Takayuki Tachibana

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.

An energy-tunable positronium beam produced using the photodetachment of the positronium negative ion

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.

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.

Desorption of cluster ions from solid Ne by low-energy ion impact

We investigated Ne(+) ions and Ne(n)(+) (n = 2-20) cluster ions desorbed from the surface of solid Ne by 1.0 keV Ar(+) ion impact. Kinetic energy analysis shows a considerably narrower energy distribution for Ne(n)(+) (n ≥ 3) ions than for Ne(n)(+) (n = 1, 2) ions. The dependence of ion yields on Ne film thickness indicates that cluster ions (n ≥ 3) are desorbed only from relatively thick films. We conclude that desorbed ions grow into large cluster ions during the outflow of deep bulk atoms to the vacuum.

Absolute sputtering yields from solid Ne by low energy He+ and Arq+ (1 ≥ q ≥ 6) impact

Absolute sputtering yields from the surface of solid Ne by low energy He+ and Arq+ (1 ≥ q ≥ 6) impact are measured. Very large sputtering yields (300 atoms/ion for 1 keV He+ impact, and 3000 atoms/ion for 1keV Ar+ impact) have been observed. A significant dependence of the sputtering yields on the chage state, i.e. the potential energy, of the incident ion for Arq+ has not been observed because it is estimated to be much smaller than that of the kinetic sputtering, which suggests that the mechanism of potential sputtering is similar to those known for the electron- and photon-stimulated desorption processes.

Efficient and surface site-selective ion desorption by positron annihilation

We compared positron- and electron-stimulated desorption (e+SD and ESD) of positive ions from a TiO2(110) surface. Although desorption of O+ ions was observed in both experiments, the desorption efficiency caused by positron bombardment was larger by one order of magnitude than that caused by electron bombardment at an incident energy of 500 eV. e+SD of O+ ions remained highly efficient with incident positron energies between 10 eV and 600 eV. The results indicate that e+SD of O+ ions is predominantly caused by pair annihilation of surface-trapped positrons with inner-shell electrons. We also tested e+SD from water chemisorbed on the TiO2 surface and found that the desorption of specific ions was enhanced by positron annihilation, above the ion yield with electron bombardment. This finding corroborates our conclusion that annihilation-site selectivity of positrons results in site-selective ion desorption from a bombarded surface.