Solid electron sources for the energy scale monitoring in the KATRIN experiment

The KArlsruhe TRItium Neutrino (KATRIN) experiment represents a next-generation tritium $\beta$-decay experiment designed to perform a high precision direct measurement of the electron anti-neutrino mass m($\nu_e$) with the sensitivity of 0.2 eV c$^{-2}$ (90 % C.L.). KATRIN is a successor experiment of the neutrino mass experiments carried out in Mainz (Germany), and Troitsk (Russia), which set the upper limit on m($\nu_e$) of 2 eV c$^{-2}$. Therefore, the aim of the KATRIN experiment represents the improvement of the neutrino mass sensitivity by one order of magnitude. The investigation of the neutrino mass scale with the sub-eV sensitivity is of particular interest for particle physics, astrophysics and cosmology. In contrast to other methods, such as the search for neutrino-less double $\beta$-decay or cosmological neutrino mass studies using large scale structures and cosmic microwave background radiation data, KATRIN will provide a completely model-independent measurement of the neutrino mass, based only on kinematic relations and energy-momentum conservation. For the observation of a non-zero neutrino mass signature in the endpoint region of the $\beta$-spectrum (endpoint energy Q = 18.6 keV) the methods of high-resolution electron spectroscopy are necessary together with a very low background level. For this purpose the upcoming KATRIN experiment uses two successive electrostatic retardation filters with the magnetic adiabatic collimation (called “MAC-E filters”). However, high resolution and low background are only two of many stringent requirements which are connected to the challenging realization of the KATRIN experiment. The stability of the energy scale of the KATRIN spectrometers is one of the main systematic effects: the principle of the MAC-E filter technique relies on the precise knowledge of the retarding potential which is experienced by the $\beta$-electrons on their path through the spectrometer. Therefore, the challenge of knowing the retarding potential precisely enough in every moment during the measurement is inevitable in the KATRIN experiment. Besides the use of the state-of-the-art equipment for a direct measurement of the high voltage, including specially developed precision high voltage dividers, several very stable calibration electron sources will be utilized in KATRIN, based on atomic/nuclear standards. One of the electron sources will be continuously measured by an additional MAC-E filter spectrometer (“monitor spectrometer”) to which the high voltage will be applied, corresponding at the same time to the filtering potential of KATRIN. This way a two-fold monitoring system will be formed. In this work the feasibility of solid electron source based on the metastable isotope krypton-83m ($^{83m}$Kr, t$_{1/2}$ = 1.83 h) was successfully tested. In this type of source the process of internal conversion of $^{83m}$Kr is utilized, where $^{83m}$Kr is continuously generated by rubidium-83 ($^{83}$Rb, t$_{1/2}$ ≃ 86 d). The monitoring task of KATRIN demands the energy stability ΔE/E of the K-32 conversion electron line (kinetic energy E = 17.8 keV, line width $\Gamma$ = 2.7 eV) of $\pm$1.6 per month. In the course of this dissertation project altogether eight samples of the solid $^{83}$Rb/$^{83m}$Kr sources, produced by two different techniques of vacuum evaporation and ion implantation, were investigated with the help of the former Mainz MAC-E filter spectrometer. In the course of the measurement campaign the stringent demand on the energy stability was fulfilled. In addition, the shapes and the absolute kinetic energies of the $^{83m}$Kr conversion electron lines were studied in detail and their dependence on the $^{83m}$Kr atom environment was investigated. The ion-implanted $^{83}$Rb/$^{83m}$Kr sources can be recommended as a standard tool for continuous monitoring of the KATRIN energy scale stability with the sub-ppm precision.