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Abstract

The existence of neutrinos and their properties have wide-ranging implications for particle physics and cosmology. Neutrino oscillation experiments prove that at least two neutrino mass eigenstates are non-zero and provide lower limits. Yet according to present knowledge the value of the absolute mass scale still remains undetermined. The KArlsruhe TRItium Neutrino experiment (KATRIN) is designed to measure the value of the effective electron neutrino mass with a sensitivity of 200 meV/c 2 (90 % CL). For that beta decays of gaseous tritium are observed. Since tritium circulation started, parts of the KATRIN experimental setup (in particular the rear wall ) are unintentionally gradually being radioactively contaminated by charged and neutral particles from the tritium source. Beta decays stemming from this contamination comprise a time-dependent non-uniform background at the < 1 % level in neutrino mass measurements. As the detailed shape of this spectrum is unknown, the goal of this thesis was to find a suitable description of the rear wall spectrum for correcting the corresponding background in KATRIN neutrino mass campaigns from June 2020 to June 2021. Low statistics data from multiple dedicated rear wall characterization measurements was used to develop empirical and partly empirical models based on features of typical beta decay spectra and to evaluate their impact on the central value and the uncertainty of the KATRIN neutrino mass result. The rear wall spectrum was found to cause a significant bias of the neutrino mass result which could be corrected. The additional systematic uncertainty was estimated to be smaller than 0.01 eV 2 on the squared neutrino mass in the current energy range setting. These results form the basis of further analyses for future data releases.