Nagayasu Oshima

Radiation effect on polypropylene studied by the relaxational behaviour at low temperature using positron annihilation

Abstract The irradiation effect of γ-rays on polypropylene (PP) has been studied through relaxational behaviour, using the positron annihilation lifetime (PAL) method. At low temperature, the intensity of a long-lived component of positronium, I3, for unirradiated PP samples increased due to a termination of the local thermal motion of polymer structures. At 100 K, the increase in I3 of 1 MGy γ-ray irradiated samples became very small. However, the increase was observed again in the PP samples after 48 h of irradiation by positrons during the PAL experiment. This suggests a restructuring of the short polymer chains cut by γ-ray irradiation may occur.

In-situ characterization of free-volume holes in polymer thin films under controlled humidity conditions with an atmospheric positron probe microanalyzer

A pulsed, slow positron beam, with a diameter of 200u2009μm, was extracted into air through a thin SiN window of an atmospheric positron probe microanalyzer (PPMA), and used to measure the ortho-positronium lifetimes τ in polyvinyl alcohol and polycaprolactam sub-μm-thick films. By measuring the variation of τ as a function of relative humidity, the effect of water molecules on the hole sizes, deduced from τ, was examined for the films with consideration to the chain mobility. The results demonstrate the usefulness of the atmospheric PPMA to the in-situ characterization of nanoscopic holes in thin films under practical conditions.

Design of a high-efficiency short-pulsed positron beam system

Abstract A conceptual design of a new pulsing system for slow positron beams is described. Although the principle resembles the conventional pulsing system using rf power, the present method applies ideal waveform to the moderator for adjusting the time-of-flight of positrons using a fast arbitrary waveform generator. Requisites for the new design are the monochromaticity (less than about 1 eV) of the initial dc positrons and the ultra-fast precise waveform to be applied to accelerate the positrons. It is shown that these requisites are within the present status of art of positron technology and electronics. A preliminary result on the monochromaticity of the positron beam is also presented.

Positron-Annihilation Studies of Frank-Kasper-Type Quasicrystals of the Mg–Zn–Ga–Al and the Mg–Zn–Rare-Earth System

We have performed positron-annihilation experiments for a Frank-Kasper (F-K)-type icosahedral quasicrystal, 2/1 and 1/1 approximant crystals of Mg–Zn–Ga–Al, and the stable F-K-type icosahedral quasicrystals of Mg–Zn–RE (RE=rare-earth metal, Y, Ho and Gd) alloys. It is shown that a high density of vacant centers in triacontahedral clusters exists in both icosahedral and approximant phases of Mg–Zn–Ga–Al. Furthermore, in the present study, we report a very interesting finding that stable non-Al-based quasicrystals such as the icosahedral quasicrystal Mg–Zn–RE system also contain a high density of structural vacancies, similar to the stable Al-based quasicrystals. In particular, it is estimated experimentally that the structural vacancy densities for icosahedral quasicrystal Mg34Zn58Y8 and Mg40Zn52Ho8 are 3 ×1020 and 4 ×1020 cm-3, respectively. These results provide important information on the structural modeling of the stable icosahedral quasicrystal Mg–Zn–RE, which is a recent research topic of the icosahedral quasicrystals.

Development of a pulsed slow-positron beam using time-varying pulsing-bias

Abstract In order to study the near-surface characterization of polymers using positron-annihilation lifetime spectroscopy (PALS), a pulsed slow-positron beam system is under development. Positrons emitted from 22 Na were injected periodically into a target by adjusting the time of flight of positrons between the moderator and the target using a time-varying electric field. By increasing the slope of the pulsing bias, the time resolution of this system, a full width at half maximum (FWHM) of 0.82 ns has been achieved. The lifetimes of ortho-positronium (o-Ps) in high-density polyethylene (HDPE) and polytetrafluoroethylene (PTFE) were obtained as 2.5 and 4.1 ns, respectively, by PALS using our new system. These values agree with those obtained by the conventional PALS.

Application of a pulsed slow-positron beam to polymers

Pulsed slow positrons were produced using a time-varying moderator bias with an interval of 82 ns; 97% of the positrons were compressed within 2 ns width at the target position. Both the positron annihilation lifetime and Doppler broadening of the positron annihilation radiation (DBPR) of polytetrafluoroethylene (PTFE) were measured as a function of the incident energy of slow positrons. It was shown that the lifetime and intensity of the long-lived component of positron annihilation are independent of the positron incident energy above 1.2 keV. However, the width of the Doppler-broadened annihilation γ-ray increased in the energy region below 1.2 keV.

Free volume of cyanate resins studied by positron annihilation

The polymerization process of bisphenol-A dicyanate (BADCy) has been studied using a positron-annihilation lifetime technique (PAL). The polymerization was conducted at 150°C, and the process was followed by PAL. Seven kinds of samples with different curing times were also formed at 150°C, and the relation between the period of the curing time and the degree of polymerization was studied. It has been shown that theo-Ps lifetime increases in samples with a higher polymerization than 85%, which is consistent with measurements of the specific volume of BADCy.

Design Study of Linear Accelerator-Based Positron Re-Emission Microscopy

The positron re-emission microscope (PRM) [1,2] images positrons that are re-emitted from surface after thermalization and diffusion of positrons implanted into material. By using PRM, it is able to nondestructively observe spatial distribution of vacancy-type defects at atomic levels below the detection limits of electron microscopy. Compared with scanning positron microscopy, PRM potentially has better lateral resolution with faster imaging time by more than one order of magnitude. However, in the previous studies of PRM [1-3], a long imaging time (more than 8 hours) was needed to acquire one image because radioisotope (RI) was used as a positron source. For practical use, a substantial reduction of the imaging time is required.

Residual Defects in Low-dose Arsenic Implanted Si after High-temperature Rapid Thermal Annealing: Their Behavior and Influence on CCD Image Sensors

Abstract The behavior of the residual defects in low-dose (10 cm) arsenic implanted Si after high-temperature (1100 °C) annealing and their influence on the properties of CCD image sensors have been investigated. The presence of residual damage was shown when using rapid thermal process as heat treatment. This damage was identified as vacancy-type defects (V4), and transformed to be oxygen-related defects (CiOi, VmOn) combined with oxygen introduced in annealing chamber. These defects made a critical impact on the transfer efficiency and white defects of CCD image sensors.

Positron Trapping Sites Originating from Oxide Interfaces on 4H-SiC C(0001)- and Si(0001)-Faces

Gate oxide layers grown by dry and wet oxidation on 4H-SiC C(0001)- and Si(0001)-faces have been measured by positron annihilation spectroscopy. The incident positron energy, which corresponds to the peak of Doppler-broadening W-parameter curves, was chosen to characterize the SiC/oxide interfaces. Positron trapping sites with lifetimes in the range of 0.16–0.32 ns were detected, and the lifetimes for the C(0001) face were longer than those for the Si(0001) face. Age–momentum correlation measurements revealed that the origin of these trapping sites is neither the SiC bulk nor the SiC vacancies, suggesting that defective oxide layers exist at the interface.