Neutron Irradiation Effects of Iron Alloys and Ceramics

Positron annihilation angular correlation measurements have been performed for the neutron irradiated various metals and ceramics in order to obtain the information of the microvoids and positronium formation in them. Positronium (Ps) formation was observed in Nb containing a small amount of oxygen and Fe-15%Cr-16%Ni-0.006%B^. In practical steels such as JPCA and JFMS no Ps formation was observed. High temperature deformation might induce microvoids into metals, but the positron annihilation angular correlation measurements could not confirm this. In non-metallic materials neutron irradiated no Ps formation has so far been observed

Positron Annihilation Study of Irradiated HOPG-type Graphite

Positron annihilation angular correlation measurements (ACAR : angular correlation of annihilation radiation) were made for HOPG (highly oriented pyrolytic graphite) and PGCCL (HOPG-type graphite, Le Carbone-Lorraine) irradiated by various particles, such as electrons, neutrons and deuterium particles. The ACAR measurements made under the condition of P_Z//c-axis for unirradiated specimens showed a clear minimum at θ=0°, but P_Z⊥c-axis measurement did not show this minimum. This minimum is considered to be due to the annihilation of positrons with π electrons the momentum distribution of which is extended to the direction parallel to c-axis. On the other hand, by neutron irradiation this minimum disappeared, which suggests that positrons are trapped at radiation-induced defects, probably vacancy sites, and the probability of the annihilation with π electrons was decreased to the large extent. But, in the case of hydrogen irradiation the minimum was not affected so much, which suggests hydrogen atoms do not disturb the π electrons, probably because hydrogen atoms are trapped at boundary regions, such as those between adjacent crystallites or region of stacking disorder

Possible activation by the green tea amino acid theanine of mammalian target of rapamycin signaling in undifferentiated neural progenitor cells in vitro

AbstractWe have shown marked promotion of both proliferation and neuronal differentiation in pluripotent P19 cells exposed to the green tea amino acid theanine, which is a good substrate for SLC38A1 responsible for glutamine transport. In this study, we evaluated the activity of the mammalian target of rapamycin (mTOR) kinase pathway, which participates in protein translation, cell growth and autophagy in a manner relevant to intracellular glutamine levels, in murine neural progenitor cells exposed to theanine. Exposure to theanine promoted the phosphorylation of mTOR and downstream proteins in neurospheres from embryonic mouse neocortex. Although stable overexpression of SLC38A1 similarly facilitated phosphorylation of mTOR-relevant proteins in undifferentiated P19 cells, theanine failed to additionally accelerate the increased phosphorylation in these stable transfectants. Theanine accelerated the formation of neurospheres from murine embryonic neocortex and adult hippocampus, along with facilitation of both 5-bromo-2’-deoxyuridine incorporation and 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide reduction in embryonic neurospheres. In embryonic neurospheres previously exposed to theanine, a significant increase was seen in the number of cells immunoreactive for a neuronal marker protein after spontaneous differentiation. These results suggest that theanine activates the mTOR signaling pathway for proliferation together with accelerated neurogenesis in murine undifferentiated neural progenitor cells

Analysis of the Thermal Activation of High-Mobility Dislocation Loops

We report thermally activated transport of highly mobile dislocation loops in terms of a line tension model where the dislocation loops are assumed to be a flexible string. The activation energy and transition rate are calculated on the basis of a classical rate theory. The activation energy merely increases with the length of the dislocation loops. However, the activation process and temperature dependence of the transition rate qualitatively change at a critical length L c . If the dislocation loops are longer than the critical length, the thermal activation occurs through the conventional double-kink formation process on the dislocation lines. On the other hand, if the dislocation loops are shorter than that, the saddle point configuration is not the double-kink type but non-deformation one. Therefore, the critical length L c is a plausible criterion for the dislocation loops to distinguish dislocation like from point-defect like in size

Positron Annihilation Study of Radiation Defects in Zinc Oxide

Positron annihilation studies have been carried out to clarify the radiation induced defects in ZnO single crystals. Vapor-grown ZnO crystals were irradiated at 77 K with 28 MeV electrons. Before irradiation as-grown specimens showed the mean positron lifetime in the range of 160-195 ps. Electron irradiation increased the mean positron lifetime up to 205-210 ps.This long-lifetime disappeared until 473 K during successive isochronal annealing steps. The radiation-induced coloration was anneale out around 523 K. During this stage, the radiation induced oxygen vacancies are thought to be moved away and disapeared. The second recovery stage at around 823 K is thought to be caused by the recovery of small ZnO interstitial clusters fomed through clustering of interstital atoms

Positron Annihilation Study of Irradiated Iron Alloys

Positron annihilation lifetime measurements and angular correlation measurements have been performed for pure Fe and Fe alloys to investigate the fundamental behaviors of radiation induced defects and alloying elements. It was found that Cu atoms interact with vacancies and form complexes of Cu and vacancies, which can be considered to be radiation enhanced precipitation of Cu atoms in Fe matrix. Angular correlation measurements of neutron irradiated Fe-Cr-Ni alloys and fatigued 316 ss showed that some vacancy clustering occurred. Calculation of positron annihilation lifetime was performed for a vacancy in Fe on the basis of local density approximation (LDA) with and without lattice relaxation and showed that lattice relaxation considerably reduced the lifetime, resulting in the reasonable fitting with the experimental value