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Transmission Function of the Pre-Spectrometer and Systematic Tests of the Main-Spectrometer Wire Electrode (PHD thesis)
The KArlsruhe TRItium Neutrino experiment, KATRIN will determine the mass of the electron anti-neutrino with a sensitivity of 0.2 eV (90% C.L.) via a measurement of the beta-spectrum of tritium decaying in a windowless gaseous molecular tritium source near its endpoint of 18.57 keV. This approach relies exclusively on the relativistic kinematics of the decay products rendering the experiment model independent and reducing the systematic uncertainty. An ultra-low background of a few mHz and an energy resolution of 0.93 eV are among the requirements to reach the sensitivity. These demands are fulfilled with the main spectrometer (MS). While the beta-decay electrons are guided by a magnetic field through the experiment, the MS acts as a high-pass filter for the beta-decay electrons. Only those above an energy barrier, the retarding potential, are transmitted to the detector. The last about 30 eV of the T2 beta-spectrum will be scanned in this way. The MS is equipped with a 650 m2, two-layered, UHV compatible and quasi-massless wire electrode suppressing secondary electron background originating at the main-spectrometer walls and caused by residual radioactivity and cosmic muons. Its energy resolution of 0.93 eV is only achieved, if a large part of the 248 wire electrode modules, which determine the electric field inside the MS, has a mechanical precision of 0.2 mm. Not a single of the about 28.000 wires of the electrode must break during the lifetime of KATRIN. A 2-dimensional laser sensor for contact-less position (precision about 0.01 mm) and tension (precision about 0.04 N) measurements was developed and applied, to firstly, verify the mechanical precision of the electrode modules and secondly, to examine their reliability. A 3-dimensional coordinate measurement table was automated to perform these measurements in a clean room. This table was also used to verify the precision of components using a camera system and image recognition methods (0.05 mm precision). These measurements, together with the results of various quality assurance tests are stored in a database allowing to reconstruct the properties and history of each single electrode module. The UHV-compatible, fail-save, non-magnetic high voltage distribution, routing 46 voltages to the electrode modules inside the MS, was designed, tested systematically and its installation was started. The pre-spectrometer (PS), which has the same working principle as the MS is placed in front of it in the KATRIN experiment. It is foreseen to operate the MS at a potential of about 18.57 keV and the PS at about 18.3 keV reducing the rate of beta-decay electrons entering the MS to about 103 per second. This measure reduces the electron scattering probability in the MS and thus the background rate. Like this, however, electrons are confined longitudinally between the spectrometer potentials and radially by the strong magnetic field of a solenoid, placed between them. The electrons trapped in this Penning trap can also produce background. This work investigates the possibility to diminish this background source by reducing the PS potential by several keV, worsening the trapping conditions. In this configuration, however, the PS transmission probability could be reduced as an adiabaticity requirement guaranteeing that electrons follow the magnetic field lines through the PS and are transmitted, is potentially violated. This phenomenon would introduce an additional systematic uncertainty for KATRIN. Therefore, the pre-spectrometer transmission at several magnetic fields settings. These investigations show that there are no non-adiabatic transmission losses for magnetic fields larger, or equal to 2.25 T (50% KATRIN design value).