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48 4 Scintillation detectors gap. Though this reduces the transmission from the scintillator to the WLS, it also reduces the transmission of the re-emitted light into the scintillator and increases so the part of detected light. Whether WLSs improve the detected signal depends strongly on the scintillator geometry and produced scintillation light. For scintillators with an emission spectrum higher than the optimal wavelength of the readout electronic a shift can improve the detection efficiency significantly. A stepwise shifting with more than one WLS, to reduce the readout area further, is also possible. SIZE OF SENSITIVE AREA. The just discussed techniques do improve the detected signal. Nevertheless, especially for big scintillators, the only way to obtain a usable signal is often to increase the readout area. Therefore either more or bigger readout devices are necessary. Multiple readout devices at opposite scintillator sites improve the uniformity of light collection. With coincidence analysis, background due to dark current of the readout devices can be reduced. Moreover a position reconstruction in addition to an energy detection is also possible with more than one readout device. Lee et al. [42] reported position accuracy of less than one cm with 40 cm long CsI:Tl coupled to two red enhanced PMT. 4.2.3 Energy resolution The energy resolution of a scintillation detector depends on the resolution of the scintillator, the collection efficiency of the scintillation light and the intrinsic resolution of the electronic readout device. The resolution of the scintillator is determined by the amount of produced scintillation photons and their variation. Thereby, the efficiency of radiation absorption limits the amount of ionizing particles which can deposit their full energy. This is especially for gamma ray detection at low count rates a limiting factor and materials with high Z and high density are preferred. Non-uniform distributions of the luminescence centres in the lattice produce local variations of the intrinsic scintillation efficiency. As was discussed previously, L also depends on dE/dx. Accordingly, the light yield for photons of the same energy varies in dependence of the amount and energy of produced photo, Compton, pair and Augerelectrons. Initially produced photoelectrons have higher energies than Compton electrons and a subsequent photoelectron, produced by inci-
4.2 Detector assembly 49 dent photon of the same energy. The scintillation yield L/E decreases with increasing particle energy E. This leads to more produced scintillation photons for Compton scattered gamma rays than for photons absorbed directly by the photo effect. The full energy line width depends on the contribution of Compton scattering and the photo effect. It gets wider the more both processes contribute. With an increasing scintillator size the contribution by Compton effect rises. Therefore the intrinsic resolution decreases while the amount of fully absorbed gamma rays increases. For high count rates scintillators of small crystal sizes have better energy resolution. The efficiency for the collection of scintillation light depends in general on the scintillator dimensions, the size and position of the readout device and the opacity of the crystal. Thus, it also depends on the wave length of the emission spectrum, the refractive index of the scintillator, on the optical coupling, on the refractor and on the entrance window of the readout device at these wavelengths. Ultimately, absorption and loss reduce the detectable scintillation light and worsens the resolution. Readout devices at opposite sides of the scintillator increase the collection efficiency in general and increase the uniformity of the light collection, leading to decreasing variance. Birks [27] quoted that larger crystals have worse resolution due to difficult uniform light collection. Non-proportional scintillator response contributes in the same order of magnitude to a decrease in resolution as nonuniform light collection. The intrinsic resolution of the readout device is determined by the photoelectron emission efficiency and the electron multiplication process. The quantum efficiency of the photocathode limits the amount of produced photoelectrons. It depends on the wavelength of the incident photon and on the point of impact. A nonuniformity of the photocathode leads only for very low count rates to a wider line width and does not contribute otherwise. Thus large diameter PMTs should be avoided in this case. Finally variations in the multiplication process of the photoelectron in the case of PMT, APD and Si-PMT contribute to the resolution. All readout divices loose resolution due to dark current and PIN diodes especially due to electronic noise [27, 30]. For the simulation, described in the next chapter, the energy resolution, reported by Kim et al. [43], was used. The KIMS Collaboration used CsI:Tl crystals with surfaces from 55x55 to 70x70 mm 2 and 300 mm length. The crystals were wrapped in two 0.2 mm thick Teflon layers and couplet to two three inch PMT’s type D726UK, with RbCs
Page 1 and 2:
A DESIGN STUDY FOR A COBRA UPGRADE
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I hereby declare that I have create
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vi Abstract
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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 36 and 37: 24 4 Scintillation detectors Figure
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 50 and 51: 38 4 Scintillation detectors group
Page 56 and 57: 44 4 Scintillation detectors AVALAN
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
Page 72 and 73: 60 5 Scintillator upgrade three dif
Page 74 and 75: 62 5 Scintillator upgrade vice. The
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 %
Page 86 and 87: 74 5 Scintillator upgrade were repo
Page 90 and 91: 78 5 Scintillator upgrade crystal p
Page 92 and 93: 80 5 Scintillator upgrade and gamma
Page 94 and 95: 82 5 Scintillator upgrade Backgroun
Page 96 and 97: 84 5 Scintillator upgrade source of
Page 98 and 99: 86 6 Summary and future work dimens
Page 100 and 101: 88 A FIRST APPENDIX Scintillator De
Page 102 and 103: 90 A FIRST APPENDIX A.2 Reports abo
Page 104 and 105: 92 A FIRST APPENDIX trons than a st
Page 106 and 107: 94 B SECOND APPENDIX Figure B.1: Am
Page 108 and 109: 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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