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44 4 Scintillation detectors AVALANCHE PHOTO DIODES (APDs) are diodes with an internal multiplication process. The small amount of charge, which is produced in conventional diodes, is increased with the avalanche effect. The charge carriers are accelerated sufficiently between collisions with the lattice to produce additional electron hole pairs. The gain is independent of the amount of incident light for applied voltages higher than the breakdown voltage. APDs are used with applied reverse voltage near below the breakdown voltage. There, the gain is proportional to the applied voltage. With a gain of a few hundred a signal is produced that is bigger than the electronic noise level. The quantum efficiency is 80 %. The signal level is very sensitive to temperature and the applied voltage. Statistical variations of the avalanche effect lead to considerable fluctuations in the multiplication process. These are larger than the fluctuations in the multiplication process of PMTs and can significantly contribute to the energy resolution. It becomes usually inferior to conventional silicon photodiodes. A better energy resolution for lower particle energies, as can be achieved with conventional diodes, is nevertheless possible, because the noise level of the preamplifier is usually much higher than the noise level of diodes. The dark current depends on the diode size and is between 0.5 and 50 nA [29, 41]. SI-PMT are pixelated avalanche diodes used in Geiger mode. This is the operation at break down voltage, where the gain is independent of the bias voltage. Si-PMT are with a gain of 10 5 to 10 6 competitive to PMTs. The signal of a Si-PMT is the sum of each APD pixel output. Single photons produce detectable charge avalanches. This allows the counting of single photons or the detection of pulses of multiple photons. Due to the limited pixel number, saturation occurs for high count rates. Room temperature operation is possible. They have good time resolution and can work in magnetic fields. 4.2.2 Detector mounting Having a scintillator and an electronic readout device, the latter must be mounted onto the scintillator in a way that allows the detection of as much scintillation light as possible. Self-absorption and losses at the scintillator surfaces impede the collection of the entire produced scintillation light. The amount of self-absorption depends on the scintillator size and opacity. It becomes a non-negligible factor for scintillator length that is in the order of the attenuation length of the crystal. This can be, depending on the quality of the crystal, up to meters.
4.2 Detector assembly 45 advantages PMT PIN APD Si-PMT quantum efficiency 20-30% 60-80% 60-80% 60-80% gain 10 6 − 10 9 1 500 10 6 big active area � - - - compact size - � � � disadvantages sensitive to magnetic fields � - - - bias/supply voltage � - - - high temperature dependence � � � - high voltage dependence � � � - high noise level - � � - Table 4.3: Advantages and disadvantages of different readout devices Scintillation light is emitted isotropically from the track of the ionizing particle and only a small fraction reaches the photocathode or diode directly. The rest impinges at the boundary and is reflected or transmitted. The total internal reflexion occurs for light with incident angles greater than the Brewster angle θB = sin −1 (nout/nscint) with nscint and nout the refraction index of the scintillator and the surrounding material. For smaller angles partial reflection occurs where a part of the light is reflected and the remainder is transmitted. This losses reduce the efficiency of scintillation light detection and consequently the energy resolution. They also lead to a non-linearity of the detected signal depending on the point of emission. There are several methods to increase the efficiency of light collection. SCINTILLATOR WRAPPING AND SURFACE FINISH. The amount of reflected light can be increased by using a reflector that redirects the light back into the scintillator volume where it reaches the readout directly or via more reflections. With each reflection some degradations occur due to absorption and transmission, thus the number of reflections is limited. The reflector surface can be specular or diffuse. At specular reflexion the incident angle θi equals the reflection angle θr. At diffuse reflexion θr is independent of θi and follows Lambert’s cosine law dI/dθr ∝ cosθr with I the intensity of reflected light. Common aluminium foil is used as specular reflector and MGO, TiO2 and aluminium oxide are good diffuse reflectors. The difference between specular and diffuse reflection is small and depends on the scintillator geometry. Care must be taken for scintillator with emission spectrum smaller than 400 nm. The reflectivity of TiO2
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A DESIGN STUDY FOR A COBRA UPGRADE
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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 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 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 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
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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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