Simulation of the Sputtering of Ions from the KATRIN Main Spectrometer Surface with Geant4

The story of the Karlsruhe Tritium Neutrino Experiment (KATRIN) begins with the unconventional lines "Liebe Radioaktiven Damen und Herren, [. ] (ich bin) auf einem verzweifelten Ausweg verfallen um den Wechselsatz der Statistik und den Energiesatz zu retten". These words introduce the letter from W. Pauli to the Gruppe der Radioaktiven Gauvereins-Tagung in Tübingen in 1930 [1]. In this historical document Pauli postulated out of necessity a particle which is known today as neutrino. In the beginning, the neutrino corresponded to a hyopthesis, but it soon became clear that it really exists. The rst proof was given in 1956 in the Cowan and Reines experiment [2]. The rst experimental milestone in neutrino research was created. And more were to follow. So is the proof of all three avour types. The Standard Model was con rmed in an exemplary manner. However, towards the end of the twentieth century, it quickly became clear that neutrino physics was to be equated with physics beyond the Standard Model. The proof of the neutrino oscillation in 1998 by the Super Kamiokande experiment [3] shows that neutrinos have a mass, although the Standard Model contains a mass equal to zero. The KATRIN experiment is based on this experimental nding. The neutrino mass is the linchpin of current neutrino research. The goal of the KATRIN experiment is to measure the electron anti neutrino mass with an unprecedented accuracy of 0.2 eV/c2 [4]. The basis is the energy spectrum of the tritium β decay, which, depending on the square of the neutrino mass, undergoes a slight shift towards low energies. Since the magnitude of the shift is in the sub-eV range, a high level of statistics is required to prove this experimental. Two central components of the experiment are decisive for this. The rst is the windowless gaseous tritium source (WGTS), which guarantees a stable high luminosity and the MAC-E lter principle, which allows an excellent energy resolution. Depending on the electromagnetic con guration of the main spectrometer the background exceeds the design value by a factor of 20-50 [5]. It is precisely to this problem that the present thesis is attached. A milestone in KATRIN’s background reduction was reached with the installation of the copper ba es. This has reduced the background caused by Radon by 95% [6]. This success allows a more detailed characterisation of the remaining background. The dependence of the background on pressure, Beld setting, voltage of the inner electrodes and spectrometer temperature was investigated. The central nding is that the desired generation mechanism must produce low energy electrons homogeneously in the spectrometer. This leads to the concept of the Rydberg background model. Rydberg atoms are electrically neutral highly excited atomic states. A potential creation mechanism of these electrically neutral, highly excited states is related to sputtering due to fast ions after radioactive decays. Further measurements have shown that a 210Pb activity is detectable inside KATRIN. This is due to a 222Rn contamination of the inner main spectrometer wall during