Investigation and Reduction of Rear Wall Systematics relevant for keV sterile Neutrino Search with KATRIN

The Karlsruhe Tritium Neutrino (KATRIN) experiment performs high-precision $\beta$-spectroscopy near the kinematic endpoint of tritium decay, with the aim to measure the effective electron antineutrino mass with a sensitivity of better than 0.3 eV. After achieving this goal, the setup will be upgraded to search for keV-scale sterile neutrinos in the scope of the Tritium Investigation on Sterile to Active Neutrino Mixing (TRISTAN) project. Reaching the target sensitivity on the mixing amplitude of $10^{-6}$ requires precise modelling of all the systematic effects. The subject of this thesis is the Rear Wall (RW), which is a gold-coated stainless steel disk and the physical upstream end of the tritium source. There are two systematic effects associated with the RW: inelastic electron backscattering and surface activity. Modelling the measured spectrum more accurately required replacing the currently available RW datasets describing these effects. Using the Monte Carlo toolkit Geant4, a new simulation code was created to study the interactions of electrons with the RW surface. Due to developing performance-enhancing options, new RW datasets could be generated by simulating up to 280 times more electrons than before. ... mehrPossibilities to reduce both electron backscattering from the RW, and RW activity were explored experimentally. Surface activity can be removed by in-situ UV/ozone decontamination, which is not yet fully understood. To improve upon the knowledge on the underlying mechanism, and to extend the method to tritium systems in general, a dedicated experiment was set up to study the properties of ozone in vacuum vessels. By applying the findings to a tritiated sample, 60% of its surface activity could be removed. However, no inherent decontamination effect of ozone without simultaneous UV irradiation was found. Electron backscattering from the RW can be reduced significantly by replacing it with a material possessing a lower nuclear charge number. One of the possible upgrades for the TRISTAN project is installing a new RW made of beryllium. However, it has not been clear yet whether beryllium is compatible with the conditions inside the tritium source. An experimental routine, including e.g. tests on tritium accumulation, was developed, with which alternative RW materials can be qualified. The first tests performed on beryllium did not provide any reason to exclude its use as an alternative RW.