Gyuseong Cho

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

Are you Gyuseong Cho?

Claim your profile

Publications (115)103.58 Total impact

  • Woo-Suk Sul, Hyoungtaek Kim, Gyuseong Cho
    [Show abstract] [Hide abstract]
    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.
    Journal of Instrumentation 04/2015; 10(04):C04002-C04002. DOI:10.1088/1748-0221/10/04/C04002 · 1.53 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    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.
    Nuclear Engineering and Technology 03/2015; 567. DOI:10.1016/j.net.2015.02.001 · 0.76 Impact Factor
  • [Show abstract] [Hide abstract]
    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.
    Journal- Korean Physical Society 02/2015; 66(4):571-577. DOI:10.3938/jkps.66.571 · 0.43 Impact Factor
  • Hyoungtaek Kim, Woo Suk Sul, Gyuseong Cho
    [Show abstract] [Hide abstract]
    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.
    Review of Scientific Instruments 10/2014; 85(10):103107. DOI:10.1063/1.4896757 · 1.58 Impact Factor
  • [Show abstract] [Hide abstract]
    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
  • [Show abstract] [Hide abstract]
    ABSTRACT: Some scattered neutron fields for the calibration of personal dosemeters and survey meters used for radiation protection purpose were constructed in the Radiation Calibration Laboratory (RCL) of Korea Atomic Energy Research Institute (KAERI). The radionuclide sources of 252Cf and 241AmBe were used for producing the neutron calibration fields with not so much different method recommended by ISO. The calibration points of interest were behind the shadow objects and the concrete wall in the irradiation room. In order to characterize the neutron calibration fields at the point of test, the spectral neutron fluence rate was determined by means of the Bonner Multi-sphere Spectrometry System (BMSS) and the measured spectra unfolded using the BUNKI code. This paper shows the dosimetric data available for calibration and the feasibility of use as the Realistic Neutron Calibration Fields (RNCF) of KAERI.
    Journal of Nuclear Science and Technology 08/2014; 37(sup1):781-784. DOI:10.1080/00223131.2000.10874996 · 1.45 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Presently the gamma scintillation camera is widely used in various industrial, environmental and medical diagnostic fields. Two major performance parameters, spatial resolution and noise, are primarily determined by the collimator. The imaging performance of the simple collimator, single pin-hole, and a coded-aperture collimator, uniformly redundant array (URA), are analyzed in this study. Though parallel-hole collimators are used in some medical applications, in principle its performance is equivalent to that of a single-hole collimator and moreover its use is limited to the near-field application only. A coded aperture imaging (CAI) techniques have been proposed for gamma-ray imaging especially for far-field applications such as the astrophysics study or environmental monitoring in order to overcome these limitations of a pin-hole or a parallel-hole collimator.
    Journal of Nuclear Science and Technology 08/2014; 45(sup5):530-533. DOI:10.1080/00223131.2008.10875908 · 1.45 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The effect of scattered radiation on the transmitted gamma ray signal should be carefully taken into account because it certainly affects the quality of reconstructed tomography images. Therefore this effect was evaluated with different counting modes and gamma energies. For less dense objects such as wood and plastics, the build-up effect can be ignored and 137Cs can yield the better result than 60Co. On the contrary, for the relatively heavy material such as oil and metal the scattered radiation effect of 137Cs source should be considered because the build-up effect is more serious than 60Co. It was also found that the image quality gets better when the count data of photo-peak were used rather than the total counts from the full spectrum. The image improvement technique using precise weight matrix calculation method was suggested to remove the influence of the scattered radiation on the reconstructed image quality. From the experiment, it was concluded that the effect of scattered radiation on the image quality is varied depending on the material composition and radiation energy. Before the implementation in the field, normally the information of the process material is available so the optimal measurement condition can be predetermined on the basis of this study. The additional data processing method is also expected to improve the reconstructed image quality further.
    Journal of Nuclear Science and Technology 08/2014; 45(sup5):371-374. DOI:10.1080/00223131.2008.10875866 · 1.45 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Isotropic light spread in the scintillator film decreases the spatial resolution of scintillator-based digital X-ray imaging systems such as digital radiography and mammography. Pixelation of the scintillator film could be a good solution to overcome this limitation. This has been demonstrated with pixilated CsI:Tl layers which was made by thermal evaporation process on a pre-patterned substrate or which was post-patterned by laser after preparation. Additionally, in order to minimize cross-talk between pixels and to maximize light collection efficiency, a reflection material can be coated on the top and side surfaces of each pixilated scintillator block. In This paper, several materials such as Al, and quarter-wave multilayer reflector with SiO2 and TiO2 were considered as the reflector materials of pixilated CsI:Tl scintillator blocks. Through a serious of simulations given below on these simple 2-dimensional scintillator blocks coated by a reflector, reflectivity, cross-talk, and modulation transfer function were calculated to find the optimum reflector.
    Journal of Nuclear Science and Technology 08/2014; 45(sup5):485-488. DOI:10.1080/00223131.2008.10875896 · 1.45 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: An extended Bonner Sphere (BS) set consisting of four polyethylene (PE) spheres and three PE spheres with an inserted lead shell to increase the detection efficiency for high energy neutron was used to measure the neutron spectra from the thick Pb target bombarded by 65 MeV electron at the Pohang Accelerator Laboratory (PAL). The response function of the BS was calculated using the MCNPX code and calibrated using a 252Cf neutron source. Active BS using the LiI(Eu) scintillator for detecting of the thermal neutron was implemented outside the target room and passive BS with gold foils was used as the thermal neutron detector due to a high intense field of gamma radiation inside the room. With the results of the spectral measurement, the design and calculation of the response function of a newly modified BS of the Korea Atomic Energy Research Institute (KAERI) are also presented.
    Journal of Nuclear Science and Technology 08/2014; 41(sup4):176-179. DOI:10.1080/00223131.2004.10875673 · 1.45 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Columnar-structured cesium iodide (CsI) scintillators doped with thallium (Tl) are frequently used as x-ray converters in medical and industrial imaging. In this study we investigated the imaging characteristics of CsI:T1 films with various reflective layers aluminum (Al), chromium (Cr), and titanium dioxide (TiO2) powder coated on glass substrates. We used two effusion-cell sources in a thermal evaporator system to fabricate CsI:T1 films on substrates. The scintillators were observed via scanning electron microscopy (SEM), and scintillation characteristics were evaluated on the basis of the emission spectrum, light output, light response to x-ray dose, modulation transfer function (MTF), and x-ray images. Compared to control films without a reflective layer, CsI:T1 films with reflective layers showed better sensitivity and light collection efficiency, and the film with a TiO2 reflective layer showed the best properties.
    Journal of the Optical Society of Korea 06/2014; 18(3):256-260. DOI:10.3807/JOSK.2014.18.3.256 · 0.96 Impact Factor
  • Jongyul Kim, Seung Wook Lee, Gyuseong Cho
    [Show abstract] [Hide abstract]
    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. DOI:10.1016/j.nima.2014.01.051 · 1.32 Impact Factor
  • [Show abstract] [Hide abstract]
    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). DOI:10.3938/jkps.64.510 · 0.43 Impact Factor
  • [Show abstract] [Hide abstract]
    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. DOI:10.1063/1.4810014 · 1.58 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The prototype SiPM was designed and fabricated for MRI compatible PET using the customized CMOS process at National Nanofab Center in KAIST. The SiPM was designed to have a size of 3x3 mm(2) composed of micro-cells of 65x65 mu m(2) with a fill factor of 68 %. The size of a micro-cell was determined by optimization between the photon detection efficiency (PDE) and the dynamic range for the photons of 511 keV from LYSO crystal. In the micro-cell structure, a specially designed quenching capacitor (QC) is added parallel to quenching resistor using the Metal-Insulator-Metal (MIM) process. This QC integrated SiPMs (QC-SiPM) was devised to realize rapid response of output pulses and to enhance the timing resolution of SiPM. Coincidence timing resolution of PET detectors depends on the output pulse shapes which are the convolution of the intrinsic pulse shape of scintillation crystals and the single photon pulse shape at the micro-cell in a SiPM. A quenching capacitor parallel to a quenching resistor provides a fast current path at the beginning stage of avalanche process, than reduces rising time of single photon pulse shape. In this study the rise time of the QC-SiPM signal was analyzed to be 22.5 ns while that for the regular SiPM was 34.3 ns. (C) 2012 Published by Elsevier B.V. Selection and/or peer review under responsibility of the organizing committee for TIPP 11.
    Physics Procedia 12/2012; 37:1511-1517. DOI:10.1016/j.phpro.2012.05.327
  • [Show abstract] [Hide abstract]
    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. DOI:10.1016/j.nima.2012.07.046 · 1.32 Impact Factor
  • [Show abstract] [Hide abstract]
    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). DOI:10.3938/jkps.60.1413 · 0.43 Impact Factor
  • Journal of Scientific Research 01/2012; 5(1). DOI:10.3329/jsr.v5i1.11935
  • [Show abstract] [Hide abstract]
    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. DOI:10.1016/j.apradiso.2011.09.019 · 1.06 Impact Factor
  • [Show abstract] [Hide abstract]
    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. DOI:10.1016/j.nima.2011.01.024 · 1.32 Impact Factor

Publication Stats

391 Citations
103.58 Total Impact Points

Institutions

  • 1997–2015
    • Korea Advanced Institute of Science and Technology
      • Department of Nuclear and Quantum Engineering
      Sŏul, Seoul, South Korea
  • 2004–2014
    • Korea Atomic Energy Research Institute (KAERI)
      • Health Physics Department
      Daiden, Daejeon, South Korea
    • Sungkyunkwan University
      • Department of Nuclear Medicine
      Sŏul, Seoul, South Korea
  • 2005
    • Pusan National University
      • Department of Mechanical Engineering
      Pusan, Busan, South Korea
  • 2003
    • Kyung Hee University
      Sŏul, Seoul, South Korea