirmed that the χ-borophene layer has a flat structure. The distance from the topmost layer of the metal crystal is 2.

Observation of a shape resonance of the positronium negative ion.

When an electron binds to its anti-matter counterpart, the positron, it forms the exotic atom positronium (Ps). Ps can further bind to another electron to form the positronium negative ion, Ps(-) (e(-)e(+)e(-)). Since its constituents are solely point-like particles with the same mass, this system provides an excellent testing ground for the three-body problem in quantum mechanics.

Electrochemical transfer of (18)F from (18)O water to aprotic polar solvent.

Electrochemical transfer of (18)F(-) from enriched [(18)O]-water to pure acetonitrile was investigated to develop a simple and effective synthesis of (18)F-radiopharmaceuticals. The transfer of (18)F is composed of two steps: first step is electro-deposition on a graphite anode and the next step is electro-emission into pure acetonitrile by inversing the polarity of the electric tension. A sufficiently high fraction of the electro-emission, 73%, was achieved.

Experimental evidence for spin-orbit interactions in positronium-Xe collisions.

Spin-orbit interaction of positronium (Ps) with the surrounding atoms, predicted by Mitroy and Novikov [Phys. Rev. Lett.

Measurement of Cu K-shell and Ag L-shell ionization cross sections by low-energy positron impact.

Inner shell ionization cross sections by low-energy positron impact have been measured. Development of an x-ray detector with thin Si(Li) crystals has enabled the first measurements of the absolute cross sections for the positron impacts in the energy range below 30 keV. Threshold behavior of the measured cross sections for the Cu K shell and Ag L shell are compared with the theoretical results of Gryzinski and Kowalski [Phys.

Direct measurement of the parapositronium lifetime in alpha-SiO2.

The mean lifetime of delocalized parapositronium in alpha-SiO2 has been determined directly for the first time using a newly developed positron lifetime spectrometer based on BaF2 scintillators and a fast digital oscilloscope. The lifetime is found to be 156+/-4 ps, which is much longer than its intrinsic lifetime of 125 ps. This indicates clearly that the primary many-body effect on positronium in alpha-SiO2 is the screening of the Coulomb interaction between the constituent particles by electrons of the medium and that the average distance between the electron and positron in positronium becomes larger than its vacuum value.