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					전기광변조 방식을 이용한
방사선 모니터링 시스템 연구




          한국국제대학교 방사선학과

             발 표 자:최영준
             지도교수 : 박 지 군
Kinds of Radiation Detector

    Gas Detector      Scintillation Detector          Semiconductor Detector




- 원리 : 기체 전리현상     - 원리 : 고체 여기현상                     - 원리 : 고체 전리현상
- 시간 응답특성이 나쁨      - 광산란에 의한 낮은 분해능                   - 제조공정 어려움  고가
- 흡수율/검출효율이 낮음     - PMT(photomultiplier) 사용  고전압 요구 - 높은 열적잡음  냉각장치 요구



                                               MIST (Medical Imaging System & Technology)
Purpose of Research




                                 Liquid Crystal
     Photoconductor
                              As a Light Modulator




        New Concept of Radiation Detector
      Apply for personal monitoring system
                                MIST (Medical Imaging System & Technology)
 Basic concept

1.   X-ray photoconductor
      입사되는 외부 에너지에 의해서 전자ㆍ정공쌍 (EHPs)을 발생시키는 물질


      방사선 검출물질로써의 광도전체의 구비요건


              Physical properties                             comments
               Atomic Number (Z)                      Absorption increases with Z

            Energy Band Gap (Eg), eV                 Wide gap reduces dark current
                                                     Lower W increase the gain
       Charge Pair Creation Energy (W), eV
                                                     Good energy resolution power
      Mobility –Life time Product (mt), cm2/V   Higher mt increases the charge collection

         Operation Electric Field (E) V/mm       Lower E reduces electrical breakdown

           Processing Temperature (oC)           Lower temperature → simple process


                                                          MIST (Medical Imaging System & Technology)
Basic concept

  Kinds of X-ray photoconductor as a radiation conversion
   material
 Photoconductor          Si      GaAs         a-Se              PbI2             HgI2
   Atomic number         14       31/33         34               82/53           80/53
   Density (g/cm3)      2.33       5.32         4.3               6.2              6.4
  Band-gap Energy
                        1.12       1.4          2.2               2.4              2.1
       (eV)
   Work function
                        3.6        4.2          50                4.9              4.2
    (eV/EHP)
  Mobility-lifetime
                       0.4/0.2   10-5/10-6   10-7/10-6         10-5/10-6        10-4/10-6
     (cm2/V)
     Resistivity
                        104        107       1012~1016           1012             1013
     (W at RT)

  Melting point (ºC)   1414       1240         221                400              75

                                                      Frequently use as a radiation
                                                          conversion material
                                                     MIST (Medical Imaging System & Technology)
Basic concept

2.   액정 (Liquid crystal)
      결정 (Crystal)과 액체 (Liquid)의 성질을 모두 가지는 물질로 전기장의 인가에
         의해서 외부에서 조사되는 광원의 투과 정도를 조절할 수 있음.


                    Light

     V




                                V-T (Voltage vs Transmission) Curve


                                       MIST (Medical Imaging System & Technology)
 액정 셀의 V-T characteristics


                 12



                 10



                  8



                  6
                                                                          Series1


                  4



                  2



                  0
                      0   2   4   6           8         10        12




                                      MIST (Medical Imaging System & Technology)
  Concept of Radiation detector

 신개념의 방사선 검출기술 개발
  : 전기광변조 방식 (광도전층 + 액정 기반)                                   X-ray


   Top electrode
       layer
 Photoconductor         Electric field
      layer
Dielectric reflective
        layer
   Liquid crystal
       layer
 Bottom electrode
        layer
    Polarization
       layer

                                         Read-out light

                                                 MIST (Medical Imaging System & Technology)
       1st Concept of Radiation detector

    Radiation Source                          Light Source




     Photoconductor
                                                                     LC Cell


V                            Vpho
    Vpho
                                                                  Oscilloscope




    R  V pho  Twisting Angleliquid crystal  V photo diode


                                            MIST (Medical Imaging System & Technology)
 연구 개발 과정


                    전기광변조 방식의 방사선 검출기 모델 설계
1차 연구과정
(본 발표내용)


       광도전체 물질의 특성 분석                     액정 셀의 특성 분석



           제작 공정 연구 및 확립              액정 셀 제작 공정 연구 및 확립



       광도전체 필름의 특성 평가                  제작된 액정 셀의 특성 평가




                    광도전체 필름과 액정 셀의 접합 공정 연구



                    액정 기반 방사선 검출기의 제작 및 성능 평가


                                        MIST (Medical Imaging System & Technology)
연구 수행 방법(광도전체 제작연구)

 광도전체 층의 제작 공정 연구

        Thermal vapor evaporation              Screen printing




   • a-Se film                      • HgI2, PbI2 film
   • Thickness : 500 mm             • Thickness : 150 mm



                                            MIST (Medical Imaging System & Technology)
 연구 수행 방법

 제작된 광도전체 TP (Test pattern)의 물리적 특성 평가
   XRD (X-ray diffraction system) 분석
   SEM (Scanning Electron Microscope) 분석


 제작된 TP (Test Pattern)의 전기적 특성 평가
   암전류 (Dark current)


                                  Dark Box




                              Photoconductor




         Power Supply                                    Electrometer




                                               MIST (Medical Imaging System & Technology)
연구 수행 방법

 제작된 광도전체 TP (Test Pattern)의 전기적 특성 평가
   광민감도 (Photosensitivity)


                                               X-ray Unit


                                  Collimater

                                      Dark Box
         Current Amplifier

           Power Supply
                                                                       ITO
                                            Photoconductor
                                                                       ITO
                +    _

                                  voltage follow
                                      circuit



                             RL
                                                         Oscilloscope




                                               MIST (Medical Imaging System & Technology)
 광도전체 (Photoconductor)

 Results of Dark current

                                  3.0



                                  2.5
      Dark Current [ nA / cm2 ]




                                  2.0

                                                                          HgI2
                                  1.5                        PbI2



                                  1.0

                                                                                    a-Se
                                  0.5



                                  0.0
                                     0.1                 1                                    10

                                             Applied Voltage [ V / mm ]



                                           Dark current measurement



                                                                    MIST (Medical Imaging System & Technology)
 광도전체 (Photoconductor)

 Results of Photosensitivity

                                                2.5
          Photo sensitivity [ nC / mR / cm2 ]




                                                2.0




                                                1.5




                                                1.0




                                                0.5




                                                0.0
                                                      0   2          4           6             8            10

                                                                   Applied bias [ V / mm ]



                                                              Photosensitivity in a-Se film



                                                                                             MIST (Medical Imaging System & Technology)
 광도전체 (Photoconductor)

 Results of Photosensitivity

                                              14
        Photo sensitivity [ nC / mR / cm2 ]



                                              12


                                              10            HgI2


                                              8


                                              6


                                              4                          PbI2


                                              2


                                              0
                                               0.0    0.5                1.0               1.5               2.0

                                                               Applied bias [ V / mm ]


                                                     Photosensitivity in HgI2, PbI2 film



                                                                                         MIST (Medical Imaging System & Technology)
향후 연구 내용

 LC cell의 V-T curve와 Vpho의 매칭을 위한 증폭회로 구성



                       1.0
                                     Linear response


                       0.8
        Transmission




                       0.6



                       0.4



                       0.2



                       0.0
                             0   1   2             3        4           5

                                         Potential [ V ]




                                                           MIST (Medical Imaging System & Technology)
     참고 문헌
1.     Z. L. Xie., 0º ~ 360º bistable nematic liquid crystal display with    11.   H. Takano, S. Suzuki, H. Hatoh, Cell design of gray-scale thin-film-
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      88, number 4, 15 august 2000                                                 VOL. 36 NO. 1 JANUARY 1992
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      charge coupled device detector: performance considerations and               and G. Myer, A new real time noncoherent to coherent light image
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      Med. Phys. 19, 1195~1199 (1992)                                              Eng.14, 217~225 (1975)
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      4763~4771 (1989)                                                       14.   C Haugen, S O Kasap, J Rowlands. Charge transport and electron-
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      2432, 228 (1995)                                                             the Theoretical X-ray Sensitivity with HgI2 Direct Detection Image
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      Materials (Claredon, Oxford, 1979), p. 531                             16.   M. Schieber, H. Hermon, A. Zuck, et al. Polycrystalline mercuric
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      Ltd./BDH Publication, Darmstadt, 1992)                                 17.   N. M. Allinson, Solid-state Imaging Arrays for X-ray Detection,
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      Fronts and localized structures in a liquid-crystal-light-valve with   20.   G. Zentai, L. Partain, R. Pavlyuchkova, C. Proano, et al. Mercuric
      optical feedback, Physica D 199 (2004) 149~165                               Iodide and Lead Iodide X-Ray Detecotrs for Radiographic and
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                                                                                              MIST (Medical Imaging System & Technology)
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