Fuminori Saito

Positronium formation reaction of trapped electrons and free positrons: delayed formation studied by AMOC

Abstract Positronium (Ps) can be formed by the reaction of trapped electrons and free positrons at low temperatures in molecular solids. While Ps formation by spur process must be fast, Ps formation by trapped electrons and free positrons is possible even at positron age of several hundreds pico-seconds. Age–momentum correlation measurement of electron–positron pair annihilation γ -rays was applied to investigate the delayed Ps formation, and an evidence for the existence of the delayed Ps formation was successfully observed.

Recovery of 18F from [18O] water by electrochemical method

An electrochemical method for producing 18F sources for the slow positron beam was applied to the recovery of 18F from H2(18)O water. The 18F of activities 150-227 mCi (5.55-8.40 GBq) was electro-deposited on a graphite rod and then emitted into pure water. The best result of the efficiency for the electro-deposition for 5 min was 97% and that for the electro-emission for 5 min was 89%. The H2(18)O water is expected to be reused much more easily by this method than by the ion exchange resin method. The metal impurities contained in the 18F solution were considerably reduced by using this method.

18F intense spot positron source for spin polarized positron beam

Abstract An intense positron source, 18 F , with a half life of 110 min has been developed for spin polarized positron beams. The radioactive 18 F (ca. 30 GBq) is produced in 1 ml 18 O -water target via the 18 O (p,n) 18 F reaction with a 14 MeV proton beam at a current of 20 μA in the AVF cyclotron vault. An irradiated 18 O -water is transferred to an experimental vault downstairs to be trapped and placed just in front of the moderator. The electrodeposition method was used to collect and fix 18 F on a small spot from 18 O -water. Recovery of 18 F on the graphite cylinder of 5 mm ∅ is 69% and 35% for 3 mm ∅ cylinder, respectively.

Measurement of Characteristic X-Rays by Positron Impact

An X-ray detector with thin Si(Li) crystals has been fabricated and employed to detect the characteristic X-rays by positron impact. Use of thin detector crystals is essential for the measurements of the characteristic X-rays induced by positron impact. Otherwise the background produced in the crystals by the annihilation -rays is too large to isolate the X-ray peaks. The data has been analyzed to obtain the inner-shell ionization cross sections by positron impact. Introduction When electrons, ions, or photons with energies higher than the binding energy of inner-shell electrons are incident on a target, characteristic X-rays are emitted by inner-shell ionization. These X-rays are widely used for elemental analysis of materials. The characteristic X-rays are also emitted by positron impact [1-7]. However, the X-rays by positron impact are not used for elemental analysis because a background in the X-ray spectra is far higher than that in the case of other particles. The background is produced by the -rays emitted from positron annihilation in the targets which subsequently deposit part of their energy in the X-ray detector crystal. The background can be suppressed if an X-ray detector with thin crystals is used. In the present work, we have fabricated an X-ray detector with thin Si(Li) crystals and employed it to detect the characteristic X-rays from several targets. We have also measured the X-rays from thin film targets and determined the inner-shell ionization cross sections. X-ray Detector The thickness of the Si(Li) crystals is 0.25 mm, which is one order of magnitude thinner than the crystals of conventional Si(Li) detectors. The effective area was limited to be 20mm 2 to optimize the energy resolution of the detector which depends on the capacitance of the crystal. In order to obtain high efficiency, two crystals of 20 mm 2 area were placed and each crystal was connected to a separate pre-amplifier. The crystals were cooled to 77 K using liquid nitrogen. The energy resolution was 300 eV at 5.9 keV. The background caused in the newly developed detector by the positron annihilation -rays was tested by observing K X-rays from Ti (4.5keV) induced by the -rays. A -ray standard source ( 22 Na) was attached to a Ti plate. (Positrons were confined and annihilated in the source because 22 Na was encapsulated in a lucite plate of 2 mm thickness.) The spectra obtained from *Present address : 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan. Electric address: ynaga@rs.kagu.tus.ac.jp Materials Science Forum Online: 2004-01-15 ISSN: 1662-9752, Vols. 445-446, pp 440-442 doi:10.4028/www.scientific.net/MSF.445-446.440

The design of a spin-polarized slow positron beam

Abstract Spin-polarized low-energy positrons were considered as useful probes for studying electron spin states of both surface and bulk materials. Due to the spin-dependent interactions between electrons and positrons, the formation of positronium (Ps), an electron-positron bound system, can be distinguished from different electron spin states. Recently, a positron source of 18 F has been developed for a spin polarized slow positron beam at the institute of physical and chemical research (RIKEN). The design of an electrostatic positron beam will be discussed in conjunction with a spin rotator.

Positronium-atom/Molecule interactions: Momentum-Transfer Cross Sections and Z eff

An overview is presented of current results on the measurements of the momentum-transfer cross sections for positronium - atom/molecule scattering and the effective numbers of electrons per atom/molecule for positronium pickoff annihilation (1 Z eff).