• Journal of Radiation Protection and Research
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• v.31 no.2
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• pp.97-104
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• 2006

In recent years, the MOSFET dosimeter has been widely used in various medical applications such as dose verification in radiation therapeutic and diagnostic applications. The MOSFET dosimeter is, however, mainly made of silicon and shows some energy dependence for low energy Photons. Therefore, the MOSFET dosimeter tends to overestimate the dose for low energy scattered photons in a phantom. This study determines the correction factors to compensate these dependences of the MOSFET dosimeter in ATOM phantom. For this, we first constructed a computational model of the ATOM phantom based on the 3D CT image data of the phantom. The voxel phantom was then implemented in a Monte Carlo simulation code and used to calculate the energy spectrum of the photon field at each of the MOSFET dosimeter locations in the phantom. Finally, the correction factors were calculated based on the energy spectrum of the photon field at the dosimeter locations and the pre-determined energy and directional dependence of the MOSFET dosimeter. Our result for $^{60}Co$ and $^{137}Cs$ photon fields shows that the correction factors are distributed within the range of 0.89 and 0.97 considering all the MOSFET dosimeter locations in the phantom.

• Proceedings of the Korean Magnestics Society Conference
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• 2012.11a
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• pp.104-105
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• 2012

The temperature dependence of the effective magnetic anisotropy constant K(T) of ferrite nanoparticles is obtained based on the measurements of SQUID magnetometry. For this end, a very simple but intuitive and direct method for determining the temperature dependence of anisotropy constant K(T) in nanoparticles is introduced in this study. The anisotropy constant at a given temperature is determined by associating the particle size distribution f(r) with the anisotropy energy barrier distribution $f_A(T)$. In order to estimate the particle size distribution f(r), the first quadrant part of the hysteresis loop is fitted to the classical Langevin function weight-averaged with the log?normal distribution, slightly modified from the original Chantrell's distribution function. In order to get an anisotropy energy barrier distribution $f_A(T)$, the temperature dependence of magnetization decay $M_{TD}$ of the sample is measured. For this measurement, the sample is cooled from room temperature to 5 K in a magnetic field of 100 G. Then the applied field is turned off and the remanent magnetization is measured on stepwise increasing the temperature. And the energy barrier distribution $f_A(T)$ is obtained by differentiating the magnetization decay curve at any temperature. It decreases with increasing temperature and finally vanishes when all the particles in the sample are unblocked. As a next step, a relation between r and $T_B$ is determined from the particle size distribution f(r) and the anisotropy energy barrier distribution $f_A(T)$. Under the simple assumption that the superparamagnetic fraction of cumulative area in particle size distribution at a temperature is equal to the fraction of anisotropy energy barrier overcome at that temperature in the anisotropy energy barrier distribution, we can get a relation between r and $T_B$, from which the temperature dependence of the magnetic anisotropy constant was determined, as is represented in the inset of Fig. 1. Substituting the values of r and $T_B$ into the $N{\acute{e}}el$-Arrhenius equation with the attempt time fixed to $10^{-9}s$ and measuring time being 100 s which is suitable for conventional magnetic measurement, the anisotropy constant K(T) is estimated as a function of temperature (Fig. 1). As an example, the resultant effective magnetic anisotropy constant K(T) of manganese ferrite decreases with increasing temperature from $8.5{\times}10^4J/m^3$ at 5 K to $0.35{\times}10^4J/m^3$ at 125 K. The reported value for K in the literatures is $0.25{\times}10^4J/m^3$. The anisotropy constant at low temperature region is far more than one order of magnitude larger than that at 125 K, indicative of the effects of inter?particle interaction, which is more pronounced for smaller particles.

• Progress in Superconductivity
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• v.9 no.1
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• pp.5-10
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• 2007

We prepared four different $MgB_2$ films on $Al_2O_3$ by hybrid physical chemical vapor deposition method with thicknesses ranging from $0.65\;{\mu}m$ to $1.2\;{\mu}m$. X-ray diffraction patterns confirm that all the $MgB_2$ films are c-axis oriented perpendicular to $Al_2O_3$ substrates. The superconducting onset temperature of $MgB_2$ films were between 39.39K and 40.72K. The residual resistivity ratio of the $MgB_2$ films was in the range between 3.13 and 37.3. We measured the angle dependence of critical current density ($J_c$) and resistivity, and determined the upper critical field ($H_{c2}$) from the temperature dependence of the resistivity curves. The anisotropy ratios defined as the ratio of the $H_{c2}$ parallel to the ab-plane to that perpendicular to the ab-plane were in the range of 2.13 to 4.5 and were increased as the temperature was decreased. Some samples showed increase of $J_c$ and decrease of resistivity when a magnetic field in applied parallel to the c-axis. We interpret this angle dependence in terms of enhanced flux pinning due to columnar growth of $MgB_2$ along the c-axis.

• Journal of Astronomy and Space Sciences
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• v.27 no.4
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• pp.329-335
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• 2010

We investigate the sources of the variation of the high-latitude thermospheric neutral mass density depending on the interplanetary magnetic field (IMF) conditions. For this purpose, we have carried out the National Center for Atmospheric Research Thermosphere-Ionosphere Electrodynamics General Circulation Model (NCAR-TIEGCM) simulations for various IMF conditions under summer condition in the southern hemisphere. The NCAR-TIEGCM is combined with a new empirical model that provides a forcing to the thermosphere in high latitudes. The difference of the high-latitude thermospheric neutral mass density (subtraction of the values for zero IMF condition from the values for non-zero IMF conditions) shows a dependence on the IMF condition: For negative $B_y$ condition, there are significantly enhanced difference densities in the dusk sector and around midnight. Under the positive-$B_y$ condition, there is a decrease in the early morning hours including the dawn side poleward of $-70^{\circ}$. For negative $B_z$, the difference of the thermospheric densities shows a strong enhancement in the cusp region and around midnight, but decreases in the dawn sector. In the dusk sector, those values are relatively larger than those in the dawn sector. The density difference under positive-$B_z$ condition shows decreases generally. The density difference is more significant under negative-$B_z$ condition than under positive-$B_z$ condition. The dependence of the density difference on the IMF conditions in high latitudes, especially, in the dawn and dusk sectors can be explained by the effect of thermospheric winds that are associated with the ionospheric convection and vary following the direction of the IMF. In auroral and cusp regions, heating of thermosphere by ionospheric currents and/or auroral particle precipitation can be also the source of the dependence of the density difference on the IMF conditions.

• The Bulletin of The Korean Astronomical Society
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• v.40 no.1
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• pp.47.2-47.2
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• 2015

At local, galaxy properties are well known to be clearly different in different environments. However, it is still an open question how this environment-dependent trend has been shaped. In this presentation, we will show the results of our investigation about the evolution of star-formation properties of galaxies over a wide redshift range, from z~2 to z~0.5, focusing its dependence on their stellar mass and environment. In the UKIDSS/UDS region, we estimated photometric redshifts and stellar population properties, such as stellar masses and star-formation rates, using the deep optical and near-infrared data available in this field. Then, we identified galaxy cluster candidates at z~0.5-2. Through the analysis and comparison of star-formation (SF) properties of galaxies in clusters and in field, we found interesting results regarding the evolution of SF properties of galaxies: (1) regardless of redshifts, stellar mass is a key parameter controlling quenching of star formation in galaxies; (2) At z<1, environmental effects become important at quenching star formation regardless of stellar mass of galaxies; and (3) However, the result of the environmental quenching is prominent only for low mass galaxies (M* < $10^{10}M_{\odot}$) since the star formation in most of high mass galaxies are already quenched at z > 1.

• 한국신재생에너지학회:학술대회논문집
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• 2005.11a
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• pp.579-582
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• 2005

The sound measurement techniques in IEC 61400-11 are applied to field test and evaluation of noise emission from 1.5 MW wind turbine generator (WTG) at Yongdang-Lee and 650 kW WTG at Hangwon-Lee in Jeju Island. Apparent sound power level, wind speed dependence and third-octave band levels are evaluated for both of WTGs. 1.5 MW WTG at Yongdang is found to emit lower sound power than 660 kW one at Hangwon, which seems to be due to lower rotating speed of the rotor of WTG at Yongdang. Equivalent continuous sound pressure level s (ECSPL) of 650 kW WTG at Hangwon vary more widely with speed than those of 1.5 MW WTG at Yongdang. The reason for this is believed to be the fixed blade-rotating speed of WTG at Yongdang. One-third octave band analysis of the measured data show that the band components around 400-500 Hz are dominant for 1.5 MW WTG at Yongdang and those around 1K Hz are dominant for 660 kW WTG at Hangwon.

• Proceedings of the Korean Operations and Management Science Society Conference
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• 2005.10a
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• pp.268-296
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• 2005

Given the growing expectations on the value and application of mobile office, we need to understand the factors which affect its successful use and implementation. We in this paper present a parsimonious model which integrates the IS success model and the TAM. The main feature of the model is that it allows one to explore the role of mobility of mobile office systems in determining the level of system usage and dependence. Although mobility is the key property of any mobile system, there is little research effort exerted to understand how mobility affects the traditional IS factors such as perceived ease of use and usefulness. In addition to the system usage, this model also uses dependence as the dependent variable. This is expected to provide richer understanding of the study context. We collected survey data from 1,614 field service workers of a major domestic electronic company. They use PDAs to retrieve information regarding the specifics of the customer service requests and to report what has been done for each service job. The results from SEM analysis show that mobility improves the perceived ease of use which then leads the field service workers to depend more on the PDA systems. Interestingly enough, however, the workers’ satisfaction does not cause more frequent use of the system.

• Journal of Magnetics
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• v.17 no.4
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• pp.245-250
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• 2012

Ferromagnetic resonance and X-ray specular reflectivity measurements were performed on $Ni_{81}Fe_{19}/Ir_{20}Mn_{80}/Co_{90}Fe_{10}$ exchange bias trilayers, which were grown using the pulsed-DC magnetron sputtering technique on Si(100)/$SiO_2$(1000 nm) substrates, to investigate the evolution of the interface roughness and exchange bias and their dependence on the NiFe layer thickness. The interface roughness values of the samples decrease with increasing NiFe thickness. The in-plane ferromagnetic resonance measurements indicate that the exchange bias field and the peak-to-peak line widths of the resonance curves are inversely proportional to the NiFe thickness. Furthermore, both the exchange bias field and the interface roughness show almost the same dependence on the NiFe layer thickness. The out-of plane angular dependent measurements indicate that the exchange bias arises predominantly from a variation of exchange anisotropy due to changes in interfacial structure. The correlation between the exchange bias and the interface roughness is discussed.

• Asia pacific journal of information systems
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• v.16 no.4
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• pp.179-202
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• 2006

The objective of this study is to identify the impacts of communication reinforcement on performance of learning in Web-PBL. Communication reinforcement is defined as the combination of information sharing and co-construction. As factors facilitating communication reinforcement, we propose learner's characteristics, task characteristics, and group characteristics. Learner's characteristics are collaboration-orientation, openness, holistic approach, and online community-orientation which reflects e-learning environment. Collaboration-oriented tasks as group projects were developed and given to groups with 5-6 members. The group characteristics are categorized into 'horizontal' and 'vertical', according to the patterns of communication between a group leader and members. To verify empirically the proposed research model, an experimental design was performed to learners who took on-line and off-line courses with group projects. We found important results as follows; First, field dependence has positive impacts on information sharing, and online community-orientation has positive impacts on co-construction. These results correspond with prior studies on relationship between field dependence and collaborative learning. Second, collaboration-oriented task directly impacts on information sharing, and indirectly affects co-construction, This result implicates that information sharing is pre-requisite of co-construction. Third, 'horizontal' was identified as a factor giving positive effects on information sharing and co-construction. This result implies that horizontal communication is very important to facilitate communication reinforcement.

• Korean Journal of Materials Research
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• v.20 no.5
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• pp.271-277
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• 2010

In this paper two aspects of the percolation and conductivity of carbon black-filled polyethylene matrix composites will be discussed. Firstly, the percolation behavior, the critical exponent of conductivity of these composites, are discussed based on studying the whole change of resistivity, the relationship between frequency and relative permittivity or ac conductivity. There are two transitions of resistivity for carbon black filling. Below the first transition, resistivity shows an ohmic behavior and its value is almost the same as that of the matrix. Between the first and second transition, the change in resistivity is very sharp, and a non-ohmic electric field dependence of current has been observed. Secondly, the electrical conduction property of the carbon black-filled polyethylene matrix composites below the percolation threshold is discussed with the hopping conduction model. This study investigates the electrical conduction property of the composites below the percolation threshold based on the frequency dependence of conductivity in the range of 20 Hz to 1 MHz. There are two components for the observed ac loss current. One is independent of frequency that becomes prevalent in low frequencies just below the percolation threshold and under a high electrical field. The other is proportional to the frequency of the applied ac voltage in high frequencies and its origin is not clear. These results support the conclusion that the electrical conduction mechanism below the percolation threshold is tunneling.