title: Inelastic electron scattering in tritium gas and ppm-precise energy scale at the KATRIN experiment
Creators: name: Rodenbeck, Caroline
Publisher: name: University of Muenster, Institutional Repository MIAMI
Year: 2024
Subjects: subject: 530 Physics

description: Das Karlsruhe Tritium Neutrino (KATRIN) Experiment bestimmt die Neutrinomasse
durch Messung der Endpunktregion des Tritiumbetazerfallsspektrums. Gegenwärtig
wird bei KATRIN Statistik gesammelt und gleichzeitig wird versucht, Systematiken
zu verringern, um eine Sensitivität von 0,2 eV/c² (90% C.L.) erreichen zu
können.
Einer der systematischen Effekte ist die inelastische Streuung von
Betazerfallselektronen an Tritiummolekülen. Dieser Effekt wird in der
vorliegenden Dissertation durch In-situ-Messungen mit einer Elektronenkanone
charakterisiert.
Ein weiterer Effekt sind Instabilitäten im Retardierungspotential des
Spektrometers, welche zu einer Verbreiterung der Transmissionsfunktion
führen. Das Potential wird im Rahmen der Dissertation mit einer Regelschleife
stabilisiert und die gesamte Messkette wird kalibriert.
Des Weiteren wird eine neuartige Methode zur Bestimmung der Übergangsenergien
von Krypton-83m entworfen. Außerhalb von KATRIN gemessene Werte für diese
Energien dienten bereits als Referenz bei der Kalibrierung der Energieskala,
doch KATRINs volles Potential lässt sich mit ihnen wegen ihrer vergleichsweise
geringeren Genauigkeit nicht ausschöpfen. Bei erfolgreicher Ausführung der
vorgeschlagenen Messung wäre eine Präzisionsverbesserung um eine Größenordnung
erreicht.
description: The Karlsruhe Tritium Neutrino (KATRIN) experiment determines the neutrino mass
by measuring the endpoint region of the tritium beta-decay spectrum. Currently,
statistics are being accumulated at KATRIN, and efforts are being made to reduce
systematic effects in order to reach a sensitivity of 0.2 eV/c² (90% C.L.).
One of the systematic effects is the inelastic scattering of beta-decay
electrons on tritium molecules, which is characterized in the present thesis by
way of in-situ measurements with an electron gun.
Another effect arises from instabilities of the spectrometer's retarding
potential, which lead to a broadening of the transmission function. The
potential is in the context of the present thesis stabilized with a regulation
loop and the full measurement chain is calibrated.
Furthermore, a novel way of determining the transition energies of krypton-83m
is devised. Values for these energies, measured outside of KATRIN, served
already as references for calibrating the energy scale, but KATRIN's full
potential cannot be unleashed with them because of their relatively lower
precision. Successful execution of the proposed measurement would yield an
order-of-magnitude improvement in precision.
description: Introduction 11
1 Neutrino physics 15
1.1 Neutrino observations and their integration into the Standard Model ..... 16
1.2 Discovery of neutrino flavor oscillations ..... 18
1.2.1 Solar neutrinos ..... 18
1.2.2 Atmospheric neutrinos ..... 19
1.2.3 Framework for neutrino oscillations ..... 20
1.3 Neutrino mass generation ..... 23
1.3.1 Dirac and Majorana mass terms ..... 24
1.3.2 Seesaw mechanism ..... 25
1.3.3 Scotogenic model ..... 26
1.4 Neutrino mass measurement ..... 29
1.4.1 Cosmological observations ..... 31
1.4.2 Search for neutrinoless double beta decay ..... 32
1.4.3 Direct kinematic approaches ..... 34
2 KATRIN 39
2.1 Experimental setup ..... 39
2.1.1 Windowless gaseous tritium source ..... 39
2.1.2 Rear wall ..... 42
2.1.3 Transport and pumping section ..... 42
2.1.4 Spectrometer principle ..... 43
2.1.5 Spectrometer hardware ..... 48
2.1.6 Detector system ..... 50
2.2 Tools for monitoring and calibrating the energy scale ..... 50
2.2.1 Electron gun ..... 51
2.2.2 Precision high-voltage dividers ..... 52
2.2.3 Conversion electrons from krypton-83m ..... 53
2.3 Neutrino mass results ..... 56
2.3.1 Systematic effects ..... 57
3 Inelastic scattering of electrons inside the tritium source 59
3.1 Electron scattering on hydrogen isotopologs outside of KATRIN ..... 60
3.2 Time-of-flight measurement principle ..... 63
3.2.1 Measurements ..... 65
3.2.2 Time-of-flight selection ..... 66
3.2.3 Energy scale ..... 70
3.2.4 Energy-loss spectra ..... 71
3.2.5 Background ..... 74
3.2.6 Event multiplicity ..... 78
3.3 Scattering of electrons on tritium ..... 79
3.3.1 Fit to the differential data ..... 80
3.3.2 Combined fit of integral and differential data ..... 84
3.3.3 Impact on the neutrino mass ..... 87
3.3.4 Summary ..... 88
3.4 Conclusion ..... 89
4 Wideband precision stabilization of the retarding potential 91
4.1 Influence of a time-dependent noise on the retarding potential ..... 92
4.1.1 Testing with a Heaviside step function ..... 94
4.1.2 Sinusoidal ripple ..... 98
4.1.3 Isotropic angular distribution ..... 99
4.2 Post-regulation ..... 101
4.2.1 Setup ..... 102
4.2.2 Dynamic setpoint control ..... 103
4.3 Performance measurements ..... 104
4.3.1 Ripple probe ..... 104
4.3.2 Conversion electrons from krypton-83m ..... 107
4.3.3 Precision high-voltage divider ..... 110
4.3.4 Evaluation over the full frequency range ..... 111
4.4 Performance during neutrino mass measurements ..... 113
4.4.1 Stability ..... 114
4.4.2 Reproducibility ..... 115
4.4.3 Settling time ..... 115
4.5 Calibration of the high-voltage divider K35 ..... 116
4.5.1 Low-voltage calibration ..... 116
4.5.2 High-voltage calibration ..... 118
4.5.3 Comparison with other calibration methods ..... 122
4.6 Conclusion ..... 123
5 Improving the energy scale calibration 125
5.1 Energy scale calibration at KATRIN using krypton-83m ..... 125
5.1.1 Shift of the energy scale ..... 126
5.2 Novel method for determining the transition energies ..... 127
5.2.1 Leveraging the cross-over transition ..... 128
5.2.2 Calculating the transition energies ..... 130
5.3 Transferring the gains to routine calibrations ..... 131
5.4 Challenges ..... 132
5.5 Conclusion ..... 133
6 Conclusion and outlook 135
Appendix 137
A.1 Flight time simulation code ..... 137
A.2 Temperature dependency of the high-voltage divider K65 ..... 140
A.2.1 First cross calibration with the K35 ..... 140
A.2.2 Low-voltage calibration ..... 143
A.2.3 Mechanical stability tests ..... 146
A.2.4 Second cross calibration with the K35 ..... 147
A.2.5 Impact on neutrino mass measurements ..... 148
A.2.6 Summary and next steps ..... 150
A.3 Supplemental figures and tables ..... 151
References 163
List of figures 177
List of tables 179