Studies of Background Reduction via angular-selective Electron Detection in the KATRIN Experiment

The phenomenon of neutrino oscillations predicts massive neutrinos, whose exact mass remains unknown in particle physics. A model-independent method for a more precise determination is applied in the kinematic measurement of the end-point energy of tritium decay: The Karlsruhe Tritium Neutrino Experiment (KATRIN) performs high-precision endpoint spectroscopy of the tritium -spectrum and has been able to constrain the neutrino mass to m¿ < 0.8 eV/c² (90% C.L.) since its beginnings. The electrons in KATRIN are guided adiabatically from the tritium source from a ∼ 2.5T magnetic field to the focal plane detector (FPD) in a similarly strong magnetic field. In between, an approximately 0.6mT magnetic field, combined with a gradually changing electrical retardation potential, forms the spectrometer. Electrons with sufficient energy can overcome the retardation potential and are accelerated to the FPD. Low-energy electrons in the spectrometer volume from decays of Rydberg and autoionizing atoms, which enter the spectrometer volume due to radioactive contamination, can also be accelerated from there to the FPD. The energy of the background, therefore, does not differ from the energy of the tritium -decay electrons within the energy resolution of the FPD. However, the background has a significantly lower transverse energy and, thus, a cyclotron motion with smaller pitch angles compared to most -decay electrons. This work focuses on the development of a modified detector with strongly angle-selective electron detection efficiency to replace or complement the original FPD. This principle is called ”active Transverse Energy Filter” (aTEF). The aTEF is intended to suppress electrons with a low pitch angle and preferentially measure electrons with a large pitch angle. In this work, commercial microchannel plates (MCPs) were used as aTEF detectors in a laboratory experiment in M¨unster. After the differentiation of electrons based on their pitch angle was successful, the development of an aTEF based on Si-PIN diodes (Si-aTEF) was pursued. Hexagonal channels were introduced into the surface of commercial Si-PIN detectors to a certain depth. Up to this depth, a large part (∼ 90%) of the detector material was removed via deep silicon etching. The remaining surfaces, which were inserted vertically into the surface, were intended to primarily detect electrons with large pitch angles. Electrons with low pitch angles should be stopped in the inactive bottom of the channels. The angular selectivity of the electron detection was measured using a specially designed setup. A photoelectron source with electrons with energies of a similar order of magnitude as in KATRIN was used. The electrons were magnetically guided onto the Si-PIN detector or Si-aTEF prototype at a variable angle. Angular selectivity was measured in two of the prototypes presented, representing a milestone in the development of the Si-aTEF. By cooling, a reduction of the reverse current and the intrinsic background as well as an improvement of the charge collection efficiency of the Si-aTEF prototypes could be achieved. The detector perfomance was limited due to nanofabrication issues, which will be resolved in the future. The experimentally found angular-selectivity was supported by semiconductor simulations that predicted the potential curve and the propagation of the depletion zone in microstructured Si-aTEF prototypes. An important difference between the commercial diodes of the Si-aTEF prototypes and the FPD used in KATRIN is their reversed doping order. The simulations suggest that the bottoms of the channels in an aTEF with FPDlike doping order should be active, which requires further investigation and, possibly, an additional blocking layer.