A Natural Standard Source for a Permanent Longterm Voltage Monitoring in KATRIN
Beatrix Ostrick (Institut für Kernphysik, Universität Münster)

Although the knowledge of the mass scale of the neutrinos is in principle accessible via basic considerations concerning the kinematics of the beta decay its experimental realisation is still an ongoing challenge. The Karlsruhe Tritium Neutrino-Experiment (KATRIN) being the successor of the Mainz and Troitsk neutrino experiments is aiming at a sensitivity on the neutrino's mass down to 0,2 eV/c2 which is an improvement of an order of magnitude with respect to its precursors.
The parabola-like shape of the endpoint region of the beta spectrum comprises a particular difficulty for measuring the neutrino mass aside from its intimate dependence on low background: Any unkown fluctuation of the spectrum leads wrongly to smaller neutrino mass sqares. KATRIN consists of a MAC-E-Filter, that is a spectrometer which collimates the electron impulses magnetically before analysing their energy electrostically. The stability of this filter potential is as to aforesaid reasons crucial of the success of KATRIN.
The concept of this longterm stability monitoring of a potential of -18 keV, i.e. the endpoint region of the Tritium beta spectrum, on a 3ppm level is based on a precision high voltage divider, which is permanently cross checked by an conversion electron source. Thus the voltage measurement is provided with a stability stemming from a nuclear transition as a natural standard. The results of measuring phases with a condensed 83mKr monitoring source at the updated Mainz neutrino mass spectrometer complemented with the KATRIN HV divider will be presented.