Ion and plasma systematics during the first KATRIN neutrino mass measurements

The Karlsruhe Tritium Neutrino (KATRIN) experiment is targeted to determine the effective mass of the electron antineutrino with an unprecedented sensitivity of 200$\,$meV/$c^2$ (90\% C.L.). This will be achieved by measuring the beta-spectrum of tritium close to the kinematic endpoint at 18.6$\,$keV. Gaseous tritium is contained in the Windowless Gaseous Tritium Source (WGTS) at an activity of $10^{11}\,$Bq in a magnetic field of 2.5$\,$T and at a temperature of 100$\,$K or less, depending on the operation mode. Inside the WGTS a cold, magnetized plasma is formed due to the continuous creation of ions and electrons via beta-decay and subsequent inelastic scattering of beta-electrons off gas molecules. The space charge potential of this plasma defines the reference energy scale for the neutrino mass measurement and therefore stringent requirements regarding homogeneity and temporal stability have to be met in order to reach the KATRIN design sensitivity. In addition, the neutrino mass sensitivity potentially could be worsened by ions leaving the WGTS and inducing background events. This thesis presents experimental studies of these effects in the course of various KATRIN measurement campaigns performed in the years 2017 to 2019. By operating a dedicated hardware setup for ion blocking, removal and detection, the ion-induced background could be minimized. Extensive investigations of the interplay between the plasma potential and various hardware devices - most importantly the rear wall disc mounted at the WGTS rear end - allowed for the determination of optimum operational parameters to minimize systematic plasma effects during the first neutrino mass measurements

Effect of ionized impurity screening on spin decoherence at low and intermediate temperatures in GaAs

We study the effect of charged impurity screening on spin decoherence in bulk n-type GaAs, and analyze in details the effect of the use of different Born approximations applied to a linearized Thomas-Fermi screening theory. The spin relaxation times are calculated by ensemble Monte Carlo techniques, including electron-electron, electron-impurities, and electron-phonons scattering. We carefully choose a parameter region so that all the physical approximations hold, and, in particular, a Yukawa-type potential can be used to describe the screened Coulomb interaction and the Born series converges. Our results show that including the second order Born approximation yields much shorter spin relaxation times compared to the commonly implemented first Born approximation: spin relaxation times may be reduced by hundreds of picoseconds, with the first Born approximation overestimating results by 30% or more for a large region of parameters. Though our ensemble Monte Carlo simulations include electron-electron and electron-phonon interactions, when considering low to intermediate carrier densities and T > 50 K, but T smaller than the Fermi temperature, our results are in good agreement with Dyakonov-Perel theory when this includes electron-impurity interactions only, which supports this to be the most relevant scattering mechanism for bulk GaAs in this low-intermediate temperature regime

Quantum simulations of spin-relaxation and transport in copper

A quantum equation of motion method is applied to simulate conduction electron spin-relaxation and transport in the presence of the spin-orbit interaction and disorder. A spin-relaxation time of 25ps is calculated for Cu with a realistic low temperature resistivity of 3.2 μΩ cm – corresponding to a spin-diffusion length of about 0.4 μm. Spin-relaxation in a finite nanocrystallite of Cu is also simulated and a short spin-relaxation time (0.47 ps) is calculated for a crystallite with 7% surface atoms. The spin-relaxation calculated for bulk Cu is in good agreement with experimental evidence, and the dramatic nanocrystallite effect observed has important implications for nano-spintronic devices. Copyright EDP Sciences/Società Italiana di Fisica/Springer-Verlag 200772.25.Ba Spin polarized transport in metals, 73.63.-b Electronic transport in nanoscale materials and structures, 72.25.Hg Electrical injection of spin polarized carriers,

First operation of the KATRIN experiment with tritium

The determination of the neutrino mass is one of the major challenges in astroparticle physics today. Direct neutrino mass experiments, based solely on the kinematics of β β -decay, provide a largely model-independent probe to the neutrino mass scale. The Karlsruhe Tritium Neutrino (KATRIN) experiment is designed to directly measure the effective electron antineutrino mass with a sensitivity of 0.2 eV 0.2 eV (90% 90% CL). In this work we report on the first operation of KATRIN with tritium which took place in 2018. During this commissioning phase of the tritium circulation system, excellent agreement of the theoretical prediction with the recorded spectra was found and stable conditions over a time period of 13 days could be established. These results are an essential prerequisite for the subsequent neutrino mass measurements with KATRIN in 2019