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68 5 Scintillator upgrade Figure 5.5: Heprep visualisation of scint64 with (left) scintillator crystals and inner cavity, containing the Nest and Nest air and (right) whole constructed assembly with (from out to inside) lead and copper shield, scintillator crystals, delrin Nest, containing delrin crystals holders and CZT crystals. stopping power, is recorded after each step. The kind of the ionising particle is also important for a possible background reduction with pulse shape discrimination. 99 % of all alpha particles in the CsI scintillator can be identified with a pulse shape discrimination [32]. The particle type information can therefore be used to veto the majority of all alpha decays in the scintillator volume. It is recorded in this package in addition to the energy and scintillator ID for the sensitive scintillators, but not used in this analysis. Different kinds of radioactive decays in the extended COBRA detector can be simulated with VENOM, using macros. These macros define the used VENOM package, change the constructed geometry, define the kind of particle gun and confine the area, where events are initialised. A set of macros for the Scint64 package, which can be further used as set of default macros, were prepared. Accordingly, they initialise the Scint64 geometry, which is not changed by other commands. Three different particle guns are used for initialising double beta decays (Decay0), decays of the natural decay chain (Chaingen) and events caused by decays of single isotopes (Bkkgen). The area of these decays can be confined to certain geometric areas, points or logical volumes. The double beta decays are initialised in the CZT detector volumes. For this thesis, the background sources were only initialised in the CsI detector volumes, the CZT detector passivation, the Nest air and the PMT glass volumes. Of these, only the contamination of the CsI detector volumes were considered in detail and the other sources were just studied super-
5.3 Monte Carlo studies of the designed upgrade 69 ficially. A more detailed background study, as done by M. Heine [54] and previous works was not performed. They require a more detailed knowledge about the efficiency of the nitrogen flushing and the CZT colourless paint and scintillator read out contamination level. These examinations or not necessary at the current stage, where primary the suitability of the scintillator is to be determined. Detector processes, leading to an energy resolution or even position dependent energy resolution are not implemented in VENOM. The energy spectra, produced by VENOM, were convoluted by a Gaussian with an energy dependent standard deviation for each detector type. The used detector resolutions for the CZT and CsI:Tl detectors are: CZT : σ E CsI : σ E = 3.787286 E + 0.013377 [54] (5.1) = 0.58 √ E + 0.019 [43] (5.2) The CsI resolution function was published by Kim et al. [43] for similar crystal sizes and PMT readout. It was used for both sizes of scintillation detectors. Although the upgrade is planned with semiconductor readout, this resolution was used to estimate the expectable efficiency. The used resolution function of the CZT crystals is expected to be worse than the resolutions of the soon installed 64 CZT crystals. For this study, the raw VENOM data is convoluted with a Root routine. Afterwards the convoluted data is analysed with regard to the amount of deposited energy in a single or all detectors and the number of involved detectors. It is analysed in order to find cuts with high detection efficiency for the decay signal and a low background count rate. The emphasis lies on coincident events in the CZT and CsI detector, which are unlikely to be imitated by the majority of background sources. 5.3.2 2νββ signal At first the signal, the neutrino accompanied double beta decay of 116 Cd into the first excited state, was studied. 116 Cd → 116 Sn(2 + ) + 2e − + 2¯ν, E ∗ = 1294 keV. (5.3) This transition was not measured for 116 Cd yet. The current upper limit of the half life is 2.4·10 21 years (90 %C.L.) [12]. Accordingly, the amount
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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.
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
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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