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50 4 Scintillation detectors photocathode, fabricated by Electron Tubes Ltd.. The energy resolution at 662 keV was around 4.2 % (RMS) and the resolution curve was parametrised by σ = 0.58 · � E (keV ) + 0.0191 · E (keV ). The relative standard derivation was evaluated with radioactive sources like 55 Fe, 241 Am, 57 Co, 137 Cs, 22 Na and 60 Co positioned in the centre of the crystal and a 60 Co source with collimator positioned in 5 cm distances. In this way the energy dependent and also position depending resolution due to doping nonuniformity and different collection efficiency was regarded [43]. The announced standard derivation σ determines the FWHM of any Gaussian through the relation FWHM= 2 √ 2ln2σ ≈ 2.35σ. The requirements for the COBRA upgrade are, as is discussed in the previous chapter 3, a low radioactive contamination, a high energy resolution, a moderate price and a high detection efficiency at compact size. The four scintillators NaI:Tl, CsI:Tl, BGO and CWO can provide some of these properties. All four scintillators can have low radioactive contamination, depending on the used scintillator powder and crystal production process. CWO contains the intrinsic 113 Cd, which has a low Q-value, BGO can contain 207 Bi, which has a problematically high Q-value and CsI:Tl contains the anthropogenic radioisotope 137 Cs. CWO and BGO have high density and high Z, but a lower light yield than NaI:Tl and CsI:Tl. Especially NaI:Tl has low density and Z, whereas the level of radioactive contamination can be very small. The drawback of NaI:Tl is also the emission spectrum, matching well with bialkali PMTs. CWO, BGO and CsI:Tl can be read out with semiconductor readouts. The reached energy resolution depends, beside the light yield of the scintillator and the matching of the scintillation emission spectrum with the quantum efficiency of the readout, also on the crystal and readout dimensions. It is not possible to deduce the reachable energy resolution of a scintillator of several hundred cm 3 from the resolution obtained with a small scintillator. The required compact size of the planned COBRA upgrade favours the scintillator readout with semiconductors, providing, the detection efficiency and energy resolution are high enough and the background contamination is small enough. The consideration, which of these scintillators is most suitable, requires studies, which are presented in the following chapter.
Chapter 5 Scintillator upgrade After discussing possible scintillator materials and readout devices in the previous chapter, this chapter will determine the kind of designed scintillation detector. Therefore, Monte Carlo simulations were performed to optimize the design of the scintillator upgrade. These simulations help to choose the optimal crystal size and readout electronic and are described in 5.1—“Monte Carlo studies for the upgrade design”. Accordingly, in 5.2—“The design” a detector upgrade with scintillation detectors is presented and the detection efficiency for a COBRA set up with a CsI:Tl detector component was studied with Monte Carlo simulations, described in 5.3—“Monte Carlo studies of the designed upgrade”. For this thesis, three Geant4 simulations were written, two independent Geant4 applications (”ScintBlock” and ”CsI”) and one package within the Venom simulation framework. The simulations were performed with the Geant4 software toolkit for detector simulations. The resulting simulated data were analysed with the ROOT data analysis framework. Geant4 is an object-oriented toolkit for the simulation of the passage of particles through matter, written in C++. It provides a realistic reproduction of physical processes for an energy range from 250 eV up to a few GeV [44]. The Geant4 toolkit includes facilities for the handling of geometry, the tracking, the detector response and the visualization. The software is used by research projects at application areas including high en- 51
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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
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 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
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
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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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