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Calibration Systems of the KATRIN Experiment and Systematics Inherent in the Search for Sterile Neutrinos at the keV Scale


The presence of non-zero neutrino mass is one of the most important discoveries of physics beyond the standard model. Experiments in neutrino oscillation have established the mass splittings and mixing parameters of the (as of now) three known mass states, but the absolute scale of neutrino masses remains a mystery. In addition, evidence has mounted suggesting that the current picture of the three neutrino species may not be complete.

The KATRIN (Karlsruhe Tritium Neutrino) experiment aims to measure the neutrino mass to 0.2 eV, representing an order of magnitude improvement in any direct measurement of the absolute scale. KATRIN will accomplish this task by measuring the endpoint spectrum of beta decay electrons from molecular tritium. The high precision beta spectroscopy will be performed via magnetic collimation electrostatic (MAC-E) filter spectroscopy. This measurement technique presents several sources of systematic error. In particular, it is of utmost importance that the activity and column density is of tritium is known to high precision. The presence of backscattered electrons also have the potential to alter the shape of the tritium endpoint. This document contains discussion of these systematic uncertainties, along with the design and construction of calibration systems that aim to reduce those systematics.

The question of electron backscattering, and how it applies to the KATRIN measurement has also been explored experimentally for the first time. A measurement has been performed using a scanning electron microscope and custom-designed electrostatic spectrometer to measure the energy spectrum of backscattered electrons close to the tritium endpoint energy. An analysis has also been performed to explore the possible systematics originating from backscattered electrons on a possible keV-scale sterile neutrino search.

The electron backscattering experiment has measured Auger electrons from gold in the energy range 6-11 keV produced at a rate of $10^{-4}$ Auger electrons per incident electron. An investigation of proposed keV sterile neutrino search analysis methods show that this rate of Auger electron production is problematic, and work needs to be done to reduce this source of background if a keV-scale sterile neutrino search is to be successful.

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