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46 4 Scintillation detectors Figure 4.11: Reflectivity of various reflector materials [28]. Figure 4.12: Scintillation detector with and without reflector, coupled directly or via a light guide to a PMT.
4.2 Detector assembly 47 drops sharply at 400 nm and the reflectivity of Al also decreases (figure 4.11). MgO is a suitable reflector also for light of lower wavelength. These external reflectors yield a good light collection efficiency which can be further improved by increasing the internal reflection. The material surrounding the scintillator should therefore have a low refractive index. A gap of air between the scintillator and the refractor is a convenient and effective solution. With such an assembly θB reduces to 42 ◦ for nscint = 1.5. The surface of the scintillator can be polished or ground leading to diffuse or specular internal reflection [29, 28]. OPTICAL COUPLING. The coupling between the scintillator and the electronic readout should transmit as much light as possible. A layer of air in between should be avoided. A small air gap between a CsI:Tl crystal (nCsI:T l=1.79) and a photodiode (nP D=1.5) reduces the electron yield by 15 % [39]. An optical glue, grease or oil with an refraction index between the scintillator and the PMT or diode window is the optimal optical coupling (see [39] for more details). The PMT window mostly consists of glass or quartz with an refraction index of nwindow=1.5. The refraction index of silicon matches very good with organic scintillators. Finding a suitable optical grease for inorganic scintillator is more complicated [28, 29]. LIGHT GUIDES are coupled in between the scintillator and the readout. They are used if the direct coupling is impossible or not desired due to lack of space, an inconvenient scintillator shape, the presence of magnetic fields or for a reduction of the radioactive background induced by the readout electronics. Photons, entering the light guide reach through internal reflexion to the readout. The light guide surface is often polished and good results are obtained with external reflectors. R. L. Garwin declared with phase space arguments that the photon flux density in light guides is not compressible. Accordingly, due to the Liouville Theorem a flux of light with the intensity Iin, incident at an area Ain, cannot be compressed into a smaller cross section area at the output Aout. Due to loss, the light intensity at the output is only in the ideal case Iin · Aout/Ain and in general smaller. [28, 29, 30] WAVE LENGTH SHIFTER (WLS) play an important role for big scintillator volumes. They permit the collection of scintillation light from big areas and enable the detection of the scintillation light with readout electronics of smaller area. Wave length shifters are essentially light guides with an admixture of a fluorescent wave length shifting material. They absorb incident scintillation light and reemit light with a longer wavelength. The WLS is coupled optically to the electronic readout, but is usually separated from the scintillator crystal by an air
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
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 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 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
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
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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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