Gyuseong Cho

Korea Advanced Institute of Science and Technology , Sŏul, Seoul, South Korea

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Publications (101)88.4 Total impact

  • 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.
    The Review of scientific instruments. 10/2014; 85(10):103107.
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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.
    SPIE Optical Engineering + Applications; 09/2014
  • Journal of the Optical Society of Korea 06/2014; 18(3):256-260. · 0.96 Impact Factor
  • Jongyul Kim, Seung Wook Lee, Gyuseong Cho
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    ABSTRACT: Visibility evaluation of neutron gratings for a polychromatic thermal neutron beam was performed for a neutron grating interferometer. Four sets of neutron gratings designed for neutron wavelengths of 2.0 Å, 2.7 Å, 3.5 Å, and 4.4 Å were fabricated and tested to find the neutron grating interferometer setup with maximum visibility. The measurements were carried out at the thermal neutron beamline of the Ex-core Neutron irradiation Facility (ENF) of the High-flux Advanced Neutron Application Reactor. The maximum visibility was obtained at the neutron grating set designed for a neutron wavelength of 2.7 Å among the four sets, and the visibility was 9.7%. The experimental data can be the basis for an optimization of the neutron grating interferometer at the thermal neutron beamline, and can be further optimized for neutron dark-field imaging with high spatial resolution and a shorter data acquisition time.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 05/2014; 746:26–32. · 1.32 Impact Factor
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    ABSTRACT: The single-slope analog-to-digital converter (SS-ADC) is the most commonly used column-level ADC for high-speed industrial, complementary metal-oxide semiconductor (CMOS)-based X-ray image sensors because of its small chip area (the width of a pixel), its simple circuit structure, and its low power consumption. However, it generally has a long conversion time, so we propose an innovative design: a complimentary dual-slope ADC (CDS-ADC) that uses two opposite ramp signals instead of a single ramp to double the conversion speed. This CDS-ADC occupies only 15% more area than the original SS-ADC. A prototype 12-bit CDS-ADC and a 12-bit SS-ADC were fabricated using a 0.35-µm 1P 4M CMOS process. During comparison of the two, the measured maximum differential non-linearity (DNL) of the CDS-ADC was a 0.49 least significant bit (LSB), the maximum integral non-linearity (INL) was a 0.43 LSB, the effective number of bits (ENOB) was 9.18 bits, and the figure of merit (FOM) was 0.03 pJ/conversion. The total power consumption was 0.031 uW. The conversion time of the new CDS-ADC was half that of the SS-ADC. The proposed dual-slope concept can be extended to further multiply the conversion speed by using multiple pairs of dual-slope ramps.
    Journal- Korean Physical Society 01/2014; 64(4). · 0.43 Impact Factor
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    ABSTRACT: The fabrication of gratings including metal deposition processes for highly neutron absorbing lines is a critical issue to achieve a good visibility of the grating-based phase imaging system. The source grating for a neutron Talbot-Lau interferometer is an array of Gadolinium (Gd) structures that are generally made by sputtering, photo-lithography, and chemical wet etching. However, it is very challenging to fabricate a Gd structure with sufficient neutron attenuation of approximately more than 20 μm using a conventional metal deposition method because of the slow Gd deposition rate, film stress, high material cost, and so on. In this article, we fabricated the source gratings for neutron Talbot-Lau interferometers by filling the silicon structure with Gadox particles. The new fabrication method allowed us a very stable and efficient way to achieve a much higher Gadox filled structure than a Gd film structure, and is even more suitable for thermal polychromatic neutrons, which are more difficult to stop than cold neutrons. The newly fabricated source gratings were tested at the polychromatic thermal neutron grating interferometer system of HANARO at the Korea Atomic Energy Research Institute, and the visibilities and images from the neutron phase imaging system with the new source gratings were compared with those fabricated by a Gd deposition method.
    The Review of scientific instruments 06/2013; 84(6):063705. · 1.58 Impact Factor
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    ABSTRACT: This paper introduces a gamma-ray tomographic system which is transportable and can be used for on-line systems such as a pipeline operation. In a previous study, a feasibility study on a gamma-ray tomographic system with a scanning geometry of Electron Beam CT was carried out by Monte Carlo simulation. This paper contains a successive work on a previous study by developing and evaluating a real system. To construct a gamma-ray CT, 137Cs was used as a gamma-ray source and radiation measurement system with 72 channel CsI detectors whose crystal is a 12 mm×12 mm×20 mm rectangular parallelepiped was developed to operate jointly with a motion control system. ML-EM algorithm was used for image reconstruction of experimental data. Using the developed transportable gamma-ray system, laboratory and field experiments were carried out successfully. The field experiment results show that a gamma-ray CT with an Electron Beam CT scanning geometry can be a transportable gantry for objects which are parts of processes.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 11/2012; 693:203–208. · 1.32 Impact Factor
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    ABSTRACT: Recently, large-size 3-transistors (3-Tr) active pixel complementary metal-oxide silicon (CMOS) image sensors have been being used for medium-size digital X-ray radiography, such as dental computed tomography (CT), mammography and nondestructive testing (NDT) for consumer products. We designed and fabricated 50 µm × 50 µm 3-Tr test pixels having a pixel photodiode with various structures and shapes by using the TSMC 0.25-m standard CMOS process to compare their optical characteristics. The pixel photodiode output was continuously sampled while a test pixel was continuously illuminated by using 550-nm light at a constant intensity. The measurement was repeated 300 times for each test pixel to obtain reliable results on the mean and the variance of the pixel output at each sampling time. The sampling rate was 50 kHz, and the reset period was 200 msec. To estimate the conversion gain, we used the mean-variance method. From the measured results, the n-well/p-substrate photodiode, among 3 photodiode structures available in a standard CMOS process, showed the best performance at a low illumination equivalent to the typical X-ray signal range. The quantum efficiencies of the n+/p-well, n-well/p-substrate, and n+/p-substrate photodiodes were 18.5%, 62.1%, and 51.5%, respectively. From a comparison of pixels with rounded and rectangular corners, we found that a rounded corner structure could reduce the dark current in large-size pixels. A pixel with four rounded corners showed a reduced dark current of about 200fA compared to a pixel with four rectangular corners in our pixel sample size. Photodiodes with round p-implant openings showed about 5% higher dark current, but about 34% higher sensitivities, than the conventional photodiodes.
    Journal- Korean Physical Society 05/2012; 60(9). · 0.43 Impact Factor
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    ABSTRACT: The electron beam X-ray tomographic scanner has been used in industrial and medical field since it was developed two decades ago. However, X-ray electron beam tomography has remained as indoor equipment because of its bulky hardware of X-ray generation devices. By replacing X-ray devices of electron beam CT with a gamma-ray source, a tomographic system can be a portable device. This paper introduces analysis and simulation results on industrial gamma-ray tomographic system with scanning geometry similar to electron beam CT. The gamma-ray tomographic system is introduced through the geometrical layout and analysis on non-uniformly distributed problem. The proposed system adopts clamp-on type device to actualize portable industrial system. MCNPx is used to generate virtual experimental data. Pulse height spectra from F8 tally of MCNPx are obtained for single channel counting data of photo-peak and gross counting. Photo-peak and gross counting data are reconstructed for the cross-sectional image of simulation phantoms by ART, Total Variation algorithm and ML-EM. Image reconstruction results from Monte Carlo simulation show that the proposed tomographic system can provide the image solution for industrial objects. Those results provide the preliminary data for the tomographic scanner, which will be developed in future work.
    Applied radiation and isotopes: including data, instrumentation and methods for use in agriculture, industry and medicine 10/2011; 70(2):404-14. · 1.09 Impact Factor
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    ABSTRACT: We developed a novel pixel-structured scintillation screen with nanocrystalline Gd2O3:Eu particle sizes for high spatial resolution X-ray imaging detectors. Nanocrystalline Gd2O3:Eu scintillators were successfully synthesized with a hydrothermal method and a subsequent calcination treatment, which were used as a material for converting incident X-rays into visible light. In this work, silicon-based pixel structures with different 100, 50 and 30μm pixel sizes, a 10μm wall width and a 120μm thickness were prepared with the standard photolithography and the deep reactive ion etching (DRIE) process. Subsequently, a micro-pixel-structured scintillation screen was fabricated by adding the synthesized nanocrystalline Gd2O3:Eu scintillating phosphor to pixel-structured silicon arrays. Additionally, X-ray imaging performance such as relative light intensity, X-ray to light response and the spatial resolution in terms of modulation transfer function (MTF) were measured by using an X-ray source and a lens-coupled charge coupled device (CCD) camera system. The light intensity of the pixel-structured nanocrystalline Gd2O3:Eu screen was much higher than that of a pixel-structured sample made with a commercial microcrystalline Gd2O3:Eu product due to the density of the nanocrystalline Gd2O3:Eu scintillating powder-filled silicon structure. As the pixel size of the pixel-structured silicon decreased, the light intensity decreased. However, as the pixel size decreased, the spatial resolution significantly improved with no evident crosstalk from the emitted optical photons between adjacent scintillating pixels. The MTF of pixel-structured nanocrystalline Gd2O3:Eu screens with a 100 and a 50μm pixel size was 20% and 30% at 6lp/mm, respectively. As a result, this new technology showed that a microchannel structure based on a nanocrystalline Gd2O3:Eu scintillator could provide higher light intensity and high spatial resolution imaging compared to conventional microcrystalline scintillating phosphor.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 10/2011; 652(1):717-720. · 1.32 Impact Factor
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    ABSTRACT: Nanocrystalline Gd2O3:Eu scintillators were successfully synthesized using a hydrothermal method and subsequent calcination treatment in the electrical furnace as an X-ray to visible light conversion material for an indirect X-ray image sensor. In this work, various Gd2O3:Eu scintillators were prepared in accordance with different synthesis conditions such as doped-Eu concentration, different calcination temperatures of 600–1400°C and calcination time of 1–10h. The transition of morphology from nanorods to particles was observed as the calcination temperature of Gd2O3:Eu scintillator increased. And the phase transformation of the sample from cubic to monoclinic structure was discovered at 1300°C calcination temperature. In addition, scintillation properties such as luminescent spectra and light intensity under 266nm UV illumination were measured as a function of calcination condition of as-synthesized Gd2O3:Eu powder. The nanocrystalline Gd2O3:Eu scintillator with a strong red light emission at near 611nm wavelength under photo- and X-ray excitation will be employed for its potential X-ray image sensor applications in the future.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 10/2011; 652(1):212-215. · 1.32 Impact Factor
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    ABSTRACT: A SiPM is an array of several thousand micro-Geiger-mode APDs (GAPD), and a GAPD generally consists of a PN diode (Cd), a quenching resistor (Rq) and any parasitic capacitor (Cq) in parallel to the quenching resistor. The single-photon pulse shape is determined by a RC decay time, a product of Ctotal (Cd+Cq) and Rq. A large fraction of current in the long tailed single-photon pulse shape causes Scintillation detectors with SiPM to exhibit a poor timing resolution compared to PMTs, which have very narrow Gaussian single-photon pulse shape. In this paper, a ∼40fF Metal–Insulator–Metal (MIM) quenching capacitor (Cq-MIM) parallel to a quenching resistor in a Geiger mode APD is proposed to modify the single-photon pulse shape of a SiPM, in order to improve the timing performance of PET detectors. A single-photon pulse shape with Cq-MIM has a very fast pulse with a low slow tail, consequently the increasing initial current.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 09/2011; 650(1):125–128. · 1.32 Impact Factor
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    ABSTRACT: A novel quasi-pixel structured scintillation screen with nanocrystalline Gd2O3:Eu particle sizes was introduced for indirect X-ray imaging sensors with high sensitivity and high spatial resolution. A nanocrystalline Gd2O3:Eu scintillating phosphor with average 100nm sizes was used as a conversion material for incident X-rays into optical photons. In this work, silicon-based pixel structures with different 100 and 50μm pixel sizes, 10μm wall width and 120μm thickness were fabricated by a standard photolithography and deep reactive ion etching (DRIE) process. The pixelated scintillation screen was fabricated by filling the synthesized nanocrystalline Gd2O3:Eu scintillating phosphor into pixel-structured silicon arrays, and X-ray imaging performance such as relative light intensity, X-ray to light response and spatial resolution in terms of modulation transfer function (MTF) of the fabricated samples were measured. Although high spatial resolution imaging was largely achieved by pixel-structured nanocrystalline Gd2O3:Eu scintillation screens, X-ray sensitivity was still low for medical imaging applications. As a result, novel quasi-pixel structured screens with additional thin Gd2O2S:Tb scintillating layer were proposed for X-ray imaging detector with suitable sensitivity and spatial resolution in comparison with pixel-structured screens, and X-ray imaging performance of quasi-pixel structured nanocrystalline Gd2O3:Eu scintillating screens was investigated.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 08/2011; 648. · 1.32 Impact Factor
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    ABSTRACT: In this paper, a tomographic scan method with fixed installed detectors and a rotating gamma-ray source system is presented to diagnose industrial plants, which were impossible to examine by conventional tomographic systems. Monte Carlo simulations had been performed for two kinds of phantoms. Lab-scale experiments with the same condition as one of the phantoms, have been carried out. Algebraic and statistical reconstruction methods were applied for the reconstruction of simulation and experimental data. The reconstruction results from different algorithms were compared. Simulation results showed that reconstruction from the photopeak counting measurement gave better results than the gross counting measurement, although the photopeak counting measurement had large statistical errors. The statistical algorithm gives better results for tomographic scan methods than the algebraic method for the simulation data. Simulation and experimental data showed that this work demonstrates the feasibility of the fixed detection and rotating source system for scanning of industrial plants. Those results appear to be promising for industrial tomographic applications, especially for petrochemical industries.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 06/2011; 640(1):139-150. · 1.32 Impact Factor
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    ABSTRACT: We have synthesized europium-doped gadolinium oxide (Gd2O3:Eu) scintillators by a precipitation method with advantages of simple process and low-temperature calcinations for high resolution X-ray imaging detectors. The powders with about 15–30 nm particle sizes were obtained by subsequent calcinations of the precursor at a different temperature of 600–800 °C for 10 h. The wavelength of the main emission peak was about 610 nm and as temperature increased, light intensity of the scintillator increased in the X-ray luminescence case. Imaging performance such as X-ray linearity and spatial resolution of both commercial bulk and nanocrystalline-Gd2O3:Eu scintillator screens was measured and compared with the X-ray imaging system after coupling these to a CCD image sensor.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 05/2011; 633:S294–S296. · 1.32 Impact Factor
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    ABSTRACT: Due to the uncertainty of the annihilation event’s position along a given line-of-response (LOR), there is a large amount of statistical noise that is inherent to conventional PET image reconstruction. To overcome the poor spatial resolution of PET, time-of-flight positron emission tomography (TOF-PET) has been suggested. The use of the time-of-flight PET provides significant improvements to the image quality and 3 dimensional imaging. In this study, 2×2 mm2 SiPMs, operated in the Geiger avalanche mode, were designed and fabricated on epitaxial silicon wafers. Fabricated SiPMs whose doping structure is N+/P/π/P+, have a high geometric fill-factor with quenching resistors of the high resistive poly-silicon layer and a high sensitivity to short-wavelength light with the shallow-junction. Fundamental characteristics of SiPM were measured and analyzed. Achievable timing resolution was investigated and quantified with a LYSO crystal and SiPMs.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 05/2011; 633. · 1.32 Impact Factor
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    ABSTRACT: Thallium doped cesium iodide (CsI:Tl) scintillator films for the use as a converter for X-ray imaging detectors were fabricated by the thermal deposition method. The microstructures of these scintillating layers were affected by various deposition conditions such as vapor pressure, substrate temperature and post-heat treatment or rapid thermal annealing (RTA). CsI(Tl) scintillator films with various polycrystalline structures were manufactured under different process conditions and prepared for experiments. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to investigate the crystal structure and morphology properties. Light output and spatial resolution of the samples were strongly affected by the microstructures, which are determined by the deposition conditions and post-heat treatment. Imaging characteristics of the various CsI:Tl films were also measured under X-ray exposure conditions by coupling them to a CCD image sensor.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 05/2011; 633. · 1.32 Impact Factor
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    ABSTRACT: X-ray imaging detectors in combination with scintillator screens have been widely used in digital X-ray imaging applications. Gd2O2S:Tb was used as scintillation material for pixelated scintillator screens based on silicon substrates (wafer) with a micropore array of various dimensions fabricated using the photolithography and deep reactive ion etching (DRIE) process. The relative light output and the modulation transfer function (MTF) of each fabricated scintillator screen were measured by a cooled CCD and compared with those of Lanex screens. The spatial resolution of our scintillator screens was higher but their light outputs were lower than those of Lanex screen probably due to the loss of light at the wall surfaces. Therefore further treatment of the wall surface, such as reflective coating, seems necessary to compensate the light loss.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 05/2011; 633. · 1.32 Impact Factor
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    ABSTRACT: Recent advances in of silicon-based CMOS (complementary metal-oxide-semiconductor) flat panel detectors have resulted in an attractive use of cost-effective radiation imaging devices for X-ray and neutron radiography/tomography system. Indirect detection methods consisted of an X-ray converter (or a scintillator screen) and photodiode arrays are more widely used in high resolution micro-CT (computed tomography), dental and industrial NDT applications. In this study, The terbium-doped gadolinium oxysulfide (Gd<sub>2</sub>O<sub>2</sub> S:Tb, Gadox) scintillator screens with different thickness (several 30-140 m thickness) were directly coupled with a fiber-optic plate (FOP) of a commercially available CMOS imaging device for high resolution X-ray and thermal neutron radiography. The RadEye1 CMOS APS (active pixel sensor) imager having a large active area of 25 × 50 mm<sup>2</sup> and 48 m pixel pitch was selected for X-ray and thermal neutron imaging with high resolution. The scintillation properties and imaging performance such as relative light output, linearity and spatial resolution were measured and evaluated under X-ray and thermal neutron beam exposure. The good linearity and high spatial resolution characteristics in X-ray and thermal neutron imaging experiments were achieved by using compact, cost-effective imaging detector with exchangeable Gadox scintillator screens.
    IEEE Transactions on Nuclear Science 07/2010; · 1.46 Impact Factor
  • Source
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    ABSTRACT: Gd2O3:Eu scintillators with nano-crystalline structures were successfully synthesized through a precipitation method and subsequent calcination treatment as a converter for X-ray imaging detectors. In this work, a simple precipitation process was carried out using diethanolamine (DEA) as a precipitant to prepare nano-crystalline Eu-doped Gd2O3 powders. Scintillation properties such as luminescent spectra, light intensity and decay time were measured by varying the calcination temperature in heat-treatment of the synthesized powder. The sample prepared at 1200°C calcination temperature showed the highest light intensity. And the scintillator emitted a strong red light at near 611nm under photo- and X-ray luminescence for its potential X-ray imaging detector applications.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 07/2010; 619(1):174-176. · 1.32 Impact Factor

Publication Stats

250 Citations
88.40 Total Impact Points

Institutions

  • 1997–2014
    • Korea Advanced Institute of Science and Technology
      • Department of Nuclear and Quantum Engineering
      Sŏul, Seoul, South Korea
  • 2011
    • Korea Atomic Energy Research Institute (KAERI)
      Daiden, Daejeon, South Korea
  • 2005
    • Pusan National University
      • Department of Mechanical Engineering
      Pusan, Busan, South Korea
  • 2004–2005
    • Sungkyunkwan University
      • Department of Nuclear Medicine
      Sŏul, Seoul, South Korea
  • 2003
    • Kyung Hee University
      Sŏul, Seoul, South Korea