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The Korea Institute of Geoscience and Mineral Resources (KIGAM) is constructing a reference neutron field facility as a national project. Neutron fields consist of mono-energetic sources of 144 keV, 250 keV, 565 keV, and 2.5 MeV have a fluence range from 102 neutrons/cm2/sec to 103 neutrons/cm2/sec. The systems for the reference neutron fields, such as a duo-plasmatron ion source, a 4-MHz beam bunching system, a neutron chamber, an irradiation room, a neutron time-of-flight (n-TOF) system, a long-counter, and a sample moving system, were designed and fabricated. The neutron energies of the reference neutron fields and their spreads were observed by using the n-TOF system. The neutron fluence was measured by using a long-counter for energies below 1 MeV and a proton-recoil counter for 2.5 MeV. The long-counter efficiency was calibrated by the Japan Atomic Energy Agency (JAEA) which had a traceability of mono-energetic neutron sources to both Japanese and international standards. The efficiency of the proton-recoil counter was obtained by using a calculation with detailed construction information.

A TOF system was also constructed to measure neutron cross section on a natural sample of Y at the energy range from 0.7 MeV to 1.2 MeV. The repetition time and the beam width of the proton pulsed beam were obtained to be 125 ns and about 2 ns, respectively. The neutron total cross section of 89 Y which was measured by this facility was compared with that of ENDF. In this report, an overview of the facility with its current status and prospects for the future will be presented.

Transfer of GaAs layers onto Si by helium ion implantation at room temperature and direct wafer bonding was investigated. The in uence of the ion energy (100 keV and 500 keV), the ion uence and the subsequent annealing condition on exfoliation and/or blistering was studied by using optical microscopy, Rutherford backscattering and channeling spectrometry (RBS/C), X-ray di raction (XRD) and cross-sectional transmission electron microscopy (XTEM) analysis and the optimum conditions for achieving thin layer exfoliation only after post-implantation annealing were experimentally determined. Our results suggest that the optimum uence for the GaAs ion-cut is in the range of (2 { 5) 1017 He+/cm2. Thin 400 GaAs layers were successfully transferred onto silicon after bonding of helium-implanted GaAs and Si wafers via a silicon-oxide intermediate layer and subsequent low-temperature splitting annealing at less than 220 C. Field emission scanning electron microscopy (FE-SEM) showed that a uniform transfer of the GaAs layer onto Si was obtained with no detectable defects at the GaAs/SiO2/Si bonding interfaces. XTEM observations of the as-split GaAs layers revealed that the relatively undamaged layer close to the intermediate layer could be extended by appropriate choices of the bonding process and/or implanted ion energy.

Powder cores were produced from gas- and water-atomized powders by cold pressing at a pressure of 1.5 GPa. Structure, morphology and thermal stability analyses of the powders were carried out. The magnetic properties of the cores were examined at high frequency. Gas-atomized powder cores exhibit excellent high-frequency characteristics, which are a stable permeability up to 50 MHz, a quality factor of maximum value 95 at a frequency of 400 kHz and a core loss of 650 mW/cm 3 at f = 50 kHz and B m = 0.1 T. It is considered that the excellent magnetic properties of FeCuNbSiB powder cores are due to the fact that atomized powders with nanocrystalline structure show good softness in magnetic properties and important fabrication conditions like an insulating layer and extremely high compacting pressure are well controlled.

Amorphous FeBSiNb alloy ribbons having bulk glass forming ability and high saturation magnetic flux density were produced by single-roller melt spinning apparatus in the thickness range of 22∼102㎛. With the increase of thickness, the coercive force and squareness ratio decreased, while maximum permeability and AC permeability increased. However electrical resistivity was almost constant. Furthermore refined and complex magnetic domain structure was observed in thicker ribbons owing to the change in internal magnetic anisotropy. For the alloy with the thickness of 81㎛, small coercive force of 24 mOe and high effective permeability of 12,000 at 1㎑ were obtained, those are considered to be better comparing to the conventional soft magnetic amorphous alloys (∼20 ㎛). The good soft magnetic properties of the thick FeBSiNb amorphous alloys were attributed to the decrease in surface pinning effect during wall motion, appearance of perpendicular anisotropy and resulted domain refinement.

We investigated the Si-doped InAs quantum dots (QDs) grown by molecular beam epitaxy and the annealing effects on the QD size distribution through photoluminescence (PL) spectroscopy. A double-peak feature in PL was observed from as-grown InAs QDs with Si-doping, and excitation intensity dependence of PL indicated that the double-peak feature is related to the ground-state emission from InAs QDs with bimodal size distribution. The PL spectrum from Si-doped InAs QDs subjected to annealing treatment at 800°C in nitrogen ambient showed three additional PL peaks and blue-shift of the double-peak feature observed from as-grown sample. The excitation-intensity-dependent PL and consideration of thermal stability of carriers through temperature-dependent PL measurement demonstrated that three additional peaks come from the InAs QDs with three new branches of QD occurring during annealing process.

InAs epilayers with thicknesses of 400, 500, 750, and 1500 nm were grown on GaAs by molecular beam epitaxy and their properties were investigated by reflection high-energy electron diffraction and photoluminescence (PL). For all InAs epilayers, the PL peak position measured at 10 K is blue-shifted from that of bulk InAs, which could be largely due to the residual strain in the epilayer. InAs epilayers have PL peaks red-shifted to lower energy as the thickness of InAs layer is increased. This is the first observation on PL of sub-μm thick InAs epilayers grown on GaAs substrate. While the PL peak position of 400 nm thick InAs layer is linearly blue-shifted toward higher energy with increase in excitation intensity ranging from 10 to 140 mW, those of thicker InAs epilayers are gradually blue-shifted and then, saturated above a power of 75 mW.

Fe-6.5% Si alloys have good magnetic properties due to their high electrical resistivity, very low magneto-striction, and low crystalline anisotropy. Despite their strong potential, these alloys have seldom been used in magnetic applications because of the very poor ductility of Si-steel above 3.0 wt% Si [1-4]. It is difficult to achieve compressed Fe-6.5% Si powder cores with excellent properties because of the low density due to poor ductility. In compressed powder cores, high density is essential in order to obtain high magnetization and permeability. In this study, an attempt was made to produce Fe-3%Si powder cores because the Fe-3.0 wt% Si alloys have relatively good magnetic properties and room temperature ductility. Gas atomized Fe-3.0 wt% Si powder was compressed into toroid shape cores. By reducing the Si content to 3.0 wt%, the hysteresis loss could be greatly reduced and thus the total core loss could be minimized. The total core loss is 600 ㎽/㎤ at 0.1 T and 50 ㎑.

Low temperature IR photoluminescence (PL) spectroscopy has been performed on InAs/GaAs epilayers with the thickness of 0.4, 0.5, 0.75 and 1.5 micrometers grown by molecular bema epitaxy for potential use as active layers in InAs-based IR lasers. A bulk InAs grown by Bridgeman technique is included for comparison with InAs epilayers. For the bulk InAs, three broad near-band-edge emission peaks are observed at 417 meV, 403 meV, and 383 meV at 10 K. Temperature and excitation power dependence of PL intensity give evidence that they may be related to band-to-band and/or exciton emission, conduction band-to-acceptor, and DAP recombination, respectively. For InAs/GaAs epilayers, the PL intensities of emission peaks are much smaller and emission peaks appear at higher energy side with maximum shift of about 5 meV, compared with the bulk sample. This blue shift can be attributed to strain effect due to large lattice mismatch between InAs epilayers and GaAs subset rates. As the thickness of epilayer increases the peak shift becomes small, which implies that the strain effect is reduced.

We have studied infrared photoluminescence (PL) and x-ray diffraction (XRD) of 400 nm and 1500 nm thick InAs epilayers on GaAs, and 4 nm thick InAs on graded InGaAs layer with total thickness of 300 nm grown by molecular beam epitaxy. The PL peak positions of 400 nm, 1500 nm and 4 nm InAs epilayer measured at 10 K are blue-shifted from that of InAs bulk by 6.5, 4.5, and 6 meV, respectively, which can be largely explained by the residual strain in the epilayer. The residual strain caused by the lattice mismatch between InAs and GaAs or graded InGaAs/GaAs was observed from XRD measurements. While the PL peak position of 400 nm thick InAs layer is linearly shifted toward higher energy with increase in excitation intensity ranging from 10 to 140 mW, those of 4 nm InAs epilayer on InGaAs and 1500 nm InAs layer on GaAs is gradually blue-shifted and then, saturated above a power of 75 mW. These results suggest that adopting a graded InGaAs layer between InAs and GaAs can efficiently reduce the strain due to lattice mismatch in the structure of InAs/GaAs.