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Positronium emission from wurtzite GaN(0001) and AlN(0001) surfaces was observed by positronium time-of-flight spectroscopy. The positronium energy spectra contained two positronium components distinguished by their energies. Through detailed analyses based on Monte Carlo simulations, these two components were attributed to positronium formed from valence and conduction electrons. The obtained results augment the previous arguments regarding the contribution of conduction electrons to positronium emission from 4H SiC(0001) and Si(111) surfaces. [ABSTRACT FROM AUTHOR]

We constructed a new apparatus, built upon a trap-based slow positron beam, for the production of a collimated, energy-tunable positronium beam under ultra-high vacuum conditions employing the photodetachment of positronium negative ions. A slow positron generator consisting of a 22Na radioisotope (20 mCi) combined with a buffer-gas positron trap is employed to generate high-quality, nano-second positron bursts with a repetition rate of 1 Hz–1 kHz. The positron bursts are focused onto an efficient positron-to-positronium negative ion converter, a Na-coated W thin film in a transmission geometry, using a magnetic lens system. The ions emitted from the opposite surface of the film are electrostatically accelerated to a given energy and photodetached by a pulsed infrared laser to form a mono-energetic positronium beam with kinetic energies of 0.2 keV–3.3 keV. The achieved detection rate of Ps atoms is 23 cps at the energy of 3.3 keV with a signal-to-background ratio as high as 300. The energy spread of the beam was evaluated by comparing the result of the time-of-flight measurements and particle-tracking simulations. With the use of a collimator of 1 mm diameter, a coherent beam with an angular divergence of less than 0.3° is obtained. The obtained Ps beam, having a much higher quality than those reported hitherto, will open up a new field of experimental investigations, such as Ps interacting with a variety of materials and fundamental studies on Ps spectroscopy. [ABSTRACT FROM AUTHOR]

Recent developments of the Slow Positron Facility at the Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK) are reported. We have modified the converter/moderator assembly for slow positron beam production, yielding an increase of an order of magnitude in the intensity of the beam. The first observation of the photodetachment of the positronium negative ion (Ps), and the installation of a reflection high-energy positron diffraction (RHEPD) station and the initial data obtained are reported. [ABSTRACT FROM AUTHOR]

We report here the production of an energy-tunable monoenergetic positronium beam using photodetachment of the positronium negative ion, a bound state of one positron and two electrons. The ions were produced efficiently by bombarding a Na-coated tungsten surface with a pulsed slow positron beam and accelerated using a static electric field. On irradiating the ions with a high intensity pulsed laser light, a positronium beam with a hitherto unrealized energy range of up to 1.9 keV has been produced in an ultra high vacuum environment. [ABSTRACT FROM AUTHOR]

The characteristics of a positronium beam produced using the photodetachment of positronium negative ions have been investigated. The yields of positronium atoms were in reasonable agreement with the result of particle trajectory simulation including the effects of positronium emission due to recoil of emitted electrons in the photodetachment. The angular divergence of the beam was measured to be 2° (FWHM) at an energy of 2.3 keV. [ABSTRACT FROM AUTHOR]

Abstract: A new apparatus for experimental studies on the positronium negative ions has been developed at the KEK-PF slow positron facility. The ions are emitted into vacuum from the Na deposited surface of a tungsten target bombarded with pulsed slow positrons. The apparatus will allow the study of the process of the Ps− photodetachment when it is combined with the use of an intense pulsed laser synchronized to the positron beam. [ABSTRACT FROM AUTHOR]

Atoms moving in a static periodic field experience a time-dependent oscillating field in their own rest frame. By tuning the frequency, an atomic transition can be induced. So far, this type of transition has been demonstrated in the EUV region or at higher frequencies by crystalline fields and in the microwave region by artificial fields. Here, we present the observation of the transition of positronium (Ps) in the sub-THz region by using an energy-tunable Ps beam with a multilayered magnetic grating. This grating produces a microsized periodic field, whose amplitude corresponds to a huge energy flux of ~100 MW cm-2, resulting in the efficient magnetic dipole transition. [ABSTRACT FROM AUTHOR]