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22 4 Scintillation detectors search (DAMA - NaI), neutrinoless double beta decay and neutrino detection experiments (SNO+ - LAB). They are also used in commercial fields like medical imaging applications (PET scanner - BGO), container and baggage scanning, industrial gauging and many more. This chapter will explain in 4.1—“Scintillator crystals” the main characteristics of the scintillation process and will give an overview of the conventional scintillator materials. In 4.2—“Detector assembly” the electronic readout devices and aspects of the detector mounting are discussed. 4.1 Scintillator crystals The basic part of a scintillation detector is the scintillating material, emitting the scintillation light. The process, which leads to the emission of the scintillation light, starts with ionizing radiation, penetrating a scintillating material. Section 4.1.1—“Interaction of ionizing radiation with matter” therefore discusses how incident radiation gets absorbed or attenuated in the scintillator and deposits energy. The conversion of this energy into luminescence emission is explained in 4.1.2—“Scintillation process”. Afterwards an overview of commercial scintillators is given in section 4.1.3—“Scintillating materials”. 4.1.1 Interaction of ionizing radiation with matter For categorization of its interaction with matter, ionizing radiation can be divided into direct and indirect ionizing radiation. Direct ionizing radiation are charged particles. It is convenient to treat particles (like α, p and µ) and electrons and positrons separately. Indirect ionizing radiation are electromagnetic radiation and neutrons. The sources of ionizing radiation were discussed in chapter 3—“The COBRA experiment”. CHARGED PARTICLES like α, p, µ interact mainly through the coulomb interaction with the atomic electrons. The energy loss through elastic scattering with the nuclei is possible, but of minor importance. The collisions with the constituent electrons are inelastic, and momentum and energy are transferred, raising the interacting electron to a higher lying shell (excitation) or remove it completely from the atom (ioni-
4.1 Scintillator crystals 23 sation). The transferred energy in each collision is small compared to the particle energy, but the number of interactions is high on small path lengths, leading to high ionisation densities. The path of these particles in matter is, due to the small energy and momentum per interaction, especially straight. The stopping power, which is the energy loss per differential path length due to ionisation, varies with the density and Z of the material and with the square of incident particles charge. The collisions can also produce high energy recoil electrons (δ- or knock-on electrons), that cause secondary ionisation. ELECTRONS AND POSITRONS loose their kinetic energy by excitation and ionisation processes and through emission of Cherenkov radiation and Bremsstrahlung. The energy loss by ionisation varies also with the density and Z of the material.The energy loss through Bremsstrahlung rises with Z 2 and dominates the energy loss for electrons and positrons with energies much higher than MeV. The electron path is, in contrast to heavy particles, not straight but tortuous so that the path length is up to 4 times longer than the transversed absorber [28]. Positrons annihilate at the end of their range with an constituent electron and two almost collinear 511 keV photons are emitted. [29] ELECTROMAGNETIC RADIATION Photons interact when penetrating a medium by the five fundamental mechanisms of electromagnetic interactions: photon absorption, electron-positron pair production, incoherent and coherent scattering and the nuclear photoelectric effect. They lose energy mainly by the first three processes. For photons with energies Eph < 100 keV photo absorption, for Eph ≈ 1 MeV the Compton process and for Eph > 2 MeV the pair production dominates for materials of medium Z like NaI. Figure 4.1 shows the mass attenuation coefficient µ/ρ for these processes in sodium iodide, which is proportional to the cross section. The photo effect is the absorption of a photon of energy Epe by an atom, which emits a subsequent photoelectron. Such total absorption can only occur if the electron is initially bound to the atom and momentum is conserved by the recoil of the residual atom. In the majority of events, the photoelectron is ejected from a K or L-shell with an energy Tpe = E − Be where Be is the binding energy of this electron. Subsequent characteristic X-rays or Auger electrons are emitted by the filling of the vacant energy state, left by the ejected photoelectron. The photoelectron and the subsequent emitted photon of Auger electron is also mostly absorbed by the scintillator, so that in the photo effect the complete energy of the incident photon is absorbed.
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
Page 8 and 9: viii Überblick
Page 10 and 11: x Contents 4.1.2 Scintillation proc
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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
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86 6 Summary and future work dimens
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88 A FIRST APPENDIX Scintillator De
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90 A FIRST APPENDIX A.2 Reports abo
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92 A FIRST APPENDIX trons than a st
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94 B SECOND APPENDIX Figure B.1: Am
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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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