Toshio Hyodo

MOMENTUM-TRANSFER CROSS SECTION FOR SLOW POSITRONIUM-HE SCATTERING

The momentum-transfer cross section for positronium-He scattering has been determined by analysing the thermalization process of orthopositronium in the gas. The momentum distribution of the positronium is observed by using the one-dimensional angular correlation of the annihilation radiation method. Silica aerogel is used to form a sufficient amount of positronium in a restricted region necessary for the high-resolution measurements. The average energies of orthopositronium for mean lifetimes ranging from 3.2 to 86 ns are determined by applying static magnetic fields. A thermalization model which includes the momentum-transfer cross section as an adjustable parameter is fitted to the average energy. The cross section thus obtained is for positronium in the energy range below 0.3 eV.

Enhancement of Annihilation Rate of Electron-Positron pairs in Periodic Fields

The enhancement of the partial annihilation rate of electron-positron pairs in crystals has been examined by assuming a two-band structure of electrons and using the lowest-order ladder graphs with static interaction. The intraband transitions of electrons due to the attractive force of positrons cause the enhancement of the rate over all the components of the momentum, while the interband transitions from the lower to the upper give a marked dehancement of HMCs (higher-momentum components) despite that the total annihilation rate remains still enhanced. Numerical computation has been made for several one-dimensional cases, and an asymmetry with respect to the zone face has been found even in the enhancement due to intraband transitions. An angular-correlation measurement has been made for silicon, and an evidence for the dehancement of HMCs has been found by comparing the date with the independent-particle theory of Stroud and Ehrenreich (Phys. Rev. 171 (1968) 399).

Evidence for Positronium-Like States in NaF and NaCl

The angular correlation of photons from positrons annihilating in NaF and NaCl at 300 K and 84 K has been measured. Narrow component which is characteristic of the annihilation of electron-positron pairs in a positronium-like statel has been observed on the angular correlation curves for the two alkali halides at 84 K.

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.

An Examination of the Angular Correlation of Annihilation Radiation in Silicon Single Crystal

The angular correlation of annihilation radiation along the [110]-axis in silicon has been measured at 120∼130 K with a special care in orienting the crystal. As predicted by a recent many-body theory of the annihilation of free positrons in a simplest nearly-free-electron system (K. Fujiwara, T. Hyodo and J. Ohyama: J. Phys. Soc. Japan 33 (1972) 1047), the cut-off of the narrow component at the angle corresponding to the filled zone face has been found to be sharper than that given by the independent-particle computation of Stroud and Ehrenreich.

Brightness enhanced intense slow positron beam produced using an electron linac

An intense pulsed slow positron beam was produced from a 100 MeV electron linac of the Japan Atomic Energy Research Institute and was extracted using solenoid transport tubes. The intensity of the obtained slow positron beam was 2–3×107 e+/s at 110 of the full power operation (at 10 μA average electron beam current). In order to use the beam for positron scattering, diffraction and microscope experiments, it was transferred from the solenoid magnetic field to a field-free region, and was then brightness-enhanced. The final beam size was reduced from 10 mm o in the solenoid magnetic field to 0.5 mm o after two stages of remoderation.

Conversion of ortho-positronium in low-density oxygen gas

Conservation of ortho-positronium in oxygen has been investigated by the high-resolution 2 gamma angular correlation method. The oxygen pressure was low enough to suppress possible conversion of para-positronium to ortho-positronium. Silica aerogel was used to form a sufficient number of positronium atoms in a restricted region necessary for the high-resolution angular correlation measurements. The existence of an inelastic conversion process has been reconfirmed. The cross sections for the elastic and inelastic conversion processes have been estimated to be (1.0+or-0.3)*10-19 cm2 and some 10-17 cm2, respectively.

Positronium in Alkali Halides

Momentum distribution of annihilation γ rays from several alkali halide crystals has been measured using high resolution 1D angular correlation apparatuses. Formation of Bloch-type positronium (Ps) in NaBr and RbCl has been newly confirmed, while no evidence for it has been seen in LiF, KF, and CsI. Effective mass of the Ps in the Bloch state has been determined by analyzing the low temperature behavior of the Ps momentum distribution. The effective mass is larger than the free Ps mass in vacuum in all the alkali halides investigated. Detailed analysis of the temperature dependence of the Ps momentum distribution has been made on KI and KBr. Temperature induced transition to the self-trapped state as previously reported in NaF and KCl has been observed also in these materials.

Intense positron beam at KEK

Abstract A positron beam is a useful probe for investigating the electronic states in solids, especially concerning the surface states. The advantage of utilizing positron beams is in their simpler interactions with matter, owing to the absence of any exchange forces, in contrast to the case of low-energy electrons. However, such studies as low-energy positron diffraction, positron microscopy and positronium (Ps) spectroscopy, which require high intensity slow-positron beams, are very limited due to the poor intensity obtained from a conventional radioactive-isotope-based positron source. In conventional laboratories, the slow-positron intensity is restricted to 10 6 e + /s due to the strength of the available radioactive source. An accelerator based slow-positron source is a good candidate for increasing the slow-positron intensity. One of the results using a high intensity pulsed positron beam is presented as a study of the origins of a Ps emitted from SiO 2 . We also describe the two-dimensional angular correlation of annihilation radiation (2D-ACAR) measurement system with slow-positron beams and a positron microscope.

Position sensitive scintillation detector for γ-rays using a GSO crystal

Abstract We report the development of a γ-ray imager using a 5 mm thick, 50 mm diameter Gd 2 SiO 5 (GSO) crystal coupled to a 2D position-sensitive photomultiplier tube (Hamamatsu R2486). A considerable improvement in the position linearity and resolution was achieved upon placing around the GSO scintillator a glass ring which has the same thickness and the same index of refraction as the GSO crystal. A position resolution of 2.2 mm FWHM was obtained at 511 keV.