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2 1 Introduction The CdZnTe detectors used by COBRA are 1 cm 3 small semiconductor crystals with a low gamma ray detection efficiency, leading to a correspondingly low detection efficiencies for ββ-decays into excited states. The aim of this thesis is to design a COBRA upgrade with improved detection efficiency for 2νββ-decays into first excited states. These can be done by expanding the current COBRA apparatus with scintillation detectors, providing they are suitable for a low background level experiment. The particle detection with scintillation detectors is therefore discussed in chapter 4—“Scintillation detectors”, after a short introduction into the physics of double beta decays in chapter 2—“Neutrino physics” and into the COBRA experiment in chapter 3—“The COBRA experiment”, which explains the demands on a COBRA scintillator upgrade. In chapter 5—“Scintillator upgrade”, the work of this thesis will be explained. Two developed simulations, helping to determine the size and kind of scintillation detector, will be presented. In the following, the development and analysis of a Monte Carlo simulation of the designed scintillator upgrade for evaluating the suitability of this upgrade is discussed. A future outlook follows.
Chapter 2 Neutrino physics The neutrino is the lightest elementary particle. It has no electric charge and is interacting so weakly, that several thousand light years of lead would be necessary for stopping a neutrino beam. On Earth, neutrino fluxes of the order of 10 10 per cm 2 and second are passing through, without anyone but a few neutrino oscillation experiments noticing. Neutrinos provide unique information about the present processes in the centre of our sun, do announce supernovae and can supply more knowledge about the interior of our earth. However, neither their actual mass, nor their actual particle nature, being Majorana of Dirac, is known yet. The neutrino was postulated in 1930 by Pauli [4], explaining the seemingly missing energy in nuclear beta decay instead of dismissing the conservation of energy. In 1953 the neutrino was first detected by Cowan and Reines [5], using a nuclear reactor as neutrino source. In the Standard Model of Particle Physics the neutrino was introduced as massless fermion, having three flavours (e, µ and τ) and coupling only to weak left-handed charged currents. More than ten years after its first detection, Davis [6] started the solar neutrino puzzle by detecting only one third of the expected 8 B neutrino flux from the sun, to call either his experiment, the Solar Standard Model or the understanding of neutrinos into question. This problem was solved not until 2001 by the measurements of the SNO experiment, confirming Davis’s experiment and the Solar Standard Model, by proving the neutrino flavour oscillation and therefore the existence of non-zero neutrino masses. The oscillation experiments are only sensitive the mass-square differences of the neutrino mass eigenstates ∆m2 ij and can neither determine the absolute 3
Page 1 and 2: A DESIGN STUDY FOR A COBRA UPGRADE
Page 3: I hereby declare that I have create
Page 6 and 7: vi Abstract
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Page 10 and 11: x Contents 4.1.2 Scintillation proc
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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〉
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Page 24 and 25: 12 3 The COBRA experiment CdZnTe (C
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Page 34 and 35: 22 4 Scintillation detectors search
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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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