Hiroki Terabe

Durable emission of positronium negative ions from Na- and K-coated W(100) surfaces

The emission of positronium negative ions from Na- and K-coated W(100) surfaces has been studied. The emission efficiencies (the fractions of incident slow positrons yielding the ions) for both samples were found to be as high as 1.5%. Although the efficiencies decreased with time after coating, the effects were more durable than that for Cs coating. In particular, the efficiency after Na coating was still higher than 0.5% after three days. The successful development of a durable Ps− source opens the door to a new era of experimental investigations on the Ps− ion.

New experiment stations at KEK Slow Positron Facility

Recent development of the Slow Positron Facility at the Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK) is reported. The facility, equipped with a dedicated 55 MeV linac, provides a high-intensity, pulsed slow-positron beam. The beam is produced in a production unit at a high tension of up to 35 kV and guided magnetically through a grounded beam line, and then branched using compact branching units in the experiment hall. An overview, some details of three experiments currently conducted and the outlook of the facility are described.

Positronium and positronium negative ion emission from alkali-metal coated tungsten surfaces

The emission efficiencies of positronium atoms and positronium negative ions from tungsten surfaces increase by alkali-metal coating. We have studied the positronium emission from alkali-metal coated surfaces using the positronium time-of-flight method. We have also performed the observation of the photodetachment of Ps− emitted from a Na-coated surface and the production of an energy-tunable Ps beam employing the photodetachment technique. This paper describes our recent studies using alkali-metal coated surfaces.

Towards the production of an energy-tunable positronium beam using Ps− photodetachment technique

The production of an energy-tunable positronium beam by photodetachment of positronium negative ions, which was proposed many years ago, has become feasible by the recent development of efficient formation of the ions and successful photodetachment. Here we shall describe our current efforts directed towards the production of a positronium beam using this technique.

Ps− emission from Na coated W(100) surfaces

The emission of positronium negative ions from a Na coated W(100) surface has been studied. The emission efficiency was found to increase with time following Na deposition to a maximum of 1.4% after 6 × 104 s. The efficiency then gradually decreased but was still higher than 0.5% after 3 days.

Ps? emission from Cs coated surfaces

Recently, we have observed the efficient emission of positronium negative ions from Cs coated W(100) and polycrystalline Mo and Ta surfaces. The efficient emission enables us to perform new experimental studies on these ions. The investigations of the efficient production of positronium negative ions are reviewed and a measurement of the binding energy of the ions and a photodetachment experiment are proposed.

Efficient formation of positronium negative ions

The emission of positronium negative ions from W(100) and Mo and Ta polycrystalline surfaces has been investigated. A dramatic change in the emission efficiency upto 1.5% was observed upon coating Ta with Cs. The efficiency obtained in the present work may be sufficient to enable thorough investigations of the ions.

Efficient emission of positronium atoms from an Na-coated polycrystalline tungsten surface

Time-of-flight spectra for the ortho-positronium emitted from clean and Na-coated tungsten surfaces have been measured using the pulsed slow positron beam at KEK-IMSS slow positron facility. Emission efficiency of positronium from the Na-coated sample was found to be several times greater than that from uncoated tungsten surfaces.

Positronium negative ion experiments – formation, photodetachment and production of an energy tunable positronium beam–

Recent developments in the efficient production of positronium negative ions open the door to a new era of experimental investigations of these ions. In the present paper, experimental studies so far are reviewed.

Observation of a Low Energy Component of Positronium Emitted from Alkali-Metal Coated Polycrystalline Tungsten Surfaces

We have studied the emission of ortho-positronium from alkali-metal coated polycrystalline tungsten surfaces. The positronium time-of-flight spectra show that the yield of the 5 eV positronium component increases by alkali-metal coating. In addition, a low energy positronium component appears by Cs or K coating. We suggest that this component is due to positron energy loss by inter-band transition or surface plasmon excitation.