Gyuseong Cho

Korea Advanced Institute of Science and Technology, Daiden, Daejeon, South Korea

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Publications (218)204.77 Total impact

  • K.T. Lim · H. Kim · M. Cho · Y. Kim · C. Kim · M. Kim · D. Lee · D. Kang · H. Yoo · K. Park · W.S. Sul · G. Cho
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    ABSTRACT: A typical Geiger-mode avalanche photodiode (G-APD) contains a guard ring that protects the structure from having an edge breakdown due to the lowering of electric fields at junction curvatures. In this contribution, G-APDs with a virtual guard ring (vGR) merged with n-type diffused guard ring (nGR) in various sizes were studied to find the optimal design for G-APDs fabricated at National NanoFab Center (NNFC) . The sensors were fabricated via a customized CMOS process with a micro-cell size of 65× 65 μm2 on a 200 mm p-type epitaxial layer wafer. I-V characteristic curves for proposed structures were measured on a wafer-level with an auto probing system and plotted together to compare their performance. A vGR width of 1.5 μm and a nGR width of 1.5 μm with an overlapping between vGR and nGR of 1.5 μm showed the lowest leakage current before the breakdown voltage while suppressing the edge breakdown. Furthermore, the current level of the lowest-leakage-current structure was as low as that of only vGR with a width of 2.0 μm, indicating that the structure is also area efficient. Based on these results, the design with vGR, nGR, and OL with width of 1.5 μm is determined to be the optimal structure for G-APDs fabricated at NNFC.
    No preview · Article · Jan 2016 · Journal of Instrumentation
  • M. Cho · H. Kim · K.T. Lim · G. Cho
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    ABSTRACT: Single Photon Emission Computed Tomography(SPECT) is a widely used diagnosis modality for detecting metabolic diseases. In general, SPECT system is consisted of a sensor, a pre-amplifier, position decoding circuits(PDC) and a data acquisition(DAQ) system. Due to such complexity, it is quite costly to assemble SPECT system by putting discrete components together. Moreover, using discrete components would make the system rather bulky. In this work, we designed a channel module ASIC for SPECT system. This system was composed of a transimpedance amplifier(TIA), comparators and digital logics. In this particular module, a TIA was selected as a preamplifier because the decay time and the rise time are shorter than that of other preamplifier topologies. In the proposed module, the amplified pulse from the TIA was split into two separate signals and each signal was then fed into two comparators with different reference levels, e.g., a low and high level. Then an XOR gate combined the comparator outputs and the output of XOR gate was sent to the suceeding digital logic. Furthermore, the output of each component in the module is composed of a signal packet. The packet includes the information on the energy, the time and the position of the incident photon. The energy and position information of a detected radiation can be derived from the output of the D-flipflop(DFF) in the module via time-over-threshold(TOT). The timing information was measured using a delayed rising edge from the low-level referenced comparator. There are several advantages in developing the channel module ASIC. First of all, the ASIC has only digital outputs and thus a correction circuit for analog signal distortion can be neglected. In addition, it is possible to cut down the system production cost because the volume of the system can be reduced due to the compactness of ASIC. The benefits of channel module is not only limited to SPECT but also beneficial to many other radiation detecting systems.
    No preview · Article · Jan 2016 · Journal of Instrumentation
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    ABSTRACT: Inorganic scintillators, composed of high-atomic-number materials such as the CsI(Tl) scintillator, are commonly used in commercially available a silicon diode and a scintillator embedded indirect-type electronic personal dosimeters because the light yield of the inorganic scintillator is higher than that of an organic scintillator. However, when it comes to tissue-equivalent dose measurements, a plastic scintillator such as polyvinyl toluene (PVT) is a more appropriate material than an inorganic scintillator because of the mass energy absorption coefficient. To verify the difference in the absorbed doses for each scintillator, absorbed doses from the energy spectrum and the calculated absorbed dose were compared. From the results, the absorbed dose of the plastic scintillator was almost the same as that of the tissue for the overall photon energy. However, in the case of CsI, it was similar to that of the tissue only for a photon energy from 500 to 4000 keV. Thus, the values and tendency of the mass energy absorption coefficient of the PVT are much more similar to those of human tissue than those of the CsI.
    No preview · Article · Nov 2015 · Radiation Protection Dosimetry
  • Source
    Hyunjun Yoo · Segyeong Joo · Shiyoung Yang · Gyuseong Cho
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    ABSTRACT: In this study, a compact radiation sensor for integration into mobile applications is proposed consisting of a CsI(Tl) crystal and a silicon photomultiplier with a 3 × 3 mm2 active area. Considering the size limit of mobile devices, the optimal size of the CsI(Tl) crystal was determined so that it has the best figure of merit (the absolute detection efficiency over the relative energy resolution) in the gamma energy spectrum while satisfying the criteria for the uniformity of the angular response. The final suggested dimensions consist of a cylindrically shaped crystal with a diameter of 5 mm, a total length of 5 mm and a tapered head length of 1 mm.
    Full-text · Article · Nov 2015
  • Chankyu Kim · Yewon Kim · Myungkook Moon · Gyuseong Cho
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    ABSTRACT: Plastic scintillators have been used for gamma ray detection in the fields of dosimetry and homeland security because of their desired characteristics such as a fast decay time, a low production cost, availability in a large-scale, and a tissue-equivalence. Gaussian energy broadening (GEB) in MCNP simulation is an effective treatment for tallies to calculate the broadened response function of a detector similarly to measured spectra. The full width at half maximum (FWHM) of a photopeak has been generally used to compute input parameters required for the GEB treatment. However, it is hard to find the photopeak in measured gamma spectra with plastic scintillators so that computation of the input parameters for the GEB has to be taken with another way. In this study, an iterative method for the GEB treated MCNP simulation to calculate the response function of a plastic scintillator is suggested. Instead of the photopeak, Compton maximum and Compton edge were used to estimate energy broadening in the measured spectra and to determine the GEB parameters. In a demonstration with a CsI(Tl) scintillator, the proposed iterative simulation showed the similar gamma spectra to the existing method using photopeaks. The proposed method was then applied to a polystyrene scintillator, and the simulation result were in agreement with the measured spectra with only a little iteration.
    No preview · Article · Sep 2015 · Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment
  • Source
    K. Ahn · D. Kwon · Y. Lee · J. Lee · G. Cho · S. Hong · M. Oh · H. Kim

    Preview · Article · Sep 2015 · Ultrasound in Obstetrics and Gynecology
  • G. Cho · H. Jin · S. Chung · M. Hahn · K. Ahn · S. Hong · M. Oh · H. Kim

    No preview · Article · Sep 2015 · Ultrasound in Obstetrics and Gynecology
  • K. Ahn · J. Lee · Y. Lee · D. Kwon · G. Cho · S. Hong · M. Oh · H. Kim

    No preview · Article · Sep 2015 · Ultrasound in Obstetrics and Gynecology
  • D. Oh · B. Lee · E. Kwon · S. Kim · G. Cho · S. Park · S. Lee · S. Hong
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    ABSTRACT: We have developed a reliable and predictable TCAD modeling method for retention characteristics of the charge trap NAND Flash device. This modeling method can explain various retention phenomena related to temperature, program pattern, and bake time. The temperature dependency is well described by direct tunneling and thermionic emission, the pattern dependency can be explained by carrier diffusion and the short time retention can be described by the electron-hole dynamics in the storage nitride layer
    No preview · Article · Jul 2015
  • H. Kim · H. Lee · S. Lee · G. Cho
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    ABSTRACT: Diesel burners have been used to regenerate diesel particulate filters (DPF) because of their simplicity in engine torque control and less oil dilution by fuel compared with the commonly used in-cylinder post fuel injection method. We previously developed a novel diesel burner using rotating plasma as an ignition source and found it to be effective in DPF regeneration. Here, we carry out in-depth studies on combustion efficiency of this plasma-ignited diesel burner and investigate the effects of influential factors such as plasma power, the amount of fresh air supplied, and O2 concentration in the exhaust gas on combustion characteristics of the burner. The obtained results show that fresh air supplied to the burner plays an important role in ignition and the early stage of combustion, and O2 concentration in the exhaust gas is identified as the most dominant factor for combustion efficiency. However, the results also indicate that plasma power and exhaust gas flow rates are insignificant in burner performance. Additionally, it is found that HC and CO emitted from the burner during DPF regeneration can effectively be suppressed with the DOC (Diesel Oxidation Catalyst). The PGM amount and volume of DOC should be determined to respond to even the case where the burner has the lowest possible combustion efficiency.
    No preview · Article · Apr 2015 · SAE Technical Papers
  • Woo-Suk Sul · Hyoungtaek Kim · Gyuseong Cho
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    ABSTRACT: Silicon photomultiplier (SiPM) modules were developed for use in positron emission tomography-magnetic resonance imaging (PET-MRI), which is a hybrid medical imaging technology. A PET-MRI is very efficient in the early diagnosis of representative senile diseases, including cancer, Alzheimer's disease, and Parkinson's disease. SiPMs comprise the core image sensor for MR-compatible PET applications since they have a low operational voltage, high gain, good timing resolution, ruggedness, insensitivity to magnetic fields, compactness, and low cost. In PET systems, SiPM microcells can be optimized by making a trade-off between photon detection efficiency (PDE) and dynamic range. The SiPM modules used in this study were fabricated at the National NanoFab Center (NNFC) of South Korea by using a customized CMOS processes. The SiPM modules were evaluated by first packaging them with a cost-effective PCB package instead of with a conventional ceramic package. Measurements on 1,400 SiPMs indicated a uniform breakdown voltage of 20.54 V with a standard deviation of 0.07 V. Moreover, the SiPM modules present a high and uniform energy resolution of 13.6% with a standard deviation of 0.5% at 511 keV with 3 × 3 × 20 mm3 cerium-doped lutetium-yttrium oxyorthosilicate (Lu2(1−x)Y2xSiO5:Ce, LYSO) crystal coupling. These results indicated that the proposed devices offer adequate performance to form the foundation of an image sensor technology for MRI-compatible PET.
    No preview · Article · Apr 2015 · Journal of Instrumentation
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    ABSTRACT: The NaI (Tl) scintillation detector is still preferred as a gamma spectrometer in many fields because of its general advantages. Recently, a Silicon Photomultiplier (SiPM) has been developed and expanded its application area as an alternative to photomultiplier tubes (PMT). It has merits such as low operating voltage, compact size, cheap production cost, and MR compatibility. In this study, an array of SiPMs is used to develop an NaI (Tl) gamma spectrometer. To maintain detection efficiency, a commercial NaI (Tl) 2x2 inch scintillator is used, and a light guide is used for transport and collection of generated photons from the scintillator to the SiPMs without loss. Test light guides were fabricated with PMMA and reflective materials. The systems of the gamma spectrometer were set up, including the light guides. Through a series of measurements, the characteristics of the light guides and the proposed gamma spectrometer were evaluated. Simulation of light collection was accomplished using DETECT 97 to analyze the measurement results. The system, including SiPMs and the light guide, achieved 14.11% FWHM energy resolution at 662keV.
    Preview · Article · Mar 2015 · Nuclear Engineering and Technology
  • D. Lee · D. Kang · K. Park · M. Cho · K.T. Lim · S. Cho · G. Cho
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    ABSTRACT: X-ray sensor applications in general need a high resolution, a high SNR, a low FPN and a low power consumption. Digital pixels with an ADC integrated in each pixel can satisfy not only aforementioned requirements but also a fast frame rate. Nyquist ADCs, however, have a large area to be implemented and thus need a sophisticated circuit design in order to operate properly. In the other hand, a sigma-delta ADC does not require a complicated circuit design and consumes a small area. Therefore, a second-order sigma-delta ADCs in each X-ray log pixel is chosen due to its low power consumption, high resolution and high frame rate. The designed X-ray array has a dimension of 10×20 with a pixel size of 100μm and the measured power consumption of each sigma-delta pixel is less than 700nW.
    No preview · Article · Mar 2015
  • D. Kang · D. Lee · M. Cho · K. Park · K.T. Lim · S. Cho · G. Cho
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    ABSTRACT: This study presents a design of a high-speed photon counting readout circuit for spectral computed tomography detectors. We propose a novel front-end architecture aimed to reduce a dead time by introducing dual signal paths in each pixel. A prototype chip is fabricated with 0.18um six-metal standard CMOS process that consists of 16 × 16 pixels and periphery circuits. Each pixel has 200um pixel pitch and contains two signal paths. The switching circuit in the pixel allows a single signal path to connect with the input pad at a time. As a result, the proposed dual-path readout scheme shows a 3-times higher speed than that of a conventional single-path readout scheme.
    No preview · Article · Mar 2015
  • Hyunduk Kim · Gyuseong Cho · Sun A. Kim · Bo Sun Kang
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    ABSTRACT: The production of short-lived radioisotopes for the synthesis of radiopharmaceuticals typically takes advantage of a cyclotron that accelerates a proton beam up to a few tens of MeV. The number of cyclotrons has been continuously increasing since the first operation of the MC-50 for the production of radiopharmaceuticals at the Korea Institute of Radiological & Medical Sciences (KIRAMS) in 1986, and currently 35 cyclotrons are under operation throughout the nation. As the number of operating cyclotrons has increased, concerns about radiation safety for the persons who are working at the facilities and dwelling in the vicinity of the facilities are becoming important issues. Radiation that could emit a time-dependent dose was shown to exist in a cyclotron vault after its shutdown. The calculation of the latent radiation dose rate was performed by using the MCNPX and the FISPACT. The calculated results for the activated long-lived radioisotopes in the concrete wall and the structural components of the cyclotron facility were compared with the measured data that were obtained by using gamma-ray spectroscopy with a HPGe detector.
    No preview · Article · Feb 2015 · Journal- Korean Physical Society
  • Hyoungtaek Kim · Woo Suk Sul · Gyuseong Cho
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    ABSTRACT: The silicon photomultipliers (SiPMs) were fabricated for magnetic resonance compatible positron emission tomography (PET) applications using customized CMOS processes at National NanoFab Center. Each micro-cell consists of a shallow n+/p well junction on a p-type epitaxial wafer and passive quenching circuit was applied. The size of the SiPM is 3 × 3 mm(2) and the pitch of each micro-cell is 65 μm. In this work, several thousands of SiPMs were packaged and tested to build a PET ring detector which has a 60 mm axial and 390 mm radial field of view. I-V characteristics of the SiPMs are shown good uniformity and breakdown voltage is around 20 V. The photon detection efficiency was measured via photon counting method and the maximum value was recorded as 16% at 470 nm. The gamma ray spectrum of a Ge-68 isotope showed nearly 10% energy resolution at 511 keV with a 3 × 3 × 20 mm(3) LYSO crystal.
    No preview · Article · Oct 2014 · Review of Scientific Instruments
  • Myung Soo Kim · Giyoon Kim · Dong-uk Kang · Daehee Lee · Gyuseong Cho
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    ABSTRACT: A large-area X-ray CMOS image sensor (LXCIS) is widely used in mammography, non-destructive inspection, and animal CT. For LXCIS, in spite of weakness such as low spatial and energy resolution, a Indirect method using scintillator like CsI(Tl) or Gd2O2S is still well-used because of low cost and easy manufacture. A photo-diode for X-ray imaging has large area about 50 ~ 200 um as compared with vision image sensors. That is because X-ray has feature of straight and very small light emission of a scintillator. Moreover, notwithstanding several structure like columnar, the scintillator still emit a diffusible light. This diffusible light from scintillator can make spatial crosstalk in X-ray photodiode array because of a large incidence angle. Moreover, comparing with vision image sensors, X-ray sensor doesn’t have micro lens for gathering the photons to photo-diode. In this study, we simulated the affection of spatial crosstalk in X-ray sensor by comparing optical sensor. Additionally, the chip, which was fabricated in 0.18 um 1P5M process by Hynix in Korea, was tested to know the effect of spatial crosstalk by changing design parameters. From these works, we found out that spatial crosstalk is affected by pixel pitch, incident angle of photons, and micro lens on each pixels.
    No preview · Conference Paper · Sep 2014
  • H. Jin · G. Cho · H. Hong · H. Seol · K. Ahn · S. Hong · M. Oh · H. Kim

    No preview · Article · Sep 2014 · Ultrasound in Obstetrics and Gynecology
  • G. Cho · H. Jin · M. Oh · W.H. Choi · A. Seo

    No preview · Article · Sep 2014 · Ultrasound in Obstetrics and Gynecology
  • K. Ahn · J.H. Lee · G. Cho · S. Hong · M. Oh · H. Kim

    No preview · Article · Sep 2014 · Ultrasound in Obstetrics and Gynecology

Publication Stats

1k Citations
204.77 Total Impact Points

Institutions

  • 1997-2015
    • Korea Advanced Institute of Science and Technology
      • Department of Nuclear and Quantum Engineering
      Daiden, Daejeon, South Korea
  • 2014
    • Korea Atomic Energy Research Institute (KAERI)
      • Health Physics Department
      Daiden, Daejeon, South Korea
  • 2013-2014
    • Korea University Medical Center
      Sŏul, Seoul, South Korea
  • 2010-2012
    • Korea University
      Sŏul, Seoul, South Korea
  • 2011
    • Inje University
      • Department of Biomedical Engineering
      Kŭmhae, Gyeongsangnam-do, South Korea
  • 2008
    • Hallym University
      Sŏul, Seoul, South Korea
  • 2006
    • Sacred Heart Hospital, Chicago
      Chicago, Illinois, United States
  • 2004
    • Sungkyunkwan University
      • Department of Nuclear Medicine
      Sŏul, Seoul, South Korea
  • 2001
    • Catholic University of Korea
      Sŏul, Seoul, South Korea
  • 2000
    • Yonsei University
      Sŏul, Seoul, South Korea
  • 1999-2000
    • Kyungpook National University
      • Department of Physics
      Daikyū, Daegu, South Korea
  • 1989-1996
    • Lawrence Berkeley National Laboratory
      • Physics Division
      Berkeley, California, United States
  • 1988-1994
    • University of California, Berkeley
      • Lawrence Berkeley Laboratory
      Berkeley, California, United States
  • 1990-1993
    • Palo Alto Research Center
      Palo Alto, California, United States