[en] Red light-emitting diodes (LEDs) with both a conventional bilayer structure and a double heterostructure (DH) have been investigated. In these LEDs, N,Nprime-bis-(1-naphthl)-diphenyl-1, 1prime-biphenyl-4,4prime-diamine (NPB), 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), and tris(8-quinolinolato) aluminum (Alq₃) were used as hole-transporting, hole-blocking, and electron-transporting layers, respectively. The bilayer and DH LEDs had a configuration of ITO/NPB/Alq₃:red dopant/Alq₃/MgAg and ITO/NPB/Alq₃:red dopant/BCP/Alq₃/MgAg, respectively. Three kinds of red fluorescent dyes -- nile red, DCJTB, and DCM -- were used as dopants. Compared with the bilayer structures, the luminance efficiencies of the DH LEDs were found to increase as much as 100%. We attribute the efficiency enhancement to the formation of a narrow recombination zone, in which both charge carriers and excitons were confined. High charge concentrations in the emissive layer resulted in efficient collision capture in the electron--hole recombination process. Exciton confinement led to improved energy transfer. The two factors were simultaneously operating and consequently benefitted from efficiency enhancement. copyright 2001 American Institute of Physics

[en] Cross-sectional transmission electron microscopy is used to examine silicon films deposited at 580 and 620⁰C in a low-pressure chemical vapor deposition system. The films are implanted with phosphorus and annealed at 800⁰C. The diffused phosphorus profiles are measured with secondary ion mass spectrometry and show very little enhanced grain boundary diffusion. The transmission electron microscopy shows amorphous regions remain in the films deposited at 580⁰C. These regions appear to be due to the extremely slow velocity for grain boundary migration during primary recrystallization. It is possible these amorphous regions are connected with the lack of a diffusion tail. The TEM results are also used to explain the electrical properties of the silicon films. (author)

[en] It has been found possible to utilize the plasma in the discharge lamp of a conventional ultraviolet photoelectron spectrometer as an electron source for low-energy electron impact excitation. The Nsub(4,5)OO Auger spectra of Xe, and Msub(4,5)NN Auger spectrum of Kr, and the autoionization electron spectra of K and Na vapor have been recorded with a Perkin-Elmer PS-18 photoelectron spectrometer. By comparing these spectra with those excited by filament electron guns the advantages and disadvantages of the two are illustrated. The characteristics of the discharge electron source were deduced, and some possible improvements in its performance are suggested. The autoionization electron spectrum of Na is analyzed by comparison with the data from photoabsorption and Na⁺-He collisions studies and the L₁Lsub(2,3)M₁ and L₁Lsub(2,3)N₁ Coster-Kronig transitions are identified by using equivalent-core approximations. (Auth.)

[en] A series of experiments was carried out to investigate the bubble nucleation to lift-off phenomena for subcooled boiling flow in a vertical annulus channel. A high speed digital video camera was used to capture the dynamics of bubbles. The bubble lift-off diameter and the bubble nucleation frequency were evaluated in terms of heat flux, mass flux, and degree of subcooling. The fundamental features of the lift-off diameter and the nucleation frequency (i.e., the variations across nucleation sites and the dependence on the flow and heat flux conditions) were addressed based on the present observation. A database for the bubble lift-off diameter was built by gathering and summarizing the data of Prodanovic et al., Situ et al., and the present experiments. We evaluated the predictive capabilities of Unal's model, Situ et al.'s model, and Prodanovic et al.'s correlation against the database. We obtained the best prediction results through modifying the wall superheat correlation in Unal's model. In addition, we suggested a new correlation for a combined parameter of the bubble nucleation frequency and the bubble lift-off diameter

[en] The APR+ (Advanced Power Reactor) adopts several new safety features and the PAFS (Passive Auxiliary Feedwater System) is one of the advanced safety features which can cool down the nuclear reactor without any external power supply in case of accidents. For validation of the cooling and operational performance of the PAFS, PASCAL (PAFS Condensing Heat Removal Assessment Loop) facility was constructed and experimental investigation of the condensation heat transfer and natural convection phenomena in the PAFS was experimentally investigated at KAERI (Korea Atomic Energy Research Institute). From the experimental result, it was found that a thermal hydraulic system analysis code, MARS-KS (Multi-dimensional Analysis of Reactor Safety), underestimated the condensation heat transfer coefficient compared to the experimental data. Referring to the previous condensation heat transfer models in the literatures, a new condensation heat transfer model for the nearly horizontal tube, especially for the PCHX (Passive Condensate Heat Exchanger) in the PAFS, was proposed. In this study, the modified correlation using the analytic slip ratio model was evaluated with the experimental data. The condensation heat transfer correlation using the slip ratio model for the horizontal heat exchanger tube was developed to model the heat transfer phenomenon inside the PCHX tube. The proposed slip ratio model showed improved prediction performance around 14% of the error. To increase the prediction capability of the condensation heat transfer correlations developed in this study, more experimental data in the wide ranges should be compared as the further study

[en] Oriented states of atomic krypton and xenon have been produced and found to exhibit quantum beats in fluorescence, using pulsed vacuum-ultraviolet synchrotron radiation. This experiment demonstrates for the first time the feasibility of studying a number of atomic and molecular properties using synchrotron radiation, in direct analogy with in-beam accelerator based angular distribution studies of nuclei. (Auth.)

[en] To evaluate the reactor thermal margin of APR+, reactor core flow distribution including both axial and lateral directional hydraulic resistances of fuel assemblies should be known. Test facility on reactor core flow distribution (ACOP) is being designed and constructed. ACOP will use 257 core simulators and each core simulator should have the same core flow characteristics of the PLUS-7 fuel assembly. However, data on the lateral directional hydraulic resistance of full-size fuel assemblies is rare while data on the axial resistance is currently available. To design a core simulator which will be used at the ACOP test facility, a 3-Ch cross flow test facility is being designed and constructed with three full-size fuel assemblies. The axial and lateral directional hydraulic resistances of fuel assemblies will be measured. The test results will be applied to the design of the core simulator of the ACOP. The experimental data can also be utilized for the sub-channel analysis by upgrading the CFD model

[en] DVI and FD are our own design features adopted into APR1400. This study focuses on improving their performance for the APR+. To minimize direct bypass rate of safety injection flow to the break, core barrel ducts are introduced. The performance of the core barrel ducts is evaluated by using safety analysis code. A performance change due to adaptation of four EDG design is also studied in connection to the core barrel duct study. For FD, design features to prevent early inflow of the N2 gas and to extend the injection period are studied

[en] A preparation(P.P-I) of herb mixture was designed to protect gastrointestine, hematopoietic organs and immune system against radiation damage. The herb preparation was water extract of herb mixture(Agelica gagantis Radix, Cnidii Rhizoma and Paeonia Radix) added with its polysaccharide fraction. In the present experiments, the ability of the preparation to protect hematopoietic and immune system was assessed in mice irradiated with ⁶⁰Co γ-rays. The administration of P.P-I increased regeneration of blood cells and splenic lymphocytes in irradiated mice. In the administration mice, both B and T cells of lymphocytes were repopulated to normal level following irradiation, the response of repopulated lymphocytes to mitogen was recovered and the production of antibody against injected antigen was increased. These results indicated that the preparation protect hematopoietic organs and immune system against radiation damage, and that enhanced immunity. Since the preparation is a relatively nontoxic natural product, it might be a useful radioprotector

[en] Due to their potential as optoelectronic devices, luminescing nanostructures have been among the most studied in the recent past. Room-temperature UV lasing has been demonstrated in ZnO nanowires. For highly asymmetric wurtzite structures, the orientation of the emitting luminescent dipole with respect to the excited state polarization can play a role in the luminescence yield. ZnS is an important, wide bandgap (E_g = 3.54 eV for the thermodynamically stable zinc blende form at room temperature) II-VI semiconductor. It has been developed for a number of applications including UV light-emitting diodes, injection lasers and phosphors. In this presentation we will discuss results of a study on ZnS nanostructurees using synchrotron-radiation-based, x-ray-excited optical luminescence (XEOL). Results on ZnO will be presented elsewhere. The experimental approach has been described previously. All measurements were performed on beamline 4-ID-C at the Advanced Photon Source. Samples were prepared by a high-temperature growth technique described previously. Briefly, ZnS powder was placed in the center of a horizontal alumina tube upstream of a Si wafer, which was covered with 2 nm thiol-capped gold nanoparticles (used to catalyze the growth). The tube was heated to 1000 C while an Ar/H₂ gas mixture flowed through the tube. This process resulted in the formation of nanoribbons of lengths in the range 10-100 (micro)m and widths less than 100 nm. The samples were characterized by high-resolution TEM images, which showed large areas of hexagonal wurtzite structure interspersed by nanosized regions with cubic sphalerite structure. Using XEOL, we have determined the local phase of the luminescing sites in ZnS nanowires. The inset of the accompanying figure shows the temperature-dependent optical spectrum obtained when exciting the nanowires with 1100 eV x-rays. There are three main peaks: a band-edge, exiton state at 338 nm, a defect-related emission at 430 nm, and a Au-related feature at 520 nm. (Au is used as a catalyst to grow the wire.) By monitoring the yields of each of these peaks as a function of energy at the Zn L edge, we are able to determine the phase associated with them. The x-ray fluorescence (XRF) and 338 nm peak has a wurtzite signature, while the 430 and 520 nm curves resemble sphalerite. These results show that the 338 nm exciton state originates in the dominant, wurtzite phase of the wire, while the other two peaks occur in dilute (∼0.1%) regions of sphalerite.