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58 5 Scintillator upgrade back side of the scintillator layer are favoured by the restricted space inside the Faraday cage. A readout from only two sides also reduces the amount of needed readout devices. For such a readout, 12 crystals of 20×5×5 cm 3 dimensions and 8 cubes of 5×5×5 cm 3 are necessary. The crystal bars are placed at the right, left, top and bottom side of the Nest. The readout will be attached to the front faces of the crystals. The front and back side of the Nest will be covered by four cubic crystals each. The scintillation light of the cubic crystals can be detected from only one side. The optimal readout device should provide a good energy resolution with CsI:Tl crystals, have low radioactive contamination and be compact in size. Reported resolutions of other experiments using CsI:Tl crystal with different readout devices help to estimate the expected energy resolution. However, the listed resolutions in chapter 4.2.3—“Energy resolution” can give only an orientation for these crystal sizes. Especially comparisons between different readout devices for scintillators of less than one cm 3 are not valid for crystals of a few hundred cm 3 . The limited dimensions of semiconductor readout devices in comparison to the crystal surface worsens the energy resolution. GEANT4 APPLICATIONS CSI The Geant4 application ”CsI” contains a PhysicsList similar to the previous described ”Scintblock”. An OpticalPhysicsList was introduced as additional class for simulating the scintillation process. It defines a new particle class, the optical photon, and new processes, like the absorption Process (G4OpAbsorption()), Rayleigh scattering (G4OpRayleigh()), boundary processes (G4OpBoundaryProcess()), scintillation (G4Scintillation()), Cerenkov radiation (G4Cerenkov()) and wave length shifting (G4OpWLS()). These classes are described in [48, 49]. A single CsI crystal (5×5×20 cm 3 ) was constructed in a volume filled with air. It is wrapped in a layer of MgO and attached to a readout device at the two narrow faces of the scintillator. The readout device is simulated by a thin glass plate of 0.625 mm and an Al layer of the same width. The incident photons are initialized outside of the scintillator material and hit the crystal surface in the middle between the two readout devices, perpendicular to the surface. The introduced optical photons are produced in the scintillating material after the deposition of energy by
5.1 Monte Carlo studies for the upgrade design 59 the incident ionizing particle. Therefore, the scintillation properties of a material must be defined. These properties are the spectral distribution of a possible slow and fast component of the scintillation light, the refractive index and the absorption length as a function of the photon energy. The scintillation light yield, fast and slow time constants and the ratio of these time constants must also be defined. The values of the wavelength dependent material properties are linked to the corresponding momentum of the optical photon. The emission spectrum is between 1.2399 eV (1000 nm) and 4.1329 eV (300 nm) [36]. The index of refraction increases over this range of energy from 1.760 to 1.979 [50]. A quenching factor of kB = 2.3·10 −3 gMeV −1 cm −2 , as quoted by Tretyak [31] for CsI:Tl, was used. Corresponding to these definitions, a number of Gaussian-distributed optical photons are initialized isotropically at the point of interaction. These photons can be absorbed or reach the crystal surface. At medium boundaries, optical photons can be reflected or refracted. For a perfectly smooth surface between two dielectric mediums, the refractive index is sufficient. All other surfaces must be defined separately. The Geant4 surface can be of two types, a skin surface (surrounding one logical volume) and a border surface (being the boundary between two logical volumes). An optical surface requires the definition of a G4OpticalSurfaceModel, a G4OpticalSurfaceFinish (polished, ground, polished- and ground-, back- or front-painted) and to specify the surface as boundary between dielectric-metal, or dielectric-dielectric materials. The reflectivity of the surface can be defined between 0.0 (no reflection) and 1.0 (no transmission). In this simulation, a 0.5 mm layer of MgO surrounds the crystal. This boundary surface was defined as polished-frontpainted (polished surface with reflector attached without air gap) with a reflectivity of 0.98 for MgO [28]. The optical photons can reach the readout device directly or after several reflections at the crystals boundary. The Al layer, presenting the photo cathode, is defined as sensitive detector volume. The detection of optical photons differs from that of other particle classes. The optical photons are not detected after absorption, but in the optical boundary process by passing the boundary surface. The detection is based on the definition of the detection efficiency on the skin of the sensitive volume. The boundary between the glass and the Al layer is defined as polished and dielectric-metal. The reflectivity is zero and the absorption efficiency is defined as shown in figure 4.10. The quantum efficiencies for seven readout devices are defined, which are shown in figure 5.2. There are quantum efficiencies belonging to
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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
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 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
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Typeset September 29, 2010
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