The Detector System of the KATRIN Experiment : Implementation and First Measurements with the Spectrometer

Abstract

The thesis in hand describes work performed in the context of the Karlsruhe Tritium Neutrino (KATRIN) experiment which is targeted to determine the absolute neutrinomass scale with an unrivaled sensitivity of mν = 200 meV (90 % C.L.). In the Standard Model of particle physics, neutrinos are established in three active generations and are described as electrically neutral, weakly interacting leptons. They are by far the most abundant and lightest fundamental particles of matter in the universe: each cubic centimeter contains 336 neutrinos. At present, however, their absolute mass scale is not known, but laboratory and cosmological studies imply that neutrino masses reside in the sub-eV range. The significant impact of massive neutrinos on particle physics and cosmology is a central motivation for the ongoing construction of a next-generation large-scale direct neutrino-mass experiment: the KATRIN experiment located at Tritium Laboratory Karlsruhe (TLK) at the Karlsruhe Institute of Technology (KIT) Campus North site. The experiment will investigate the electron energy spectrum of tritium β-decay close to the kinematic endpoint of E0 ≈ 18.6 keV with unprecedented precision in a direct and model-independent measurement in order to search for a minute shape distortion caused by a non-zero neutrino mass. Foray into the sub-eV level will be achieved by combining a high-luminosity windowless gaseous molecular tritium source with a large high-resolution integrating spectrometer based on the MAC-E-filter principle and a segmented 148-pixel silicon wafer housed in a complex detector system. KATRIN relies on an almost background-free, highly efficient, long-term stable, and well-understood detection technique for 18.6-keV electrons, since a generic low signal count rate of only few times 0.01 cps is expected. In addition, detailed signal parameters, such as deposited energy, arrival time, and point of detection, are vital to understand electron-transport and background-generation mechanisms along the entire beam line of the experiment. With respect to these challenges faced by the KATRIN spectrometer and detector section (SDS), the following objectives were set for this thesis: • The detector system being an integral main component was to be fully implemented and integrated into the KATRIN beam line. All subsystems were to be optimized and comprehensively characterized. Subsequently, the detector system was to be used as a diagnostic tool for the first SDS commissioning phase to allow a detailed investigation of the transmission characteristics and background behavior of the main spectrometer. • Detector-based and spectrometer-related backgrounds were to be examined with respect to identifying the specific sources and characteristics to understand the associated background generation mechanisms. In a staged approach, first the intrinsic detector background and second the electron background from the main spectrometer was to be investigated. Special emphasis was to be put on nuclear α-decays of emanated radon atoms and on the quantum-tunneling effect of field emission. The motivation of these studies was to provide a specific background model in order to establish a solid experimental base for further background optimizations, thereby reaching the ambitious design goal of 0.01 cps for the total background rate to achieve the targeted neutrino-mass sensitivity. In chapter 1, a brief overview on the history and current status of neutrino physics is given. Based on an introductory survey of natural and man-made neutrino sources, the unique particle properties of neutrinos and in particular the phenomenology of neutrino flavor oscillations are discussed. This is supplemented by a survey of cosmological and laboratory methods to access the absolute neutrino-mass scale. The focus of chapter 2 is set on a description of the working principle and the status of the main components of the KATRIN experiment. Special attention is given to a detailed description of the experimental setup for the first SDS commissioning phase during a four-month measurement campaign in the middle of 2013, as this represents an important milestone for the experiment. In addition, an overview of expected background processes occurring in the detector system and the main spectrometer is given. In chapter 3, the complex setup of the KATRIN detector system with its major functional sub-components is described. A special focus is set on the working principle and performance of each sub-component as well as on the required benchmarks and design specifications for the KATRIN experiment. The characterization of the detector response for different types of radiation is highlighted in chapter 4. In this context, the optimization works to achieve an efficient detector operation are described. Further topics include the determination of crucial detector parameters and the understanding of detector systematics. Of special interest for KATRIN is the description of the long-term detector performance during the first SDS commissioning phase. The intrinsic detector background from cosmic-ray muons, external radiation, and intrinsic radioactivity is described in chapter 5. Several passive and active strategies are outlined to minimize these background classes. In addition, the level of background contribution of the detector system to the total background rate of the combined SDS system is discussed. The focus of chapter 6 is on the electron-related background process from the spectrometer. Of particular concern here are nuclear α-decays of emanated radon atoms. In this context, detector properties, such as segmentation and good timing resolution, are used to investigate the characteristics of this background class. This culminates in a determination of the radon activity and emanation rate of the spectrometer. The chapter is concluded by examining contributions from other background sources. In chapter 7, the quantum-tunneling effect of field electron emission from elevated metal surfaces to a negative high voltage is studied. A very interesting side aspect in this context was the observation of hydrogen anions. The combination of field-emission induced electrons and anions are used as a tool to further characterize important detector parameters, but also to investigate the mapping properties of the SDS system with a well localized particle source. This demonstrates the dual purpose of the investigations of this thesis: to determine crucial detector parameters, such as its alignment relative to the spectrometer axis and the thickness of its insensitive dead-layer volume, as well as to use the excellent detector properties to study background processes and phenomena. The thesis in hand is completed with chapter 8 by giving a detailed recapitulation of the works performed and by presenting an outlook to the upcoming second SDS commissioning phase which will build on the ground-laying work of this thesis.