Browsing by Author "Kim, KH"

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    Charge transport properties of CdMnTe radiation detectors
    (EDP Sciences, 2012-04-11) Rafiei, R; Boardman, DA; Reinhard, MI; Sarbutt, A; Kim, KH; Watt, GC; Uxa, S; Prokopovich, DA; Belas, E; Bolotnikov, AE; James, RB
    Growth, fabrication and characterization of indium-doped cadmium manganese telluride (CdMnTe) radiation detectors have been described. Alpha-particle spectroscopy measurements and time resolved current transient measurements have yielded an average charge collection efficiency approaching 100 %. Spatially resolved charge collection efficiency maps have been produced for a range of detector bias voltages. Inhomogeneities in the charge transport of the CdMnTe crystals have been associated with chains of tellurium inclusions within the detector bulk. Further, it has been shown that the role of tellurium inclusions in degrading charge collection is reduced with increasing values of bias voltage. The electron drift velocity was calculated from the rise time distribution of the preamplifier output pulses at each measured bias. From the dependence of drift velocity on applied electric field the electron mobility was found to be μn = (718 ± 55) cm2/Vs at room temperature. © 2012 the Authors, published by EDP Sciences.
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    High-purity CdMnTe radiation detectors: a high-resolution spectroscopic evaluation
    (Intistute of Electrical Engineers, 2013-02-07) Rafiei, R; Reinhard, MI; Kim, KH; Prokopovich, DA; Boardman, DA; Sarbutt, A; Watt, GC; Bolotnikov, AE; Bignell, LJ; James, RB
    The charge transport properties of a high-purity CdMnTe (CMT) crystal have been measured at room temperature down to a micron-scale resolution. The CMT crystal, doped with indium, was grown by the vertical Bridgman technique. To reduce the residual impurities in the Mn source material, the growth process incorporated a five-times purification process of MnTe by a zone-refining method with molten Te solvent. The resulting 2.6 mm thick crystal exhibited an electron mobility-lifetime product of μnτn=2.9 × 10-3 cm2V-1. The velocity of electron drift was calculated from the rise time distribution of the preamplifier's output pulses at each measured bias. The electron mobility was extracted from the electric field dependence of the drift velocity and at room temperature it has a value of μn=(950±90) cm2/Vs. High-resolution maps of the charge collection efficiency have been measured using a scanning microbeam of 5.5 MeV 4He2+ ions focused to a beam diameter <; 1 μm and display large-area spatial uniformity. The evolution of charge collection uniformity across the detector has been highlighted by acquiring measurements at applied biases ranging between 50 V and 1100 V. Charge transport inhomogeneity has been associated with the presence of bulk defects. It has been demonstrated that minimizing the content of impurities in the MnTe source material is highly effective in achieving major improvements in the CMT detector's performance as compared to previous data. © 2013, IEEE.
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    Improving the representation of cross-boundary transport of anthropogenic pollution in East Asia using radon-222
    (Taiwan Association for Aerosol Research, 2016-01-01) Chambers, SD; Kang, CH; Williams, AG; Crawford, J; Griffiths, AD; Kim, KH; Kim, WH
    We report on 10 years of hourly atmospheric radon, CO, and SO2 observations at Gosan Station, Korea. An improved radon detector was installed during this period and performance of the detectors is compared. A technique is developed whereby the distribution of radon concentrations from a fetch region can be used to select air masses that have consistently been in direct contact with land-based emissions, and have been least diluted en route to the measurement site. Hourly radon concentrations are used to demonstrate and characterise contamination of remote-fetch pollution observations by local emissions at this key WMO GAW site, and a seasonally-varying 5-hour diurnal sampling window is proposed for days on which diurnal cycles are evident to minimise these effects. The seasonal variability in mixing depth and “background” pollutant concentrations are characterised. Based on a subset of observations most representative of the important regional fetch areas for this site, and least affected by local emissions, seasonal estimates of CO and SO2 in air masses originating from South China, North China, Korea and Japan are compared across the decade of observations. 2016, © Taiwan Association for Aerosol Research
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    Pollution of airborne metallic species in Seoul, Korea from 1998 to 2010
    (Elsevier, 2016-01-01) Ahmed, E; Kim, KH; Kim, JO; Park, JK; Chambers, SD; Feng, X; Jong, RS; Jeon, EC
    A comprehensive analysis was made to characterize the long-term changes in concentration of seven key heavy metal species (iron (Fe), Copper (Cu), lead (Pb), manganese (Mn), chromium (Cr), nickel (Ni), and cadmium (Cd)) bound to total suspended particles (TSP) in Seoul, Korea, from 1998 to 2010. Their mean values over this period were: 1579 ± 652, 174 ± 54.3, 63.6 ± 8.60, 46.0 ± 15.2, 11.7 ± 9.58, 9.34 ± 8.87, and 1.78 ± 0.64 ng m−3, respectively. Most of the metals exhibited a strong seasonality with maxima in spring (Mn, Fe, and Ni), winter (Pb and Cd), or fall (Cr and Cu) but minima in summer. The most prominent reductions of 71.5% (Pb) and 91.1% (Cu) were seen from early in the study period (1999–2002) to the most recent year (2010). Despite many advances in air quality, these latest concentration values were still higher than in many Western cities. It is thus still desirable to reduce their levels even further to ensure air quality improvement in the coming years. © 2017 Elsevier B.V.