Sohei Okada

ANNEALING PROCESSES OF VACANCY-TYPE DEFECTS IN ELECTRON-IRRADIATED AND AS-GROWN 6H-SIC STUDIED BY POSITRON LIFETIME SPECTROSCOPY

Annealing processes of vacancy‐type defects in 3 MeV electron‐irradiated and as‐grown 6H‐SiC have been studied by positron lifetime spectroscopy. Vacancy‐type defects giving rise to a positron lifetime of 183 ps were detected in as‐grown n‐type specimens. They were found to be annealed at around 1400 °C and were related to silicon vacancies, possibly complexes of silicon vacancies and nitrogen atoms. Defects related to carbon vacancies, silicon vacancies, and divacancies were found to be created by electron irradiation. The defects related to carbon vacancies and divacancies were found to be annealed up to 500 °C. The defects related to silicon vacancies were found to be annealed at around 750 and 1400 °C. The former annealing stage was inferred to be due to migration of silicon vacancies to internal sinks or nitrogen atoms to form complexes of silicon vacancies and nitrogen atoms. The latter annealing stage was explained as due to annihilations of the complexes as well as the case of as‐grown specimens.

Positron annihilation and ESR study of irradiation-induced defects in silica glass

Abstract The angular correlation of positron annihilation radiation (ACAR), positron lifetime and electron spin resonance (ESR) have been measured on fused and synthetic silica glass samples before and after irradiation with fast neutrons up to a dose of 8.6 × 1018 n/cm2 at about 150°C or with 3 MeV electrons up to a dose of 1 × 1018 e/cm2 below 50°C. The ACAR curves are deconvoluted into two Gaussian components: a narrow and a broad component. The narrow component is due to self-annihilation of ortho-state of positronium (p-Ps) formed in “intrinsic” structural voids with radius of about 0.3 nm, while the broad component comes from pick-off annihilation of ortho-Ps and annihilation of positrons with valence electrons presumably associated with oxygen. Detailed post-irradiation experiments reveal that two kinds of positron trapping centers (defects) are introduced. Both kinds of the defects give almost the same broad component and markedly suppress Ps formation. This suggests that they are oxygen related centers. The first type defects with the positron lifetime of about 0.25 ns anneal out at 400–500°C, while the second type defects with lifetime of 0.47 ns recover after annealing at about 600°C. Electron spin resonance (ESR) measurements were also made on the same samples to detect irradiation-induced paramagnetic center (defects): E′ centers, peroxyradicals (POR) and non-bridging oxygen hole centers (NBOHC). The correspondence between the positron trapping centers and the ESR-active paramagnetic centers is not straightforward, partly because positrons can be trapped not only at the paramagnetic defects but also at diamagnetic defects. However, possible relations between these positron trapping centers and the paramagnetic defects are discussed.

Vacancy production by 3 MeV electron irradiation in 6H-SiC studied by positron lifetime spectroscopy

The vacancy production in 6H-SiC by 3 MeV electron irradiation at room temperature was studied using positron lifetime spectroscopy combined with annealing experiments. It was found that the trapping rates of positrons in vacancies increased linearly with the fluence in the initial stage of irradiation. After the linear increase, the trapping rates were found to be proportional to the square root of the fluence. The linear and nonlinear fluence dependences of the trapping rates are explained by the reduction of vacancies due to recombination with interstitials during irradiation. The positron trapping rate for the admixture of silicon vacancies and divacancies showed a tendency to saturate in the higher fluence range. The trapping rate for carbon vacancies decreased after reaching a maximum. These results are explained in terms of the shift of the Fermi level due to the irradiation process. It was found that, for the lightly irradiated specimen, an annealing stage caused by recombination between close vac...

Production of an intense slow positron beam by using an electron LINAC and its applications

Abstract An image of reemitted positrons has been produced in a proto-type positron transmission-type reemission microscope by using the electrostatic intense slow positron beam from an electron LINAC. RHEPD experiments have then been performed on H-terminated Si(111).

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.

Effect of Mg ion implantation on electrical properties of CuInSe2 thin films

The effects of Mg ion implantation on the electrical properties of CuInSe2 epitaxial thin films have been investigated. The implantation was carried out using the multienergy implantation technique to obtain a constant profile of the Mg concentration along the depth direction. After implantation, the layer was annealed at 400 °C in N2 atmosphere for 60 min. From the results of reflection high-energy electron diffraction, it was confirmed that the damages due to ion implantation were removed by the thermal annealing. The conductivity type in all implanted films was n type, and the carrier concentration was increased with increasing Mg concentration in the films. Consequently, it is concluded that the Mg atom acts as a donor in CuInSe2.

Positron annihilation and ESR studies of electron- or neutron-irradiated silica glass

Abstract Fused and synthetic silica glass samples were irradiated with 3 MeV electrons up to a dose of l × 1018 e/cm2 or with fast neutrons up to a dose of 8.3 × 1018 n/cm2. Before and after irradiation, positron lifetime and electron spin resonance (ESR) were measured. Positron lifetime spectra were decomposed into three components with the time constant τi (i = 1, 2, 3) and their relative intensities Ii I1 + I, + I3 = 1). Before irradiation, two lifetime components derived from parapositronium (p-Ps) τ1 and ortho-positronium (o-ps) (τ3) have been observed. The intensity I1 is almost the same as one-third of I3, which is consistent with the spin-state statistics of Ps between o-Ps and p-Ps. After irradiation, τ1 increased slightly and I1 exceeds one third of I3. This suggests positron trapping at irradiation-induced defects (type I defects). τ2 shows a marked decrease at a low dose (6.9 × 10−5 dpa) irradiation but attains a constant value of 0.47 ns for the higher dose irradiation. This suggests that τ2 is due to trapping at another kind of irradiation-induced defects (type II defects). In ESR spectra of the irradiated samples, three kinds of paramagnetic defects were observed. Positron lifetime and ESR measurements were also made after post-irradiation isochronal annealing. Detailed comparison between positron lifetime and ESRspectra is made to clarify possible positron trappin at the irradiation-induced paramagnetic defects with give ESR signals.

Chemical stress relaxation of ethylene-propylene copolymer rubber by heat and radiation

Abstract In an attempt to shorten the evaluation time for the deterioration under various conditions caused by chemical reactions, we tried to extend the time-temperature superposition principle for the stress relaxation of rubber. In the case of deterioration by radiation instead of by heat, a time-dose rate reduction is proposed and the master curves obtained for chemical-stress relaxation of rubber. A new method which contains a numerical analysis of stress decay curves is proposed to obtain the rate of crosslinking and scission under irradiation for already crosslinked samples.

Development of a new Monte Carlo simulation system on positron behavior in matter

Abstract We have developed a new Monte Carlo simulation system named EGS4-SPG to design devices with a high efficiency for slow positron generation. The system is applicable for various processes in a wide range of energies. Applications are demon-strated for brightness enhancement, thermalization depth profiling and slow positron generation by energetic positron and photon bombardment onto multiple moderator assemblies.

The deterioration of mechanical properties of chloroprene rubber in various conditions

The changes of mechanical properties of chloroprene rubber, used as a jacket material for electric cables in nuclear power generating stations, were investigated under various deterioration environments. The heat resistant properties of the rubber material were measured at various temperatures using a stress relaxation method. From the view point of stress-strain behaviour and chemorheology, the addition of antioxidant and/or antirad reagent to the rubber improves the resistance to heat and/or radiation, respectively. The effects of dose-rate and surrounding atmosphere on the mechanical properties of rubber under radiation were also studied. The radiation damage of rubber was accelerated remarkably under a pressure of 90 atm of air. A vibration fatigue test under heat and radiation was also performed.