development of micro-pattern gaseous detectors – gem - LMU

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14 Chapter 1 Interactions of Charged Heavy Particle and Photons in Matter μ / ρ compton photo total Figure 1.7: The total mass attenuation coefficient for photons in Ar/CO2 at a ratio of 93/7 in the range of 10keV < Eγ < 10GeV [NIST 10]. Photoelectric effect (pink), Compton scattering (blue dotted) and pair production (blue straight line) summarize to the black envelope. Photons with higher energies (Eγ ≫ 1 MeV ) are dominantly absorbed by the nucleus for pair production: with pair γ + nucleus −→ e + + e − + nucleus, (1.24) µ ∝ const(Eγ) . (1.25) Due to momentum conservation the absorption of a photon by an atomic electron needs a third collision partner which takes the recoil momentum, in this case the atomic nucleus. In the vicinity of the nucleus, that is in the K shell, the cross section for absorption of a photon carrying energy Eγ is about 80 % of the total cross section. If the photon energy lies above the K shell binding energy EK the total photoelectric cross section is given in the Born approximation by with EK < Eγ < mec 2 . σγ ≈ 32π 3 · √ 2 · Z 5 α 4 · r 2 e · mec 2 Eγ 2 (1.26)
1.1. Primary Processes in Gaseous Detectors 15 Photoelectric Effect for 55 Fe For measurements with the GEM detector later presented, see Ch. 3, a 55 Fe source is used to analyze certain properties of the chamber. 55 Fe decays by electron capture to excited Manganese 55 Mn which emits photons by returning into the ground state. Depending on the inner rearrangement of the electrons, two different photon energies are characteristic. The Kα photon which represents the transition of an electron from the L to the K shell, carries an energy of Eγ = 5.90keV. If the 55 Mn atom returns to its ground state via M-to-K transition of an electron, a K β - photon is emitted with an energy of Eγ = 6.50keV. The line intensity proportion for Kα and K β is 24.4% and 2.86%, respectively, the Kα process dominates over K β by approximately 10 : 1 [PDG 10]. When these photons are interacting with the Argon/C02 gaseous filling of the GEM detector, the photoelectric effect is the dominating process . The Kα- photon energy is higher than the binding energy or an electron from the K shell of Argon (EK = 3.22keV). The electron can detach with a remaining kinetic energy of 2.68 keV. The excited Argon atom then returns to its ground state through emission of one ore more photons that leave the volume without being detected. The resulting X-ray spectrum consists of three line with the energies: 2.68 keV, 5.90 and 6.50 keV. By photoelectric effect the X-ray energy is transferred to an electron in the active volume of the detector. Recording of this spectrum with the triple GEM detector can be used to state the energy resolution of the device and is illustrated in Ch. 4.1. Since excitation dominates over ionization for statistical reasons, the required energy loss for an ionization is greater than the simple ionization potential. Therefore the total number nt of ionized particle is: nt = 1 Wi dE dx (1.27) where (dE/dx) is the energy loss of the incident particle and Wi represents the average energy required for the production of an ion-electron pair. For Argon the mean energy for ion-electron-pair creation is Wi = 26 eV which is nearly a factor two larger than the simple ionization potential of 15.8 eV. Since the gas used in the GEM detector is not purely mono atomic Argon but a mixture of Ar/CO2 in the ratio 93/7, average values for Wi should be taken. With the given mean energy for ion-electron creation for Argon and CO2 the gas mixture has a value of: W gas i W Ar i = 26 eV and W CO2 i = 33 eV , Ar = 0.93 · Wi + 0.07 · W CO2 i = 26.50 eV , (1.28) as lower limit for ion-electron pair production. Thus the average number of ionized atoms generated by the 55 Fe photon is:
Page 1: DEVELOPMENT OF MICRO-PATTERN GASEOU
Page 5: Dedicated to Engin Abat 29/09/1979
Page 9 and 10: Contents Introduction I 1 Interacti
Page 11 and 12: Introduction Our universe is moment
Page 13 and 14: is testing tubes with alternative d
Page 15: efficiency. Monitoring of this para
Page 18 and 19: 8 Chapter 1 Interactions of Charged
Page 20 and 21: 10 Chapter 1 Interactions of Charge
Page 32 and 33: 22 Chapter 2 The GEM Principle doub
Page 34 and 35: 24 Chapter 2 The GEM Principle mobi
Page 36 and 37: 26 Chapter 2 The GEM Principle
Page 38 and 39: 28 Chapter 3 The Triple GEM Prototy
Page 50 and 51: 40 Chapter 4 Energy Resolution and
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64 Chapter 5 Efficiency Determinati
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66 Chapter 6 Rise Time Studies 3000
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68 Chapter 6 Rise Time Studies 2 ns
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70 Chapter 6 Rise Time Studies obse
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72 Chapter 6 Rise Time Studies
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74 Chapter 7 Gas Gain Studies eff G
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76 Chapter 7 Gas Gain Studies
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78 Chapter 8 Implementation of High
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88 Chapter 9 Summary and Outlook st
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90 BIBLIOGRAPHY [Bieb 03] O. Biebel
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92 BIBLIOGRAPHY [NIST 10] NIST. “
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94 BIBLIOGRAPHY
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96 Chapter A Assumptions for Effici
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98 Chapter B Construction Drawings
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104 Chapter C Design of Prototype 2
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Acknowledgements First of all I wou
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Selbständigkeitserklärung Ich ver
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