Background systematics and extensions to the KATRIN background model

The goal of the Karlsruhe Tritium Neutrino (KATRIN) experiment is to measure the absolute mass of neutrinos with an unprecedented sensitivity of 0.2 eV/c${}^2$ in a model-independent way by precise determination of the beta spectrum of molecular tritium. To achieve this, knowledge of statistical and systematic uncertainties as well as the background processes is essential. Understanding the background processes is of great importance as the current background level exceeds the estimated design value and affects the neutrino mass sensitivity by systematics. It is hypothesised that most of the measured background events originate from ionised Rydberg states released from the surface of the main spectrometer vessel by radioactive decays of a Pb-210 contamination. To investigate this hypothesis that very low-energy electrons are produced by the ionisation of highly excited states by the black-body radiation, a transverse energy filter was used. The passive transverse energy filter (pTEF) is a gold plate with a micro-structured honeycomb structure that acts as a filter for electrons with larger polar angles (transverse energy) relative to the guiding magnetic field. The transmission of the electrons through the filter device is connected to their initial kinetic energy in the main spectrometer. In the context of this thesis, the hypothesis of the current Rydberg background model was not confirmed, so extensions of the model were incorporated.