Characterization and Optimization of the KATRIN Tritium Source

The Karlsruhe Tritium Neutrino (KATRIN) experiment aims to measure the effective electron anti-neutrino mass via high-precision spectroscopy of the energy spectrum of $\beta$-decay electrons of tritium near the 18.6 keV endpoint with an unprecedented accuracy.
The specifications of the KATRIN experiment result in an experimental setup with a target discovery potential of 5 $\sigma$ for a neutrino mass of 350 meV/c$^{-2}$ and the expected capability to push the upper limit on the neutrino mass down to a target of 200 meV/c$^{-2}$ (90 % C. L.) if no neutrino mass signal is detected.
To achieve this unprecedented sensitivity, both statistical and systematical uncertainties have to be stringently minimized.
The reduction of statistical uncertainties requires a total measurement duration of 3 years with a windowless gaseous tritium source (WGTS) capable of producing $10^{11}$ $\beta$-electrons per second.
To keep the systematical uncertainties at the level required to reach the target sensitivity, the activity of this $\beta$-electron source needs to be stable at the level of 0.1%.
This stability is influenced by several factors such as the purity and pressure of the tritium gas, as well as the temperature of the WGTS beam tube enclosing the gaseous tritium. ... mehr
Therefore, in order to minimize systematical uncertainties, a very stable injection of tritium gas into the beam tube is necessary.
This is achieved by the tritium loop system.
The main focus of this thesis is the optimization of stabilized injection of tritium gas into the WGTS, the in-depth characterization and modeling of the isotopic gas composition inside of the WGTS, as well as the measurement and continuous monitoring methods of the WGTS column density.



Summarizing the results presented in this thesis, it was shown that the stability of the tritium column density of the WGTS meets the requirement of 0.1%.
A variety of column density monitoring methods were implemented and have been used to derive the best current upper limit on the neutrino mass from direct measurement of 1.1 eV/c$^{-2}$.
This outstanding performance in both stability and monitoring can be achieved reliably and reproducibly.
This is necessary for the upcoming measurement campaigns, which are needed by the KATRIN experiment in order to meet its scientific goal of a 200 meV/c$^{-2}$ sensitivity on the neutrino mass.