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24 4 Scintillation detectors Figure 4.1: Photon mass attenuation coefficients for NaI, figure taken from [27] The total cross section varies in complex manner with E and Z. The photoelectron cross section for MeV photons varies approximately as Z to the power of 4 or 5, depending on the photon energy. So materials with high Z are preferable for γ-ray detection. Below 0.1 MeV the photo effect cross section is complicated by the presence of absorption edges at which the photoelectron cross section decreases sharply with E. There the photon energy is smaller than the binding energy Be = BK, BL, . . . and the corresponding electrons stop to contribute to the photo effect [28, 29, 30]. Compton scattering is the incoherent collision interaction of photons
4.1 Scintillator crystals 25 and quasi-free electrons. These electrons can be considered as free if the photon energies Eph are much higher than the binding energy of the electron Be. In Compton scattering the photon energy cannot be totally absorbed by the free electron. The scattered photon has an energy E ′ ph : E ′ ph = Eph , (4.1) 1 + α(1 + cosθ) which depends on the scattering angel of the photon θ and the initial energy of the photon Eph, with α = Eph/mec2 . The photon can scatter being maximal with different probabilities in all directions, with E ′ ph for small scattering angles around 0◦ and minimal for backscattering of 180◦ . Correspondingly, the transferred recoil energy of the electron can have values between zero for (no scattering) and the maximal transferred recoil energy Tcm in the case of backscattering. This continuum of recoil energies is called the Compton continuum with Tcm, being the Compton edge: Tcm = Eph 1 + (1/2α) (4.2) The total attenuation cross section for Compton scattering is energy dependent and can be divided into a component for absorbed energy σa and one for scattered energy σs. They represent the average fraction of the incident photon energy Eph which can be absorbed in the material after the conversion into the Compton electron of energy Tc or which is scattered in form of a photon in a collision. (see figure 4.1) When the photon energy exceeds twice the electron mass 2mec 2 , pair production is possible. It occurs only in the presence of an electric field of a charged particle, normally of the nucleus, but also of the electron if the photon energy exceeds 4mec 2 . The photon is completely absorbed and its energy is converted into the rest mass and kinetic energy Tpp = E − 2mec 2 of an electron-positron pair. The cross section rises approximately with Z 2 . These three effects lead to a spectrum of secondary electrons whose energies depend on the energy of the primary photon. The photoelectrons have energies Tpe = E − Be, the Compton electrons have a spectrum of energies Tcs from 0 to Tcs,max = E/(1 + mec 2 /2E), and pair production produces an electron-positron pair with energy Tpp = E − 2mec 2 . The relative number of each group of electrons depends, for a given material, on the energy of the initial photon. Figure 4.2 shows this energy
Page 1 and 2: A DESIGN STUDY FOR A COBRA UPGRADE
Page 3: I hereby declare that I have create
Page 6 and 7: vi Abstract
Page 8 and 9: viii Überblick
Page 10 and 11: x Contents 4.1.2 Scintillation proc
Page 12 and 13: xii Contents
Page 14 and 15: 2 1 Introduction The CdZnTe detecto
Page 16 and 17: 4 2 Neutrino physics mass scales no
Page 18 and 19: 6 2 Neutrino physics Ground states
Page 20 and 21: 8 2 Neutrino physics mi: 〈mνe〉
Page 22 and 23: 10 2 Neutrino physics
Page 24 and 25: 12 3 The COBRA experiment CdZnTe (C
Page 26 and 27: 14 3 The COBRA experiment 1420:480:
Page 28 and 29: 16 3 The COBRA experiment Figure 3.
Page 30 and 31: 18 3 The COBRA experiment They are
Page 32 and 33: 20 3 The COBRA experiment Therefore
Page 34 and 35: 22 4 Scintillation detectors search
Page 38 and 39: 26 4 Scintillation detectors spectr
Page 40 and 41: 28 4 Scintillation detectors SCINTI
Page 42 and 43: 30 4 Scintillation detectors emissi
Page 44 and 45: 32 4 Scintillation detectors activa
Page 46 and 47: 34 4 Scintillation detectors descri
Page 48 and 49: 36 4 Scintillation detectors Figure
Page 50 and 51: 38 4 Scintillation detectors group
Page 56 and 57: 44 4 Scintillation detectors AVALAN
Page 60 and 61: 48 4 Scintillation detectors gap. T
Page 62 and 63: 50 4 Scintillation detectors photoc
Page 64 and 65: 52 5 Scintillator upgrade ergy phys
Page 66 and 67: 54 5 Scintillator upgrade event ID
Page 68 and 69: 56 5 Scintillator upgrade Ephoton (
Page 70 and 71: 58 5 Scintillator upgrade back side
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Page 76 and 77: 64 5 Scintillator upgrade lecting e
Page 78 and 79: 66 5 Scintillator upgrade Figure 5.
Page 84 and 85: 72 5 Scintillator upgrade In 3.1 %
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74 5 Scintillator upgrade were repo
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76 5 Scintillator upgrade Figure 5.
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78 5 Scintillator upgrade crystal p
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80 5 Scintillator upgrade and gamma
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82 5 Scintillator upgrade Backgroun
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84 5 Scintillator upgrade source of
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86 6 Summary and future work dimens
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88 A FIRST APPENDIX Scintillator De
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90 A FIRST APPENDIX A.2 Reports abo
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92 A FIRST APPENDIX trons than a st
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94 B SECOND APPENDIX Figure B.1: Am
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96 B SECOND APPENDIX Figure B.5: Am
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98 B SECOND APPENDIX B.2 Geant4 Gea
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100 B SECOND APPENDIX Cube, scint64
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102 B SECOND APPENDIX
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104 Bibliography [13] H. V. Klapdor
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106 Bibliography [43] T. Y. Kim et
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Typeset September 29, 2010
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