Spectroscopic Investigations of Intermolecular Effects in the KATRIN Tritium Source Loop

The Karlsruhe Tritium Neutrino (KATRIN) experiment aims to measure the effective electron anti-neutrino mass with sensitivity of $m(\nu_\mathrm{e})=0.2$ eV (90% C.L.) by investigating the shape of the energy spectrum of the electrons released during the decay of molecular tritium near the kinematic endpoint of 18.6 keV. KATRIN uses a high luminosity tritium windowless gaseous tritium source (WGTS) with an activity of 100 GBq stabilised at the per mil level and a high voltage system with ppm stability at the spectrometer, which has an ultra-high vacuum inside.

In order to achieve the design sensitivity, it is essential to understand and quantify all the systematic effects that have an influence on the shape of the spectrum. The main focus of this work is the design, construction and set-up of a new tritium-compatible experiment for the systematic investigation of the molecular and intermolecular effects of all six hydrogen isotopologues under WGTS-like conditions. The composition of the gas at KATRIN is significantly determined by three contributions: chemical (H$_2$, HD, HT, D$_2$, DT, T$_2$), ortho-para (o-H$_2$, p-H$_2$, o-D$_2$, p-D$_2$, o-T$_2$, p-T$_2$) and dimers as the simplest form of two bound molecules (T$_2$)$_2$. ... mehrStudies in the literature on conversion rates and exchange reactions or dimer formation are only partially available for tritium or, as in the case of dimer studies, not available at all. Raman and infrared absorption spectroscopy are the methods of choice.

A total of three pre-studies were carried out, the results of which fed directly into the design of the new experiment. The first pre-investigation investigated the influence of tritium decay on the natural exchange reaction of H$_2$ and D$_2$ as well as H$_2$ and T$_2$. With an additional experiment designed and successfully commissioned during this work, both chemical and ortho-para studies of the inactive isotopologues could be investigated. The proof of principle of the newly developed measurement principle of partial catalysis at 77 K and concentration measurement by Raman spectroscopy online and in real time at room temperature was demonstrated. In the last investigation, gold- and silver-coated optics were tested for their suitability for Raman and IR spectroscopy.

Taking into account the results of the previous investigations, a new tritium compatible experiment was designed, constructed and set up. The experiment fulfils all the requirements and therefore ensures the necessary measurements to determine the intermolecular effects under KATRIN-like conditions.