• 천문학회지
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• 제47권6호
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• pp.303-309
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• 2014

We present results of our investigation of the radio intrinsic brightness temperatures of compact radio jets. The intrinsic brightness temperatures of about 100 compact radio jets at 2, 5, 8, 15, and 86 GHz are estimated based on large VLBI surveys conducted in 2001-2003 (or in 1996 for the 5 GHz sample). The multi-frequency intrinsic brightness temperatures of the sample of jets are determined by a statistical method relating the observed brightness temperatures with the maximal apparent jet speeds, assuming one representative intrinsic brightness temperature for a sample of jets at each observing frequency. By investigating the observed brightness temperatures at 15 GHz in multiple epochs, we find that the determination of the intrinsic brightness temperature for our sample is affected by the flux density variability of individual jets at time scales of a few years. This implies that it is important to use contemporaneous VLBI observations for the multi-frequency analysis of intrinsic brightness temperatures. Since our analysis is based on the VLBI observations conducted in 2001-2003, the results are not strongly affected by the flux density variability. We find that the intrinsic brightness temperature $T_0$ increases as $T_0{\propto}{\nu}^{\xi}_{obs}$ with ${\xi}=0.7$ below a critical frequency ${\nu}_c{\approx}9GHz$ where the energy loss begins to dominate the emission. Above ${\nu}_c$, $T_0$ decreases with ${\xi}=-1.2$, supporting the decelerating jet model or particle cascade model. We also find that the peak value of $T_0{\approx}3.4{\times}10^{10}$ K is close to the equipartition temperature, implying that the VLBI cores observable at 2-86 GHz may be representing jet regions where the magnetic field energy dominates the total energy in jets.

• 대한의생명과학회지
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• 제10권4호
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• pp.515-521
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• 2004

To measure regional cerebral blood volume (rCBV) with perfusion MR imaging of cerebral fat embolism by neutral fat and free fatty acids in cats. Triolein (group 1, n=15), oleic acid (group 2, n=9) and linoleic acid (group 3, n=11) were infused into unilateral internal carotid artery using microcatheter through the transfemoral approach. PVA particle was used as a non-fat embolic material in a control group (group 4, n=9). Perfusion-weighted MR image was obtained at 30 minutes and 2 hours postembolization, based on T2-and diffusion-weighted images. The data of lesion and contralateral normal area were transferred to personal computer, time-to-signal intensity curve was drawn and trans for used to △R2/sup */ curve in regular order. The process in the personal computer was done by using the author's developmental image processing program and interactive data language (IDL) softwares. Statistical significance was approved by paired t-test and ANOVA. rCBV of the lesion was decreased comparing to the normal area in all groups. The ratios of rCBV were as follows (group No, at 30 minutes, at 2 hours); group 1,32％, 51％; group 2, 30％, 44％; group 3, 39％, 61％; group 4, 21％, 36％. rCBVs of 2 hours was significantly increased compared to those of 30 minutes in all groups (P<0.005). rCBV was decreased at 30 minutes in cerebral fat embolism and recovered a little, but significantly at 2 hours. Perfusion-weighted images was useful method in offering hemodynamic information in cerebral fat embolism.

• 한국가시화정보학회지
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• 제15권1호
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• pp.47-52
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• 2017

In droplet-based microfluidic systems, in-droplet preconcentration of a sample is one of the important prerequisites for biochemical or medical analysis. There have been a few studies on preconcentration in a moving droplet, but they are limited to practical applications since 1) their method are time-consuming or 2) they require specific properties such as electric and magnetic properties. In this study, we demonstrated the position control of polystyrene particles of 5 and $10{\mu}m$ in diameter inside a moving water-in-oil droplet using traveling surface acoustic waves. Since the frequencies for effective control of each diameter were found, microparticles with no labels could be utilized. In addition, the proposed method enabled on-demand preconcentration inside a polydimethylsiloxane microchannel. In-droplet preconcentration of microparticles was realized by splitting a mother droplet with manipulated particles at a downstream bifurcation zone. Given these advantages, the proposed system is a promising acoustofluidic lab-on-a-chip platform for preconcentration inside a droplet.

• 천문학회지
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• 제48권5호
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• pp.285-298
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• 2015

Active Galactic Nuclei (AGN) with bright radio jets offer the opportunity to study the structure of and physical conditions in relativistic outflows. For such studies, multi-frequency polarimetric very long baseline interferometric (VLBI) observations are important as they directly probe particle densities, magnetic field geometries, and several other parameters. We present results from first-epoch data obtained by the Korean VLBI Network (KVN) within the frame of the Plasma Physics of Active Galactic Nuclei (PAGaN) project. We observed seven radio-bright nearby AGN at frequencies of 22, 43, 86, and 129 GHz in dual polarization mode. Our observations constrain apparent brightness temperatures of jet components and radio cores in our sample to > 10^8.01 K and > 10^9.86 K, respectively. Degrees of linear polarization m_L are relatively low overall: less than 10%. This indicates suppression of polarization by strong turbulence in the jets. We found an exceptionally high degree of polarization in a jet component of BL Lac at 43 GHz, with m_L ~ 40%. Assuming a transverse shock front propagating downstream along the jet, the shock front being almost parallel to the line of sight can explain the high degree of polarization.

• 천문학회보
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• 제35권1호
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• pp.34.2-34.2
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• 2010

Cosmic rays registered by Neutron Monitors on the surface of the Earth are believed to be coming from outer space, and sometimes also from the exotic objects of the Sun. Ground level enhancement (GLE) is the sudden, sharp and short-lived increase in cosmic rays originated from the Sun. Since GLE is the signature in solar cosmic ray intensity, different solar factors erupted from the Sun can be responsible for causing it. In this context, an attempt has been made to determine quantitative relationships of GLEs > 5% with simultaneous solar, interplanetary and geophysical factors from 1997 through 2006 thereby searching the perpetrators which seem to be causing them. The study has revealed that solar flares are stronger ($0.71{\times}10-4$ w/m2) during GLE peaks than the solar flares ($1.10{\times}10-5$ w/m2) during GLE non-peaks and backgrounds. On the average, the solar wind plasma velocity and interplanetary magnetic field are found stronger during the GLE peaks than the GLE non-peaks and backgrounds indicating that the solar flares, in conjunction with interplanetary shocks, sometimes may cause GLE peaks. Direct proportionality of GLE peaks to simultaneous solar energetic particle (SEP) fluxes imply that the GLE peaks may often be caused by SEP fluxes. Although the high intensity of SEP fluxes are also seen extended few minutes even after GLE peaks, the mean (373.62 MeV) of the GLE associated SEP fluxes is much stronger than the mean (10.35 MeV) of the non-GLE associated SEP fluxes. Evidences are also supported by corresponding SEP fluences that the the mean fluence (${\sim}5.32{\times}107/cm2$) across GLE event was more intense than the mean fluence (${\sim}2.53{\times}106/cm2$) of SEP fluxes across non-GLE event.

• 한국진공학회지
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• 제19권1호
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• pp.45-51
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• 2010

마그네트론 타겟에서 일어나는 다양한 물리적 현상에 의한 결과로 인해 발생하는 타겟 표면의 온도를 측정함으로써 그 분포가 플라즈마, 혹은 증착되는 박막에 영향을 줄 수 있는 가능성을 분석하였다. 마그네트론 스퍼터링의 타겟은 크게 원형 타겟과 사각 타겟으로 구분되는데, 사각 타겟에서는 자기장에 의한 corner effect 등에 의해 전자 집중 방전 영역이 발생하고 그것에 의해 타겟 표면에서 불균일한 온도분포가 생성됨을 확인했다. 국부적으로 온도가 높게 올라가는 지역은 비스퍼터링 지역에 비해 $10{\sim}20^{\circ}C$ 정도 높았으며, 스퍼터링 공정 시 문제점 중에 하나인 particle이 발생하면 그 부분에서 온도가 $20^{\circ}C$ 정도 더 상승함을 알 수 있었다. 이런 영향은 증착되는 박막의 균일도에도 적지 않은 영향을 주었으며 세라믹 타겟의 경우, 균열의 원인이 될 수 있고, 불균일한 타겟 침식으로 타겟의 수명을 단축시키는 문제를 유발하기도 한다.

• 한국재료학회지
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• 제5권7호
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• pp.858-868
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• 1995

다른 탄소함량을 갖는 두 Hadfield 강에서 압연에 따른 결정방위조직(Texture)과 마르텐사이트 상변태를 연구하였다(0.65wt.%, 1.35wt.%) 두 Hadfield 강의 적층결함에너지 차이가 매우 적음에도 불구하고 (약 2mJm$^{-2}$ ) 결정방위조직은 차이를 보였다. 0.65wt.% 탄소 강의 경우, 낮은 변형구간에서는 낮은 적층에너지 금속과 비슷한 결정방위조직을 보였으나 높은 변형에서는 {111}, {110}<001>과 같은 이상 결정방위조직이 나타났고, 이것은 입계 또는 일단의 전단띠에(shear bands) 형성된 $\alpha$ 마르텐사이트의 방해에 의한 것으로 생각된다. 이와는 대조적으로 1.35wt.% Hadfield강은 전변형구간에서 낮은 적층결함에너지를 갖는 금속과 비슷한 결정방위조직을 보이고 있다. 이것은 변형중에 유도된 마르텐사이트의 양이 적게 형성되는 사실에 기인하며, 변형에 따른 $\alpha$ 마르텐사이트 변태는 교류자화율과 금속입자 시험으로 알 수 있다.

• 대한지리학회지
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• 제45권5호
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• pp.541-552
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• 2010

후기 플라이스토신 및 홀로신 시기 동아시아 여름 몬순의 변화를 밝히기 위해 지화학적 분석 방법(탄소동원원소(${\delta}^{13}C$), 탄질율(C/N), 대자율, 입도 분석 등)을 활용하여 서해안 염하구 습지 퇴적물을 분석하였다. 연구 결과에 따르면 한반도에서 여름 몬순은 7,700-7,800년 전에 정점이었고 7,400년 전 경부터 약화되기 시작한다. 그리고 마지막 빙하 최성기 중 24,000-24,500년 전 경에는 몬순이 상대적으로 약했고 18,500-19,500년 전 경에는 상대적으로 강했다. 염하구 습지 퇴적물의 지화학적 분석 자료는 과거 여름 몬순의 변동을 밝힐 수 있는 새로운 고환경 자료로서 가치를 갖는다. 특히 고환경 자료가 부족한 한국 학계에 큰 도움이 될 것으로 사료된다.

• 천문학회지
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• 제48권2호
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• pp.155-164
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• 2015

In Kang (2015) we calculated the acceleration of cosmic-ray electrons at weak spherical shocks that are expected to form in the cluster outskirts, and estimated the diffuse synchrotron radiation emitted by those electrons. There we demonstrated that, at decelerating spherical shocks, the volume integrated spectra of both electrons and radiation deviate significantly from the test-particle power-laws predicted for constant planar shocks, because the shock compression ratio and the flux of inject electrons decrease in time. In this study, we consider spherical blast waves propagating through a constant density core surrounded by an isothermal halo with ρ ∝ r⁻ⁿ in order to explore how the deceleration of the shock affects the radio emission from accelerated electrons. The surface brightness profile and the volumeintegrated radio spectrum of the model shocks are calculated by assuming a ribbon-like shock surface on a spherical shell and the associated downstream region of relativistic electrons. If the postshock magnetic field strength is about 0.7 or 7 µG, at the shock age of ∼ 50 Myr, the volume-integrated radio spectrum steepens gradually with the spectral index from α_inj to α_inj + 0.5 over 0.1–10 GHz, where α_inj is the injection index at the shock position expected from the diffusive shock acceleration theory. Such gradual steepening could explain the curved radio spectrum of the radio relic in cluster A2266, which was interpreted as a broken power-law by Trasatti et al. (2015), if the relic shock is young enough so that the break frequency is around 1 GHz.

• 접착 및 계면
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• 제14권2호
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• pp.82-87
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• 2013

본 연구에서는 레졸, 질산철 그리고 트리블럭 공중합체를 이용하여 직접 탄화과정에 의해 자성체 나노입자가 분산된 탄소나노세공체를 합성하였다. 나노세공 마그네타이트/카본($Fe_3O_4$/carbon) 나노복합체는 낮은 마그네타이트 함량(1 wt%)을 가지고 잘 배열된 이차원적 육방체 구조(p6mm)를 보이며, 균일한 세공크기(3.6 nm), 높은 표면적(635 $m^2/g$)과 세공부피(0.48 $cm^3/g$)를 가진다. 작은 입자크기(10.2 nm)를 가지는 마그네타이트 나노입자는 초상자기성(7.7 emu/g)을 보이고 탄소 세공벽 내에 잘 분산되었다. 나노세공 마그네타이트/카본 물질은 최대 995 mg/g의 아이부프로펜 흡착량을 보였다. 또한, 자석을 이용하여 용액과 나노세공 마그네타이트/카본 물질의 분리가 용이하였다. 본 연구에서 제조된 나노복합체는 우수한 아이부프로펜 흡착제로 작용하였다.