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82 5 Scintillator upgrade Background 134 Cs 238 U 232 Th activity 14.1 mBq/kg 0.75 ppt 0.38 ppt events/year 12×10 6 1×10 4 2×10 3 1CZT-1&2CsI 2×10 5 13 3 cut A-3σ 19237
5.3 Monte Carlo studies of the designed upgrade 83 decay chains produce 0.03 % and 0.04 % coincident (1CZT-1&2CsI) entries, being about 15000 and 7000 coincident events. Accordingly, the PMTs could become a main source of background and their use should be avoided. PAINT: The upper limit for contamination in the colourless paint for the CZT crystal was measured to be lower than 1.5 mBq/kg for 238 U, 0.6 mBq/kg for 232 Th and 50 mBq/kg for 40 K [58], resulting in less than about 220000, 11000 and 55000 decays per year (paint mass of 0.027g/CZT crystal). Less than 5 % of these decays produce coincident entries, whereas about 100 are in the cut C-3σ region. 222 RN decays with a half-life of 3.82 days, but feeds into 210 Pb with a half-life of 22 years. The last gamma active nuclei in the 222 Rn decay chain is 214 Bi. As Rn is a gas, it diffuses into every area with lower partial pressure. The decay product will be charged and accumulate on all surfaces. The air in the Gran Sasso laboratory can have radon activities of 35±2 Bq/m 3 [20]. The cavity in the Nest contains roughly 4·10 −4 m 3 , yielding about 15 mBq or half a million 222 Rn decays in one year. These is a maximal estimation, without nitrogen flushing. Simulation of one million events of the 222 Rn decay chain in the Nest air were produced. 3 % of these decays produce coincident (1CZT-1&2CsI) entries. These decays will produce about 520 events in the C-3σ region, being less than 7 % of the minimal expected 134 Cs background. RESULTS Background PMT (K/U/Th) Paint (K/U/Th) 222 Rn activity 504/78.2/25.5 mBq/PMT 4/0.1/1 Bq/kg 35 mBq/m 3 events/year 318/50/17×10 6 0.2/0.1/0.06×10 6 0.5×10 6 1CZT-1&2CsI 0.04/0.03/0.04 % ?/3.2/4.6 % 2.95 % Table 5.11: List of different background sources The detection of the neutrinoless double beta decay into the first excited state is not possible or at least very difficult with CsI crystals. The background of 134 Cs is more than one order of magnitude higher than the upper limits of the paint and 222 Rn background. As alternative scintillator CWO or BGO should be studied. NaI:Tl is due to is small density and the preferred read out with PMTs no solution, as even background optimised PMTs are expected to be a main
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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
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x Contents 4.1.2 Scintillation proc
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xii Contents
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2 1 Introduction The CdZnTe detecto
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4 2 Neutrino physics mass scales no
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6 2 Neutrino physics Ground states
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8 2 Neutrino physics mi: 〈mνe〉
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10 2 Neutrino physics
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12 3 The COBRA experiment CdZnTe (C
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14 3 The COBRA experiment 1420:480:
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16 3 The COBRA experiment Figure 3.
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18 3 The COBRA experiment They are
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20 3 The COBRA experiment Therefore
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22 4 Scintillation detectors search
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24 4 Scintillation detectors Figure
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26 4 Scintillation detectors spectr
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28 4 Scintillation detectors SCINTI
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30 4 Scintillation detectors emissi
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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
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 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
Page 110 and 111: 98 B SECOND APPENDIX B.2 Geant4 Gea
Page 112 and 113: 100 B SECOND APPENDIX Cube, scint64
Page 114 and 115: 102 B SECOND APPENDIX
Page 116 and 117: 104 Bibliography [13] H. V. Klapdor
Page 118 and 119: 106 Bibliography [43] T. Y. Kim et
Page 120: Typeset September 29, 2010
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