title: Studies of Background Reduction via angular-selective Electron Detection in the KATRIN Experiment
Creators: name: Gauda, Kevin
Publisher: name: University of Muenster, Institutional Repository MIAMI
Year: 2024
Subjects: subject: 530 Physics

description: Das KATRIN-Experiment strebt die Messung der Neutrinomasse mittels Tritium-β-Endpunktspektroskopie an.
Niederenergetische Elektronen im Spektrometervolumen bilden den größten Anteil des Untergrunds.
Sie sind am KATRIN-Detektor energetisch nicht von Signalelektronen unterscheidbar, folgen aber einem Zyklotronpfad mit kleineren Neigungswinkeln.

In dieser Arbeit wurde die Entwicklung eines ”active Transverse Energy Filter” (aTEF) mit stark winkelselektiver Detektionseffizienz zur Unterdrückung von Elektronen geringen Neigungswinkels angestrebt.
Zunächst gelang anhand von Mikrokanalplatten die Unterscheidung von Elektronen anhand ihres Neigungswinkels in einem Testaufbau.
aTEF-Prototypen, basierend auf Si-PIN-Dioden (Si-aTEF) mit hexagonalen Kanälen in der Oberfläche, wurden vorbereitet und ihre Winkelselektivität nachgewiesen.
Der intrinischen Untergrund sowie die Ladungssammeleffizienz der Prototypen wurde durch Kühlung verbessert.
Der Potentialverlauf in Si-aTEFs wurde simuliert, um das Verhalten zu verstehen.
description: The KATRIN experiment aims for the measurement of the neutrino mass via tritium-β-endpoint spectroscopy.
Low-energetic electrons in the spectrometer volume are the main backround component.
They are indistinguishable from signal electrons at the KATRIN detector, but follow a cyclotron path with smaller pitch angles.

The development of an "active transverse energy filter" (aTEF) with strong angle-selective detection efficiency to reduce detection of electrons with small pitch angles was pursued in this work.
At first, the differentiation of electrons based on their pitch angle was achieved with microchannel plates in a test setup.
aTEF-prototypes, based on Si-PIN diodes (Si-aTEF), with hexagonal channels in their surface were prepared and angle-selectivity was shown.
The intrinsic background and the charge collection of the prototypes was enhanced by cooling.
The potential curve in Si-aTEFs was simulated to understand their behaviour.
description: 1 Introduction ..... 1
2 Introduction to Neutrino Physics ..... 3
2.1 Neutrino Oscillations ..... 3
2.2 Neutrino Mass ..... 7
2.2.1 Neutrino Masses and the Standard Model ..... 7
2.2.2 Cosmology and Supernovae ..... 10
2.2.3 Neutrinoless Double-β Decay ..... 11
2.2.4 Kinematic Measurements ..... 13
3 The KATRIN Experiment ..... 17
3.1 Setup ..... 18
3.1.1 Source and Transport Section ..... 18
3.1.2 Spectrometer and Detector Section ..... 22
3.2 Neutrino Mass Analysis ..... 26
3.2.1 β-Decay Spectrum of T2 and Experimental Response Function ..... 27
3.2.2 Neutrino Mass Analysis Methods and Results ..... 29
3.3 Background ..... 31
3.3.1 Overview ..... 31
3.3.2 Rydberg background ..... 33
3.3.3 Rydberg Background Reduction Strategies ..... 38
3.3.3.1 Shifted Analyzing Plane ..... 38
3.3.3.2 THz Radiation ..... 40
3.3.3.3 aTEF ..... 40
4 Angle-selective Electron Filtering and Detection ..... 43
4.1 Overview on passive Transverse Energy Filters for KATRIN ..... 43
4.1.1 TEF for Background Reduction ..... 44
4.1.2 pTEF for Background Characterization ..... 46
4.1.2.1 Original pTEF Idea ..... 46
4.1.2.2 Implementation in KATRIN and Results ..... 47
4.2 Microchannel Plate-based Active Transverse Energy Filter: MCP-aTEF ..... 50
4.3 Proof-of-Principle of aTEF and pTEF Concept ..... 52
4.3.1 Test Setup ..... 53
4.3.2 Experiments on MCPs as aTEF or pTEF ..... 58
4.4 aTEF-Concepts based on other Detector Types ..... 69
5 Fabrication of Microstructures in Silicon ..... 73
5.1 Sample preparation ..... 75
5.1.1 Sample Cleaning Recipes ..... 75
5.2 UV-Lithography ..... 76
5.2.1 Lithography Mask ..... 76
5.2.2 Photoresist Mask ..... 77
5.2.3 UV-Lithography ..... 77
5.3 Cryogenic Inductively Coupled Plasma Reactive Ion-Etching ..... 78
5.3.1 Etching Procedure ..... 81
5.3.2 Cryogenic ICP-RIE Parameters ..... 82
5.3.3 ICP-RIE Parameter Tuning for Deep Silicon Etch ..... 83
5.4 Photoresist removal ..... 87
5.5 Examples ..... 89
5.5.1 Silicon chips and wafers ..... 89
5.5.2 Si-PIN Diodes ..... 92
6 Si-aTEF Prototype Development and Tests ..... 97
6.1 Silicon PIN Detector Introduction ..... 97
6.2 Electric Potential in (Microstructured) Si-PIN Diodes ..... 100
6.2.1 Solution of the 1-dimensional Poisson Equation ..... 100
6.2.2 COMSOL™ Multiphysics Simulation ..... 104
6.2.3 1-dimensional PIN-Diode ..... 104
6.2.4 2-dimensional PIN-Diode ..... 105
6.3 Experimental Proof of Angle-selective Electron Detection ..... 109
6.3.1 Setup I – Modified ToF Test Setup ..... 112
6.3.2 Setup II – Dedicated aTEF Test Setup ..... 113
6.3.3 Characterization of (Microstructured) Si-PIN diodes ..... 116
6.3.3.1 Diodes A and B ..... 116
6.3.3.2 Diode C ..... 119
6.3.3.3 Diode D ..... 122
6.4 Towards an aTEF for the KATRIN Experiment ..... 132
7 Conclusion and Outlook ..... 135
A Appendix ..... 139
Bibliography ..... 143